Chapter3

Minerals and Rocks

October 2012

Introduction to rock-forming Minerals• The Earth is composed of rocks.

• Rocks are aggregates of minerals. • Minerals are composed of atoms.

• In order to understand rocks, we must first have an understanding of minerals.

• In order to understand minerals we must have some basic understanding of atoms - what they are and how they interact with one another to form minerals.

Definition of a Mineral

• Naturally formed; it forms in nature on its own,• Solid (it cannot be a liquid or a gas),• With a definite chemical composition (every

time we see the same mineral it has the same chemical composition that can be expressed by a chemical formula), and

• Characteristic crystalline structure (atoms are arranged within the mineral in a specific ordered manner).

Cont’dExamples• Glass - can be naturally formed (volcanic glass called

obsidian), is a solid, its chemical composition, however, is not always the same, and it does not have a crystalline structure. Thus, glass is not a mineral.

• Ice - is naturally formed, is solid, and does have a definite chemical composition that can be expressed by the formula H2O. Thus, ice is a mineral, but liquid water is not (since it is not solid).

• Halite (salt) - is naturally formed, is solid, does have a definite chemical composition that can be expressed by the formula NaCl, and does have a definite crystalline structure. Thus halite is a mineral.

•Therefore, a mineral is a naturally occurring, inorganic, solid with a definite composition and a regular internal crystal structure.

Atomic Chemistry and Bonding

• All matter is made up of atoms, and all atoms are made up of three main particles known as protons, neutrons and electrons.

• As summarized in the following table, protons are positively charged, neutrons are uncharged and electrons are negatively charged. The negative charge of one electron balances the positive charge of one proton. Both protons and neutrons have a mass of 1, while electrons have almost no mass.

Elementary particle

Charge Mass

Electron -1 ~0

Proton +1 1

Neutron 0 1

Cont’d

• The simplest atom is that of hydrogen, which has one proton and one electron.

• The proton forms the nucleus of hydrogen, while the electron orbits around it.

• All other elements have neutrons as well as protons in their nucleus. • The positively-charged protons tend to repel each other, and the neutrons

help to hold the nucleus together. • For most of the 16 lightest elements (up to oxygen) the number of neutrons

is equal to the number of protons. • For most of the remaining elements there are more neutrons than protons,

because with increasing numbers of protons concentrated in a very small space, more and more extra neutrons are needed to overcome the mutual repulsion of the protons in order to keep the nucleus together.

• The number of protons is the atomic number; the number of protons plus neutrons is the atomic weight.

• For example, silicon has 14 protons, 14 neutrons and 14 electrons. Its atomic number is 14 and its atomic weight is 28.

Structure of Atoms• Electrons orbit around the nucleus in different shells, labeled from the innermost shell

as K, L, M, N, etc. Each shell can have a certain number of electrons. The K-shell can have 2 Electrons, the L-shell, 8, the M-shell 18, N-shell 32.

#electrons = 2N2, where N=1 for the K shell, N=2 for the L shell, N=3 for the M shell, etc.• A Stable electronic configuration for an atom is one 8 electrons in outer shell (except in

the K shell, which is completely filled with only 2 electrons). Thus, atoms often loose electrons or gain electrons to obtain stable configuration.

• Noble gases have completely filled outer shells, so they are stable. Examples He, Ne, Ar, Kr, Xe, Rn.

• Others like Na, K loose an electron. This causes the charge balance to become unequal. In fact to become + (positive) charged atoms called ions.

• Positively charged atoms = cations. Elements like F, Cl, O gain electrons to become (-) charged. (-) charged ions are called anions.

Types of bonding:

Ionic bonding- caused by the force of attraction between ions of opposite charge. Example: Na+1 and Cl-1. Bond to form NaCl (halite or salt).

Covalent bonding - Electrons are shared between two or more atoms so that each atom has a stable electronic configuration (completely filled outermost shell) part of the time. Example: H has one electron, needs 2 to be stable. O has 6 electrons in its outer shell, needs 2 to be stable. So, 2 H atoms bond to 1 O to form H2O, with all atoms sharing electrons, and each atom having a stable electronic configuration part of the time.

Cont’d

• Metallic bonding -- Similar to covalent bonding, except innermost electrons are also shared. In materials that bond this way, electrons move freely from atom to atom and are constantly being shared. Materials bonded with metallic bonds are excellent conductors of electricity because the electrons can move freely through the material.

• Van der Waals bonding -- a weak type of bond that does not

share or transfer electrons. Usually results in a zone along which the material breaks easily (cleavage). A good example is graphite. Several different bond types can be present in a mineral, and these determine the physical properties of the mineral.

Crystal Structure• Solids having a regular, orderly arrangement of their internal atoms are

said to have crystalline structure and are known as crystals. Most solid substances, including rocks and minerals, are made up of aggregates of many small crystals.

• Crystals are characteristically bounded by flat surfaces, which are large-scale reflection of the internal arrangement of the atoms in the crystal.

• Study of the arrangement of faces in natural crystals showed that there are six basic groups, each with a characteristic symmetry of the faces.

• Packing of atoms in a crystal structure requires an orderly and repeated atomic arrangement. Such an orderly arrangement needs to fill space efficiently and keep a charge balance.

• Since the size of atoms depends largely on the number of electrons, atoms of different elements have different sizes.

• Crystal structure depends on the conditions under which the mineral forms.

The Six basic Systems of Crystal symmetry

1. Isometric: Three equal-length axes at right angles to each other. Example; Garnet Magnetite, Halite, Pyrite

2. Tetragonal: Two equal axes and a third either longer or shorter, all at right angles. Example; Zircon

3. Orthorhombic: Three unequal axes, all at right angles. Example; Olivine, Aragonite, Anhydrite, Goethite

4. Monoclinic: Three unequal axes, two at right angles and a third perpendicular to one but oblique to the other. Example; Pyroxene, Amphibole, Micas, Orthoclase, Gypsum

5. Triclinic: Three unequal axes meeting at oblique angles. Example; Plagioclase, Aluminosilcate

6. Hexagonal: Three equal axes in the same plane intersecting at 600 and a fourth perpendicular to the plane of the other three. Example; Quartz, Calcite, Hematite

Composition of MineralsThe variety of minerals we see depend on the chemical elements available to

form them. In the Earth's crust the most abundant elements are as follows:• 1. O, Oxygen 45.2% by weight• 2. Si, Silicon 27.2%• 3. Al, Aluminum 8.0%• 4. Fe, Iron 5.8%• 5. Ca, Calcium 5.1%• 6. Mg, Magnesium 2.8%• 7. Na, Sodium 2.3%• 8. K, Potassium 1.7%• 9. Ti, Titanium 0.9%• 10. H, Hydrogen 0.14%• 11. Mn, Manganese 0.1%• 12. P, Phosphorous 0.1%

The most common minerals are those based on Si and O: the Silicates. Silicates are based on SiO4 tetrahedron. 4 Oxygens covalently bonded to one

silicon atom.

ionic radii

Ionic radii of ions commonly found in rock-forming minerals.

• Polymorphs are minerals with the same chemical composition but different crystal structures. The conditions are such things as temperature (T) and pressure (P), because these affect ionic radii. At high T atoms vibrate more, and thus distances between them get larger. Crystal structure changes to accommodate the larger atoms. At even higher T substances changes to liquid and eventually to gas. Liquids and gases do not have an ordered crystal structure and are not minerals. Increase in P pushes atoms closer together. This makes for a more densely packed crystal structure.

Examples:

Carbon (C) has two different polymorphs. At low T and P pure carbon is the mineral graphite, (pencil lead), a very soft mineral.

At higher T and P the stable form is diamond, the hardest natural substance known.

Ionic Substitution (Solid Solution) Ionic substitution - (also called solid solution), occurs because

some elements (ions) have the same size and charge, and can thus substitute for one another in a crystal structure.

Examples:• Olivines Fe2SiO4 and Mg2SiO4. Fe+2 and Mg+2 are about the same

size, thus they can substitute for one another in the crystal structure and olivine thus can have a range of compositions expressed as the formula (Mg, Fe)2SiO4.

• Alkali Feldspars: KAlSi3O8 (orthoclase) and NaAlSi3O8, (albite) K+1 can substitute for Na+1

• Plagioclase Feldspars: NaAlSi3O8 (albite) and CaAl2Si2O8 (anorthite) NaSi+5 can substitutes for CaAl+5 (a complex solid solution).

Properties of MineralsPhysical properties of minerals allow us to distinguish between minerals and thus identify them, as

you will learn in lab. Among the common properties used are:• Habit - shape• Color• Streak (color of fine powder of the mineral)• Luster -- metallic, vitreous, pearly, resinous (reflection of light)• Cleavage (planes along which the mineral breaks easily)• Density (mass/volume)• Hardness: based on Mohs hardness scale as follows:

1. Talc2. Gypsum (fingernail)3. Calcite (penny)4. Fluorite5. Apatite (knife blade)6. Feldspar (Orthoclase) 7. Quartz8. Topaz9. Corundum10. Diamond

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Formation of Minerals

Minerals are formed in nature by a variety of processes. Among them are:

Crystallization from melt (igneous rocks).

Precipitation from water (chemical sedimentary rocks, hydrothermal ore deposits).

Biological activity (biochemical sedimentary rocks).

Change to more stable state - (the processes of weathering, metamorphism, and diagenesis).

Precipitation from vapor. (not common, but sometimes does occur around volcanic vents).

Group of Minerals

• We group minerals into classes on the basis of their predominant anion or anion group. These include oxides, sulphides, carbonates, silicates, and others.

• Silicates are by far the predominant group in terms of their abundance within the crust and mantle.

GROUP EXAMPLES

Oxides hematite (iron-oxide – Fe2O3), corundum (aluminum-oxide Al2O3), water-ice (H2O)

Sulphides galena (lead-sulphide - PbS), pyrite (iron-sulphide – FeS2), chalcopyrite (copper-iron-sulphide – CuFeS2)

Carbonates calcite (calcium-carbonate – CaCO3), dolomite (calcium-magnesium-carbonate – (Ca,Mg)CO3

Silicates quartz (SiO2)*, feldspar (sodium-aluminum-silicate – NaAlSi3O8), olivine (iron or magnesium-silicate - FeSiO4)

Halides fluorite (calcium-fluoride – CaF2), halite (sodium-chloride - NaCl)

Sulphates gypsum (calcium-sulphate – CaSO4·H2O), barite (barium-sulphate - BaSO4)

Phosphate The most important phosphate mineral is apatite (Ca5(PO4)3(OH))

Native elements gold (Au), diamond (C), graphite (C), sulphur (S), copper (Cu)

Silicate Minerals

• The vast majority of the minerals that make up the rocks of the earth's crust are silicate minerals.

• These include minerals such as quartz, feldspar, mica, amphibole, pyroxene, olivine, and a great variety of clay minerals.

• The building block of all of these minerals is the Silica tetrahedron, a combination of four oxygen atoms and one silicon atom.

• These are arranged such that planes drawn through the oxygen atoms describe a tetrahedron (a four-faced object)—which is a pyramid with a triangular base.

Silicon-oxygen tetrahedron

Cont’d• The bonds in a silica tetrahedron have some of the properties of covalent

bonds and some of the properties of ionic bonds.• As a result of the ionic character, silicon becomes a cation (with a charge of

+4) and oxygen becomes an anion (with a charge of -2), hence the net charge of a silica tetrahedron Si04 is -4.

• Silica tetrahedra are linked together in a variety of ways to form most of the common minerals of the crust.

• Most minerals are characterized by ionic or covalent bonds or a combination of the two, but one other type of bond which is geologically important is the metallic bond.

• Elements that behave as metals have outer electrons that are relatively loosely held. When bonds between such atoms are formed these electrons can move freely from one atom to another.

• A metal can thus be thought of as an array of positively charged nuclei immersed in a sea of mobile electrons. This characteristic accounts for two very important properties of metals: their electrical conductivity and their malleability.

Silicon-oxygen tetrahedron Structure

The silica tetrahedron and the structure of silicate minerals.

a. The silica tetrahedron consists of a central silicon atom bound to 4oxygens.

b. In orthosilicates such as olivine, the tetrahedra are separateand each oxygen is also bound to other metal ions that occupy interstitialsites between the tetrahedra.

c. In pyroxenes, the tetrahedra eachshare two oxygen and are bound together into chains. Metal ions arelocated between the chains.

d. In sheet silicates, such as talc, mica, andclays, the tetrahedra each share 3 oxygens and are bound together intosheets.

Silicon-oxygen tetrahedronName Structural Group Unit Example Typical Formula

Nesosilicates Independent tetrahedra SiO4 Olivine (Fe,Mg)2SiO4

Sorosilicates Two tetrahedra sharing one oxygen

Si2O7 Melilite Ca2MgSi2O7

Cyclosilicates Closed rings of tetrahedra each sharing two oxygens

(SiO3)nn=3,4,6

Beryl (6-fold)Axinite (4-fold)Benitoite (3-fold)

Be3Al2(SiO3)6

Ca2(Mn,Fe2)Al2BO3(SiO3)4(OH)

BaTi(SiO3)3

Inosilicates (a) Continuous single chains of tetrahedra, each sharing two oxygens(b) Continuous double chains of tetrahedra alternately sharing two and three oxygens

(SiO3)

Si4O11

PyroxenesPyroxenoidsAmphiboles

MgSiO3

CaSiO3

Mg7Si8O22(OH)2

Phyllosilicates Continuous sheets of tetrahedra sharing three oxygens

Si4O10 MicasTalc

KAl2(Si3Al)O10(OH,F)2

Mg3Si4O10(OH)2

Tektosilicates Three-dimensional framework of tetrahedra with all four oxygen atoms shared

SiO2

QuartzFeldspars

SiO2

KAlSi3O8

IGNEOUS ROCKS

Origin of Igneous rocks

• An igneous rock is any crystalline or glassy rock that forms from cooling of magma.

• Magma consists mostly of liquid rock matter, but may contain crystals of various minerals, and may contain a gas phase that may be dissolved in the liquid or may be present as a separate gas phase.

Cont’d

• Magma can cool to form an igneous rock either on the surface of the Earth - in which case it produces a volcanic or extrusive igneous rock, or beneath the surface of the Earth, in which case it produces a plutonic or intrusive igneous rock.

• At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a separate vapor phase when pressure is decreased as magma rises toward the surface.

• This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles.

• Gas gives magmas their explosive character, because volume of gas expands as pressure is reduced.

Cont’dThe composition of the gases in magma is:• Mostly H2O (water vapor) with some CO2 (carbon dioxide)• Minor amounts of Sulfur, Chlorine, and Fluorine gases• The amount of gas in magma is also related to the chemical composition of the

magma.• Rhyolitic magmas usually have higher dissolved gas contents than basaltic

magmas.Types of MagmaTypes of magma are determined by chemical composition of the magma. Three general types are recognized, but we will look at other types later in the course:1. Basaltic magma (1000-1200oC) -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na2. Andesitic magma (800-1000oC) -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K3. Rhyolitic or Granitic magma (650-800oC) -- SiO2 65-75%, low in Fe, Mg, Ca, high in K, NaViscosity of Magmas Viscosity is the resistance to flow (opposite of fluidity). Depends on composition, temperature, & gas content. Higher SiO2 content magmas have higher viscosity than lower SiO2 content magmas. Lower Temperature magmas have higher viscosity than higher temperature magmas.

SummarySummary Table

Magma Type

Solidified Volcanic

Rock

Solidified Plutonic

Rock

Chemical Composition

Temperature Viscosity Gas Content

Basaltic Basalt Gabbro 45-55 SiO2 %, high in Fe, Mg, Ca, low in K, Na

1000 - 1200 oC Low Low

Andesitic Andesite Diorite 55-65 SiO2 %, intermediate in Fe, Mg, Ca, Na, K

800 - 1000 oC Intermediate

Intermediate

Rhyolitic Rhyolite Granite 65-75 SiO2 %, low in Fe, Mg, Ca, high in K, Na

650 - 800 oC High High

Origin of Magma

• In order for magmas to form, some part of the Earth must get hot enough to melt the rocks present.

• Since rocks are mixtures of minerals, they behave somewhat differently. Unlike minerals, rocks do not melt at a single temperature, but instead melt over a range of temperatures.

• Thus, it is possible to have partial melts from which the liquid portion might be extracted to form magma.

• The two general cases are:– Melting of dry rocks is similar to melting of dry minerals, melting

temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt. The degree of partial melting can range from 0 to 100%

– Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals; melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt.

Origin of Basaltic Magma

Much evidence suggests that Basaltic magmas result from dry partial melting of mantle. Basalts contain minerals like olivine, pyroxene and plagioclase, none of which contain

water. Basalts erupt non-explosively, indicating a low gas content and therefore low water

content. The Mantle is made of garnet peridotite (a rock made up of olivine, pyroxene, and

garnet). Under normal conditions the temperature in the Earth, shown by the geothermal

gradient, is lower than the beginning of melting of the mantle. Thus in order for the mantle to melt there has to be a mechanism to raise the

geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure

or depth, carrying its heat with it. If the raised geothermal gradient becomes higher than the initial melting temperature at

any pressure, then a partial melt will form. Liquid from this partial melt can be separated from the remaining crystals because, in

general, liquids have a lower density than solids. Basaltic or gabbroic magmas appear to originate in this way.

Origin of Granitic or Rhyolitic Magma

Most Granitic or Rhyolitic magma appears to result from wet melting of continental crust. The evidence for this is:

• Most granites and rhyolites are found in areas of continental crust.

• When granitic magma erupts from volcanoes it does so very explosively, indicating high gas content.

• Solidified granite or rhyolite contains quartz, feldspar, hornblende, biotite, and muscovite. The latter minerals contain water, indicating high water content.

Origin of Andesitic Magma

• Average composition of continental crust is andesitic, but if andesite magma is produced by melting of continental crust then it requires complete melting of crust. Temperatures in crust unlikely to get high enough.

• Andesitic magmas erupt in areas above subduction zones, suggests relation between production of andesite and subduction.

• One theory involves wet partial melting of subducted oceanic crust. But, newer theories suggest wet partial melting of mantle.

Magmatic Differentiation

• When magma solidifies to form a rock it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change.

• Depending on how many minerals are lost in this fashion, a wide range of compositions can be made. The process is called magmatic differentiation.

Over the years, various processes have been suggested to explain the variation of magma compositions observed within small regions. Among the processes are:1. Distinct melting events from distinct sources.2. Various degrees of partial melting from the same source.3. Crystal fractionation.4. Mixing of 2 or more magmas.5. Assimilation/contamination of magmas by crustal rocks.6. Liquid Immiscibility.7. Combined process (a combination of one of these)

1. Distinct Melting Events• One possibility that always exists is that the magmas are not related

except by some heating event that caused melting. In such a case each magma might represent melting of a different source rock at different times during the heating event. The possibility of distinct melting events is not easy to prove or disprove.

2. Various Degrees of Partial Melting• When a multicomponent rock system melts, unless it has the composition of the eutectic, it melts over a range of temperatures at any given pressure, and during this melting, the liquid composition changes. Thus, a wide variety of liquid compositions could be made by various degrees of partial melting of the same source rock.

3. Crystal Fractionation Liquid compositions can change as a result of removing crystals from the liquid as they form. In all cases, crystallization results in a change in the composition of the liquid, and if the crystals are removed by some process, then different magma compositions can be generated from the initial parent liquid. If minerals that later react to form a new mineral or solid solution minerals are removed, then crystal fractionation can produce liquid compositions that would not otherwise have been attained by normal crystallization of the parent liquid.

Bowen's Reaction Series

• Norman L. Bowen, an experimental Petrologist in the early 1900s, realized this from his determinations of simple 2- and 3-component phase diagrams, and proposed that if an initial basaltic magma had crystals removed before they could react with the liquid, that the common suite of rocks from basalt to rhyolite could be produced.

• This is summarized as Bowen's Reaction Series.

Bowen's Reaction…cont’dBowen suggested that the common minerals that crystallize from magmas could be divided

into a continuous reaction series and a discontinuous reaction series.• The continuous reaction series is composed of the plagioclase feldspar solid solution

series. A basaltic magma would initially crystallize a Ca- rich plagioclase and upon cooling continually react with the liquid to produce more Na-rich plagioclase. If the early forming plagioclase were removed, then liquid compositions could eventually evolve to those that would crystallize a Na-rich plagioclase, such as a rhyolite liquid.

• The discontinuous reaction series consists of minerals that upon cooling eventually react with the liquid to produce a new phase. Thus, as we have seen, crystallization of olivine from a basaltic liquid would eventually reach a point where olivine would react with the liquid to produce orthopyroxene. Bowen postulated that with further cooling pyroxene would react with the liquid, which by this time had become more enriched in H2O, to produce hornblende. The hornblende would eventually react with the liquid to produce biotite. If the earlier crystallizing phases are removed before the reaction can take place, then increasingly more siliceous liquids would be produced.

This generalized idea is consistent with the temperatures observed in magmas and with the mineral assemblages we find in the various rocks. We would expect that with increasing SiO2 oxides like MgO, and CaO should decrease with higher degrees of crystal fractionation because they enter early crystallizing phases, like olivines and pyroxenes. Oxides like H2O, K2O and Na2O should increase with increasing crystal fractionation because they do not enter early crystallizing phases.

Mechanisms of Crystal FractionationCrystal Settling/Floating - In general, crystals forming from magma will have different densities than the liquid.

Inward Crystallization - Because a magma body is hot and the country rock which surrounds it is expected to be much cooler, heat will move outward away from the magma. Thus, the walls of the magma body will be coolest, and crystallization would be expected to take place first in this cooler portion of the magma near the walls. The magma would then be expected to crystallize from the walls inward.

Filter pressing - this mechanism has been proposed as a way to separate a liquid from a crystal-liquid mush. In such a situation where there is a high concentration of crystals the liquid could be forced out of the spaces between crystals by some kind of tectonic squeezing that moves the liquid into a fracture or other free space, leaving the crystals behind. It would be kind of like squeezing the water out of a sponge.

4. Magma Mixing

• If two or more magmas with different chemical compositions come in contact with one another beneath the surface of the Earth, then it is possible that they could mix with each other to produce compositions intermediate between the end members. If the compositions of the magmas are greatly different (i.e. basalt and rhyolite), there are several factors that would tend to inhibit mixing.

– Temperature contrast - basaltic and rhyolitic magmas have very different temperatures. If they come in contact with one another the basaltic magma would tend to cool or even crystallize and the rhyolitic magma would tend to heat up and begin to dissolve any crystals that it had precipitated.

– Density Contrast- basaltic magmas have densities on the order of 2600 to 2700 kg/m3, whereas rhyolitic magmas have densities of 2300 to 2500 kg/m3. This contrast in density would mean that the lighter rhyolitic magmas would tend to float on the heavier basaltic magma and inhibit mixing.

– Viscosity Contrast- basaltic magmas and rhyolitic magmas would have very different viscosities. Thus, some kind of vigorous stirring would be necessary to get the magmas to mix.

5. Crustal Assimilation/Contamination

• Because the composition of the crust is generally different from the composition of magmas which must pass through the crust to reach the surface, there is always the possibility that reactions between the crust and the magma could take place.

• If crustal rocks are picked up, incorporated into the magma, and dissolved to become part of the magma, we say that the crustal rocks have been assimilated by the magma.

• If the magma absorbs part of the rock through which it passes we say that the magma has become contaminated by the crust.

• Either of these processes would produce a change in the chemical composition of the magma unless the material being added has the same chemical composition as the magma.

• In a sense, bulk assimilation would produce some of the same effects as mixing, but it is more complicated than mixing because of the heat balance involved.

• In order to assimilate the country rock enough heat must be provided to first raise the country rock to its solidus temperature where it will begin to melt and then further heat must be added to change from the solid state to the liquid state. The only source of this heat, of course, is the magma itself.

6. Liquid Immiscibility

• Liquid immiscibility is where liquids do not mix with each other. We are all familiar with this phenomenon in the case of oil and water/vinegar in salad dressing. We have also discussed immiscibility in solids, for example in the alkali feldspar system. Just like in the alkali feldspar system, immiscibility is temperature dependent.

Two important properties of immiscible liquids.– 1. If immiscible liquids are in equilibrium with solids, both liquids must be in equilibrium with the

same solid compositions.– 2. Extreme compositions of the two the liquids will exist at the same temperature.

• Liquid immiscibility was once thought to be a mechanism to explain all magmatic differentiation. If so, requirement 2, above, would require that siliceous liquids and mafic liquids should form at the same temperature. Since basaltic magmas are generally much hotter than rhyolitic magmas, liquid immiscibility is not looked upon favorably as an explanation for wide diversity of magmatic compositions. Still, liquid immiscibility is observed in experiments conducted on simple rock systems.

There are however, three exceptions where liquid immiscibility may play a role.• 1. Sulfide liquids may separate from mafic silicate magmas.• 2. Highly alkaline magmas rich in CO2 may separate into two liquids, one rich in carbonate, and the

other rich in silica and alkalies. This process may be responsible for forming the rare carbonatite magmas.

• 3. Very Fe-rich basaltic magmas may form two separate liquids - one felsic and rich in SiO2, and the other mafic and rich in FeO.

7. Combined Processes

• As pointed out previously, if any of these processes are possible, then a combination of the process could act to produce chemical change in magmas.

• Thus, although crystal fractionation seems to be the dominant process affecting magmatic differentiation, it may not be the only processes.

• As we have seen, assimilation is likely to accompany by crystallization of magmas in order to provide the heat necessary for assimilation. If this occurs then a combination of crystal fraction and assimilation could occur.

• Similarly, magmas could mix and crystallize at the same time resulting in a combination of magma mixing and crystal fractionation. In nature, things could be quite complicated.

Mode of occurrence of Igneous bodies

Eruption of Magma• When magmas reach the surface of the Earth they

erupt from a vent.

• They may erupt explosively or non-explosively.

• Non-explosive eruptions are favored by low gas content and low viscosity magmas (basaltic to andesitic magmas). Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface and produce Pillow lavas if erupted beneath water.

Type of volcanic flows

Ropy surface of a Pahoehoe flowAA flow, the left side on the photo is a pahoehoe flow

Tephra that falls from the eruptioncolumn produces a tephra fall deposit

If eruption column collapses a pyroclastic flow may occur, wherein

gas and tephra rush down the flanks of the volcano at high speed. This is the most dangerous type of volcanic

eruption. The deposits that areproduced are called ignimbrites.

Structures and Field relationshipsVOLCANOES• Shield volcano – volcanoes that erupt low viscosity magma

(usually basaltic) that flows long distances from the vent.

• Pyroclastic cone or cinder cone – a volcano built mainly of tephra fall deposits located immediately around the vent.

Volcanoes…cont’d• Stratovolcano (composite volcano) – a volcano built of

interbedded lava flows and pyroclastic material.

• Crater - a depression caused by explosive ejection of magma or gas.

Volcanoes…cont’d• Caldera - a depression caused by collapse of a volcano into the

cavity once occupied by magma.

• Lava Dome - a steep sided volcanic structure resulting from the eruption of high viscosity, low gas content magma.

• Fissure Eruptions - An eruption that occurs along a narrow crack or fissure in the Earth's surface.

• Pillow Lava - Lavas formed by eruption beneath the surface of the ocean or a lake.

PLUTONS

• Igneous rocks cooled at depth. • Name comes from Greek god of the underworld - Pluto.

Dikes are small (<20 m wide) shallow intrusions that show a discordant relationship to the rocks in which they intrude. Discordant means that they cut across preexisting structures. They may occur as isolated bodies or may occur as swarms of dikes emanating from a large intrusive body at depth.

Sills are also small (<50 m thick) shallow intrusions that show a concordant relationship with the rocks that they intrude. Sills usually are fed by dikes, but these may not be exposed in the field.

Plutons…cont’d• Laccoliths are somewhat large intrusions that result in uplift and folding of the

preexisting rocks above the intrusion. They are also concordant types of intrusions.

• Batholiths are very large intrusive bodies, usually so large that there bottoms are rarely exposed. Sometimes they are composed of several smaller intrusions.• Stocks are smaller bodies that are likely fed from deeper level batholiths. Stocks may have been feeders for volcanic eruptions, but because large amounts of erosion are required to expose a stock or batholith, the associated volcanic rocks are rarely exposed.

RELATIONSHIPS TO PLATE TECTONICS

Diverging Plate Boundaries Diverging plate boundaries are mostly beneath the oceans and occur at oceanic

ridges. Here, basaltic magma is erupted at the oceanic ridge and is intruded beneath the

ridge where it forms new oceanic crust. Only rarely does the oceanic ridge build itself above the oceans surface. One

example of where this occurs is the island of Iceland in the northern Atlantic Ocean. Eruptions of magma in Iceland are mostly basaltic.

Converging Plate Boundaries• Where lithospheric plates converge, oceanic lithosphere subducts

beneath either another plate composed of oceanic lithosphere or another plate composed of continental lithosphere.

If an oceanic lithospheric plate subducts beneath another oceanic lithospheric plate, we find island arcs on the surface above the subduction zone. These are volcanoes built of mostly andesitic lavas pyroclastic material, although some basalts and rhyolites also occur.

If an oceanic plate subducts beneath a plate composed of continental lithosphere, we find continental margin arcs. Again, the volcanoes found here are composed mostly of andesitic lavas and pyroclastics. It is likely that some magmas cool beneath the volcanic arc to form dioritic and granitic plutons.

Reflection• Why basaltic magmas are not explosive and

rhyolitic magma is explosive?

• Are Volcanic or Plutonic igneous rocks are most common in Ethiopia?

• How plutonic rocks are exposed to the surface?

Textures of Igneous Rocks The main factor that determines the texture of an igneous rock is the cooling

rate (dT/dt)

Other factors involved are:– The diffusion rate - the rate at which atoms or molecules can move (diffuse) through

the liquid.– The rate of nucleation of new crystals - the rate at which enough of the chemical

constituents of a crystal can come together in one place without dissolving.– The rate of growth of crystals - the rate at which new constituents can arrive at the

surface of the growing crystal. This depends largely on the diffusion rate of the molecules of concern.

• In order for a crystal to form in magma enough of the chemical constituents that will

make up the crystal must be at the same place at the same time to form a nucleus of the crystal.

• Once a nucleus forms, the chemical constituents must diffuse through the liquid to arrive at the surface of the growing crystal.

• The crystal can then grow until it runs into other crystals or the supply of chemical constituents is cut off.

•Nucleation and growth cannot occur until temperatures are below the temperature at which equilibrium crystallization begins.

•The rate of crystal growth and nucleation depends on how long the magma resides at a specified degree of undercooling (ΔT = Tm - T), and thus the rate at which temperature is lowered below the crystallization temperature.

1. For small degrees of undercooling (region A in the figure) the nucleation rate will be low and the growth rate moderate. A few crystals will form and grow at a moderate rate until they run into each other. Because there are few nuclei, the crystals will be able to grow to relatively large size, and a coarse grained texture will result. This would be called a phaneritic texture.

2. At larger degrees of undercooling, the nucleation rate will be high and the growth rate also high. This will result in many crystals all growing rapidly, but because there are so many crystals, they will run into each other before they have time to grow and the resulting texture will be a fine grained texture. If the sizes of the grains are so small that crystals cannot be distinguished with a handlens, the texture is said to be aphanitic.

3. At high degrees of undercooling, both the growth rate and nucleation rate will be low. Thus few crystals will form and they will not grow to any large size. The resulting texture will be glassy, with a few tiny crystals called microlites. A completely glassy texture is called holohyaline texture.

4. Two stages of cooling, i.e. slow cooling to grow a few large crystals, followed by rapid cooling to grow many smaller crystals could result in a porphyritic texture, a texture with two or more distinct sizes of grains. Single stage cooling can also produce a porphyritic texture. In a porphyritic texture, the larger grains are called phenocrysts and the material surrounding the phenocrysts is called groundmass or matrix

Grain Size• In a rock with a phaneritic texture, where all grains are

about the same size, we use the grain size ranges shown below to describe the texture:

<1 mm fine grained 1 - 5 mm medium grained 5 - 3 cm coarse grained > 3 cm very coarse grained

• In a rock with a porphyritic texture, we use the above table to define the grain size of the groundmass or matrix, and this table to describe the phenocrysts:

0.03 - 0.3 mm microphenocrysts

0.3 - 5 mm phenocrysts

> 5 mm megaphenocrysts

FabricFabric refers to the mutual relationship between the grains. Three types of fabric are commonly referred to:• 1. If most of the grains are euhedral - that is they are bounded by well-formed

crystal faces. The fabric is said to be idomorphic granular.• 2. If most of the grains are subhedral - that is they bounded by only a few

well-formed crystal faces, the fabric is said to be hypidiomorphic granular.• 3. If most of the grains are anhedral - that is they are generally not bounded

by crystal faces, the fabric is said to be allotriomorphic granular.

ShapeSome common grain shapes are:•Tabular - a term used to describe grains with rectangular tablet shapes.•Equant - a term used to describe grains that have all of their boundaries of approximately equal length.•Fibrous - a term used to describe grains that occur as long fibers.•Acicular - a term used to describe grains that occur as long, slender crystals.•Prismatic - a term used to describe grains that show an abundance of prism faces.

Some common terms to describe texture

• Vesicular - if the rock contains numerous holes that were once occupied by a gas phase, then this term is added to the textural description of the rock.

• Glomeroporphyritic - if phenocrysts are found to occur as clusters of crystals, then the rock should be described as glomeroporphyritic instead of porphyritic.

• Amygdular - if vesicles have been filled with material (usually calcite, chalcedony, or quartz, then the term amygdular should be added to the textural description of the rock. An amygdule is defined as a refilled vesicle.

• Pumiceous - if vesicles are so abundant that they make up over 50% of the rock and the rock has a density less than 1 (i.e. it would float in water), then the rock is pumiceous.

• Scoraceous- if vesicles are so abundant that they make up over 50% of the rock and the rock has a density greater than 1, then the rock is said to be scoraceous.

• Graphic - a texture consisting of intergrowths of quartz and alkali feldspar wherein the orientation of the quartz grains resembles cuneiform writing. This texture is most commonly observed in pegmatites.

• Spherulitic - a texture commonly found in glassy rhyolites wherein spherical intergrowths of radiating quartz and feldspar replace glass as a result of devitrification.

• Obicular - a texture usually restricted to coarser grained rocks that consists of concentrically banded spheres wherein the bands consist of alternating light colored and dark colored minerals.

Textures on microscopic examination of igneous rocks • Myrmekitic texture - an intergrowth of quartz and plagioclase that shows small wormlike bodies of quartz enclosed in

plagioclase. This texture is found in granites.• Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals, wherein the plagioclase is totally

surrounded by pyroxene grains. This texture is common in diabases and gabbros.• Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not completely enclosed in a matrix of

pyroxene grains.• Poikilitic texture - smaller grains of one mineral are completely enclosed in large, optically continuous grains of another

mineral.• Intergranular texture - a texture in which the angular interstices between plagioclase grains are occupied by grains of

ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides.• Intersertal texture - a texture similar to intergranular texture except that the interstices between plagioclase grains are

occupied by glass or cryptocrystalline material.• Hyaloophitic texture - a texture similar to ophitic texture except that glass completely surrounds the plagioclase laths.• Hyalopilitic texture - a texture wherein microlites of plagioclase are more abundant than groundmass and the groundmass

consists of glass which occupies the tiny interstices between plagioclase grains.• Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage and the interstices

between plagioclase grains are occupied by glass or cryptocrystalline material.• Coronas or reaction rims - often times reaction rims or coronas surround individual crystals as a result of the crystal becoming

unstable and reacting with its surrounding crystals or melt. If such rims are present on crystals they should be noted in the textural description.

• Patchy zoning - This sometimes occurs in plagioclase crystals where irregularly shaped patches of the crystal show different compositions as evidenced by going extinct at angles different from other zones in the crystal.

• Oscillatory zoning - This sometimes occurs in plagioclase grains wherein concentric zones around the grain show thin zones of different composition as evidenced by extinction phenomena.

• Moth eaten texture (also called sieve texture)- This sometimes occurs in plagioclase wherein individual plagioclase grains show an abundance of glassy inclusions.

• Perthitic texture - Exsolution lamellae of albite occurring in orthoclase or microcline. • Antiperthitic texture – Exsolution lamellae of orthoclase or microcline occurring in albite.

Classification of Igneous rocks • Classification of igneous rocks is one of the most confusing aspects of geology. This is:-

– partly due to historical reasons, – partly due to the nature of magmas, and – partly due to the various criteria that could potentially be used to classify rocks.

There are various criteria that could be used to classify igneous rocks. Among them are:

1. Minerals Present in the Rock (the mode). The minerals present in a rock and their relative proportions in the rock depend largely on the chemical composition of the magma. This works well as a classification scheme if all of the minerals that could potentially crystallize from the magma have done so - usually the case for slowly cooled plutonic igneous rocks. But, volcanic rocks usually have their crystallization interrupted by eruption and rapid cooling on the surface. In such rocks, there is often glass or the minerals are too small to be readily identified.

2. Texture of the Rock. Rock texture depends to a large extent on cooling history of the magma. Thus rocks with the same chemical composition and same minerals present could have widely different textures. In fact we generally use textural criteria to subdivide igneous rocks in to plutonic (usually medium to coarse grained) and volcanic (usually fine grained, glassy, or porphyritic.) varieties.

4. Chemical Composition. Chemical composition of igneous rocks is the most distinguishing feature.

• The composition usually reflects the composition of the magma, and thus provides information on the source of the rock.

• The chemical composition of the magma determines the minerals that will crystallize and their proportions.

• A set of hypothetical minerals that could crystallize from a magma with the same chemical composition as the rock (called the Norm), can facilitate comparison between rocks.

• Still, because chemical composition can vary continuously, there are few natural breaks to facilitate divisions between different rocks.

• Chemical composition cannot be easily determined in the field, making classification based on chemistry impractical.

3. Color. Color of a rock depends on the minerals present and on their grain size. Generally, rocks that contain lots of feldspar and quartz are light colored, and rocks that contain lots of pyroxenes, olivines, and amphiboles (ferromagnesium minerals) are dark colored.

But color can be misleading when applied to rocks of the same composition but different grain size. For example granite consists of lots of quartz and feldspar and is generally light colored. But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black colored (i.e. an obsidian). Still we can divide rocks in general into felsic rocks (those with lots of feldspar and quartz) and mafic rocks (those with lots of ferromagnesium minerals). But, this does not allow for a very detailed classification scheme.

IUGS system of classification:-Plutonic Olivine

ClinopyroxeneOrthopyroxene

Lherzolite

Har

zbur

gite

Wehrlite

Websterite

Orthopyroxenite

Clinopyroxenite

Olivine Websterite

Peridotites

Pyroxenites

90

40

10

10

Dunite

Felsic to Intermediate rocks

Mafic rocks

Ultramafic rocks

IUGS system of classification:-Volcanic

(foid)-bearing Trachyte

(foid)-bearing Latite

(foid)-bearing Andesite/Basalt

(Foid)ites

10

60 60

35 65

10

20 20

60 60

F

A P

Q

Rhyolite Dacite

Trachyte Latite Andesite/Basalt

Phonolite Tephrite

Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmidt (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298.

IUGS-chemical classification• IUGS chemical classification of volcanic rocks (based on total alkalies [Na2O +

K2O] vs. SiO2

Other chemical classificationSiO2 (Silica) Content• > 66 wt. % - Acid• 52-66 wt% - Intermediate• 45-52 wt% - Basic• < 45 wt % - UltrabasicSilica Saturation

• Silica Undersaturated Rocks - In these rocks we should find minerals that, in general, do not occur with quartz. Such minerals are:

Nepheline- NaAlSiO4 Leucite - KAlSi2O6Forsteritic Olivine - Mg2SiO4 Sodalite - 3NaAlSiO4Perovskite - CaTiO3 Melanite - Ca2Fe+3Si3O12Melilite - (Ca,Na)2(Mg,Fe+2,Al,Si)3O7

• Silica Oversaturated Rocks. These rocks can be identified as possibly any rock that does not contain one of the minerals in the above list.

• Silica Saturated Rocks. These are rocks that contain just enough silica that quartz does not appear, and just enough silica that one of the silica undersaturated minerals does not appear.

Alumina (Al2O3) SaturationThree possible conditions exist.1. Peraluminous rocks containing an excess of Alumina over that required forming

feldspars. This condition is expressed chemically on a molecular basis as:– Al2O3 > (CaO + Na2O + K2O). In peraluminous rocks we expect to find an Al2O3-

rich mineral present as a modal mineral - such as muscovite [KAl3Si3O10(OH)2], corundum [Al2O3], topaz [Al2SiO4(OH,F)2], or an Al2SiO5- mineral like kyanite, andalusite, or sillimanite. Peraluminous rocks will have corundum [Al2O3] in the CIPW norm and no diopside in the norm.

2. Metaluminous rocks are those for which the molecular percentages are as follows:– Al2O3 < (CaO + Na2O + K2O) and Al2O3 > (Na2O + K2O). These are the more

common types of igneous rocks. They are characterized by lack of an Al2O3-rich mineral and lack of sodic pyroxenes and amphiboles in the mode.

3. Peralkaline rocks are those that are oversaturated with alkalies (Na2O + K2O), and thus undersaturated with respect to Al2O3. On a molecular basis, these rocks show:– Al2O3 < (Na2O + K2O). Peralkaline rocks are distinguished by the presence of Na-

rich minerals like aegerine [NaFe+3Si2O6], riebeckite [Na2Fe3+2Fe2+3Si8O22(OH)2], arfvedsonite[Na3Fe4+2(Al,Fe+3)Si8O22(OH)2],or aenigmatite [Na2Fe5+2TiO2Si6O18] in the mode.

Alkaline/Subalkaline Rocks

One last general classification scheme divides rocks that alkaline from those that are subalkaline. Note that this criterion is based solely on an alkali vs. silica diagram, as shown below. Alkaline rocks should not be confused with peralkaline rocks as discussed above. While most peralkaline rocks are also alkaline, alkaline rocks are not necessarily peralkaline. On the other hand, very alkaline rocks, that are those that plot well above the dividing line in the figure below, are also usually silica undersaturated.

Diagram showing Alkaline and Subalkaline division

SEDIMENTARY ROCKS

Sedimentary Rocks

Deposited on or Near Surface of Earth by Mechanical or Chemical Processes

Sedimentary Rock

• Clastic (terrigenous or detrital) – Conglomerate or Breccia – Sandstone – Siltstone – Shale

• Chemical/biochemical – Evaporites – Carbonate sedimentary rocks (limestones and

dolostone) – Siliceous sedimentary rocks

• Organic (coals) – Other - ironstones

• Volcanoclastic

Types of Sedimentary Rock

• Sandstones• Conglomerates• Breccia• Shale/mudstones

Clastic rocks Chemical & Organic rocks

Evaporites rocks

These rocks are formeddue to evaporation of saline water (sea water)eg. Gypsum, Halit (rock salt)

Carbonate rocks

Form basically from CaCO3 – both by chemical leaching and by organic source (biochemical) eg. Limestone; dolomite

Organic rocks

Form due to decomposition of organic remains under temperature and pressure eg. Coal/Lignite etc.

• Formed from broken rock fragments weathered and eroded by river, glacier, wind and sea waves. These clastic sediments are found deposited on floodplains, beaches, in desert and on the sea floors.

CLASTIC ROCKS

Classified by:• Grain Size • Grain

Composition • Texture

• Clastic rocks are classified on the basis of the grain size: conglomerate, sandstone, shale etc.

The formation of a clastic sedimentary rock involves three processes:

Transportation- Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers, or ocean currents. The distance the sediment is transported and the energy of the transporting medium all leave clues in the final sediment of the mode of transportation.

Deposition - Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes to low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium.

Diagenesis - is the process that turns sediment into rock. The first stage of the process is compaction. Compaction occurs as the weight of the overlying material increases. Compaction forces the grains closer together, reducing pore space and eliminating some of the contained water. Some of this water may carry mineral components in solution, and these constituents may later precipitate as new minerals in the pore spaces. This causes cementation, which will then start to bind the individual particles together. Further compaction and burial may cause recrystallization of the minerals to make the rock even harder

Sediment Sizes and Clastic Rock Types

Sedimentary rocks made of silt- and clay-sized particles are collectively called mudstones, and are the most abundant sedimentary rocks.

Rock Type Sediment Grain Size

Shale Clay less than 0.001 mm

Siltstone Silt .001-0.1 mm

Sandstone Sand .01-1 mm

Conglomerate Gravel 1mm +

Grain-size (mm)

Sediment

64-256

4-64

2-4

Cobble Pebble Granule

Gravel

1-2

0.5-1

0.25-0.5

0.125-0.25

0.625-0.125

Very Coarse Sand Coarse Sand Medium Sand Fine Sand Very Fine Sand

Sand

0.031-0.625

0.016-0.031

0.008-0.016

0.004-

0.008

Coarse Silt Medium Silt Fine Silt Very Fine Silt

Silt

<0.004

Clay Clay

The Udden-Wentworth grain-size scale

Clasts (larger pieces, such as sand or gravel)

• Clasts and matrix (labelled),

and iron oxide cement

(reddish brown color)

Gravel: Grain size greater than 2 mm

1. If rounded clasts = conglomerate

2. If angular clasts = breccia

Conglomerates

Breccia

Sandstone

• Sandstones ( > 50% sand-sized (0.062 - 2 mm) clastic grains ):• Sandstones are classified according to the types of clastic

grains present (quartz, feldspar, & lithic fragments) and the presence (wackes) or absence (arenites) of significant fine-grained matrix material (< 0.03 mm).

• After this subdivision they are described in terms of the types of preserved sedimentary structures, using terms like cross-bedded sandstone and relative maturity using criteria such as degree of sorting, roundness of the clasts, diversity of clast types, etc.

MudstoneMudstones: ( > 50% silt (0.062 - 004 mm) and / or clay (< 0.004 mm) • Mudstones are composed of silt-sized quartz and feldspar grains and much smaller

clay mineral particles. Depending of the relative proportions of these two types of grains, mudstones range from siltstones to shales, and claystones.

• Siltstones can be distinguish from shales and mudstones by biting a piece between your teeth. If it feels "gritty" then it is a siltstone, if it feels smooth or slick, then it is a shale or claystone.

• One of the most important features of mud rocks is their colour, an indication of their oxidation state and the paleo-environment of their deposition:– Red shales are oxidized and typically represent sub-aerial detritus derived from the

continents. They may represent in sub-aerial deposits, but also are formed by continental dust settling into organic-poor deep marine environments.

– Green shales are relatively reduced, and common in the shallow submarine environments depleted in oxygen by the decay of organic matter.

– Black shales are rich in organic matter and highly reduced, typically deposited in anoxic environments. They sometimes act as source rocks from which oil and gas are released during burial and diagenesis.

SUMMARY: Characteristics and names of some common clastic sedimentary rocks.

Particle Size Rock Name Rock Characteristics

mud(see below) Shale smooth feel, layered appearance

mud (mud sized particles:

< 0.063 mm) Mudstone smooth feel, massive to layer

silt(silt sized particles:0.063 - 0.004 mm)

Siltstone slightly gritty feel, may have layered appearance

sand(sand-sized particles:

0.0625-2.0 mm)Sandstone

rough gritty feel, constituent grains clearly visible, including quartz, feldspar, other minerals, and rock

fragments.

granules, pebbles, cobbles, boulders (granule to

bouldersized particles:

2 mm - > 256 mm)

Conglomerate large rounded fragments composed of older rock materials

granules, pebbles, cobbles, boulders (granule to

boulder sized particles:2 mm - > 256 mm)

Breccia large angular fragments composed of older rock materials

CHEMICAL ROCKS

Carbonate sediments• Limestone: It is a non-clastic rock

formed either chemically or due to precipitation of calcite (CaCO3) from organisms usually (shell). These remains will result in formation of a limestone.

•Limestones formed by chemical precipitation are usually fine grained, whereas, in case of organic limestone the grain size vary depending upon the type of organism responsible for the formation

– Fossiliferous Limestone: which medium to coarse grained, as it is formed out of cementation of Shells.

Three classification schemes are currently used, each with a different emphasis, but the third, that of Dunham, based on texture, is now used more widely.

A very simple but often useful scheme divides limestones on the basis of grain size into calcirudite (most grains >2mm), calcarenite (most grains between 2mm and 62um) and calcilutite (most grains <62um).

The classification scheme of R.L. Folk based mainly on composition, distinguishes three components: (a) the grains (allochems), (b) matrix, chiefly micrite and (c) cement, usually drusy sparite. An abbreviations for the grains used as prefixes are bio- (referring skeletal components), oo- (for ooids), pel- (for peloids), and intra- (for intraclasts) together with either sparite or micrite, whichever is the dominant. Terms can be combined if two types of grains are dominant in a rock, as in biopelsparite, or bio-oosparite. Terms can be modified to give an indication of coarse grain size, as in biosparrudite, or intramicrudite.

Dunham classification of limestones divides the rocks into: grainstone, grains without matrix (such as a bio- or oosparite); packstone, grains in contact but with considerable matrix (e.g. biomicrite); wackstone, grains are floating in a matrix (could also be a biomicrite); and a mudstone, chiefly micrite with few grains (<10%). The terms can be qualified to give information on composition, e.g. oolitic grainstone, peloidal mudstone.

Depositional Texture Recognizable Depositional texture not

recognizable

Original components not bound together during deposition

Original components were bound together during deposition as shown by intergrown skeletal matter, lamination contrary to gravity, or sedimentat-floored cavities that are roffed over by organic or questionably organic matter and are too large to be interstices.

Contains mud (particles of clay and fine silt size)

Lacks mud and is grain supported

Crystalline Carbonates

Mud-supported Grain-supported Grainstone

(mudstone<1%)

(subdivided according to classifications designed to bear on physical texture or diagenesis)

Mudstone(Grains<10%)

Wackstone(Grains>10%)

Packstone

Boundstone

Classification of Limestone, based on depositional texture (after Dunham)

EVAPORITIC ROCKS

These rocks are formed within the a depositional basin from chemical substances dissolved in the seawater or lake water.

Gypsum Halite

(NaCl)CaSO4.2H20

Chert is the most common chemical siliceous

sediment.

It is a dense rock composed of one or several

forms of silica (opal, chalcedony,

microcrystalline quartz).

It occurs either as nodular segregations, mainly

in a carbonate host rock, or as areally extensive

bedded deposits.

It has a tough, splintery to conchoidal fracture.

It may be white or variously colored gray,

green, blue, pink, red, yellow, brown, and black.

Flint (feuerstein) is a term widely used both as

a synonym for chert and as a variety of chert.

SILICEOUS ROCKS

Coals

Coal Formation

• Delta, continental environments

• Carbonized Woody Material• Often fossilized trees, leaves

present

Oil shale The term oil shale has been applied to any rock from which substantial quantities of oil can be extracted by heating.

Lithology is diverse and may include shales, marlstones, dolomitic limestones and siltstones.

Normally these rocks are fine textured and laminated. They range in color from light shades of brown, green to dark brown, gray or black.

Volcanoclastic Sediments• Fragmental volcanic rocks formed by any mechanism

or origin emplaced in any physiographic environment (on land, under water, or under ice), or mixed with any nonvolcanic fragment types in any proportion are called Volcaniclastic sediments.

• Volcaniclastic materials exhibit all possible degrees of sorting. Some are very well sorted and finely laminated; others are chaotic and unsorted and contain debris ranging from the finest ash to great blocks of either cognate or noncognate (accidental) rocks.

Volcaniclastic sediments (tuffaceous sandstone); Wonchi crater lake

Texture and Structure of Sedimentary rocks

• Clastic- composed of individual fragments that were transported and deposited as particles.

• Crystalline- results from the in situ precipitation of solid mineral crystals.

Texture- refers to the size, shape, arrangement of the grains the make up the rock.

• Grain size- grain diameter (boulders, pebbles, cobbles, sand, silt, or clay).

• Shape- is described in terms of sphericity

• Roundness or (angularity) refers to the sharpness or smoothness of their corners.

Texture

Degree of roundness helps in knowing the distance of transportation (method of erosion)

•Angular clasts- short distance transport from the source•Rounded clasts- long distance transport

Analyzing the parameters of clastic rocks, one may realize about the matrix content, sorting, roundness, and composition of a clastic rock at hand. An important aspect dealing with such variations is known as the Maturity of the rock. Two types of maturity; textural and compositional maturities are devised to analyze degree of weathering and, energy level and persistency of transporting media, during deposition. Texturally immature sediments are those with much matrix, poor sorting and angular grains. Texturally mature sediments, on the other hand, are characterized by little matrix, moderate to good sorting and subrounded to rounded grains. Sediments with no matrix, very good sorting and well-rounded grains are known as super matured. Textural maturity in sandstones is largely a reflection of the depositional process, although it can be modified by diagenetic processes. Where there has been minimal current activity, the sediments generally are texturally immature; persistent current or wind activity results in more mature sandstone.

STRUCTURES

The process of deposition usually imparts variations in layering, bed forms or other structures that point to the environments in which deposition occurred. Such things as water depth, current velocity and current direction can some times be determined from sedimentary structures.

Thus, it is important to recognize various sedimentary structures to infer the depositional environments of ancient sediments. In the study of stratigraphy, especially in the deformed and folded areas, sedimentary structures are so important to understand which way is up/down, so that able to determine the sequence of events occurred in the area.

The structural features that tell us which way is up/down are often referred to as top and bottom indicators.

1.Stratification and Bedding

A. Layering (bedding): One of the most obvious features of sedimentary rocks is layering structure or stratification. The layers are evident because of differences in mineralogy, grain size, degree of sorting, or color of the different layers. In rocks, these differences may be made more prominent by the differences in resistance to weathering or color changes brought out by weathering. Layering is usually, described on the basis of layer thickness as shown in the table below. Distinctive types of layering are described below.

Bed thicknesses (cm) >300 100-300 30-100 10-30 3-10 1-3 0.3-1 < 0.3

MassiveVery thickly beddedThickly beddedMedium beddedThinly BeddedVery thinly beddedThickly laminatedThinly laminated

B. Cross bedding: consists of sets of beds that are inclined with respect to one another. The beds are inclined in the direction that the wind or water was moving at the time of deposition. Boundaries between sets of cross beds, usually, represent an erosional surface. Cross bedding is very common in beach deposits, sand dunes, and river sediments. Individual beds within cross bedded strata are useful indicators of current direction and to indicate the top and bottom part of the bed. All cross beds have asymptotic contact in their lower contact on which they were deposited.

Bed set

Cross beds

Cross bed sets boundary

C. Graded bedding: with decreasing of current velocity, larger or more dense particles are deposited first and followed by smaller particles. If this occurred within a bed, then it results in the formation of a bed that sows decreasing of grain size, upwards. This structure is important in determination of tops and bottoms of beds. Commonly, reverse graded bedding cannot be occurred, as current velocity increased. This is because, as the velocity of the current increased, it will start to erode the surface of the bed instead of progressive deposition of coarser materials.

Graded bed

Upward direction of

the succession

2. Surface Features

These sedimentary structures are developed on the surface of the beds and tell us about water currents, wind direction and climate conditions.

A. Ripple marks: Ripple marks are characteristic of shallow water deposition. They are caused by waves or winds piling up the sediments into long ridges. Based on their geometry, two types of ripple marks are known: Symmetrical and Asymmetrical. Asymmetrical ripple marks can give an indication of current direction of water or wind direction. Symmetrical ripples formed under the condition when the water moves back and forth. Symmetrical ripple marks typify standing water with a steady back and forth movement, such as tidal action.

Back and forth movement of water

Schematic draw of symmetric ripple marks( repetition of lines is to illustrate their appearance in 3D)

Current or wind direction

Asymmetric ripple marks(nearly vertical in the windward side, and gentle slope in the leeward side)

B. Mud cracks: These structures result from the drying out of wet sediments at the surface of the earth. The cracks are due to shrinkage of the sediments (clays), on drying. In cross section, the mud cracks tend to curl up, thus becoming a good top/bottom indicator. The presence of mud cracks indicates that the sediments were exposed to the surface shortly after deposition.

Dallol salt plain

C. Casts and Molds: Any depression formed on the surface of previously deposited sediment, at the interface of water and sediment, may become a mold for any sediment that come later. The body of the newly sediment that takes on the shape of the mold is referred to as cast.

Load casts: These are bulbous protrusions that are formed when compaction causes sediment to be pushed downward in to softer sediments.

Flute casts (Sole marks): Flutes are elongated depressions formed at the surface of the formerly deposited sediment by current erosion. The flutes form an elongated mold for the new sediment. Preservation of the overlying sediment as cast resulted in flute casts, which are some times referred to as sole marks. Flute casts are excellent indicators of current direction and tops/bottoms of beds.

D. Tracks and Trails: These features result from organisms moving across the sediment as they walk, crawl, or drag their body parts through the sediments.

E. Burrow marks: Any organism that burrows in to soft sediment can disturb the sediment and destroy many of the structures. If burrowing is not extensive, the holes made by such organisms can later become filled with water that deposits new sediment in the holes. Burrow marks are also used to indicate top and bottom parts of beds. If animals were churning up and intensively burrowed through the sediment, bioturbation may be resulted. Bioturbation disrupts and even destroys primary bedding and lamination. It may produce nodularity in the sediment, with subtle grain-size differences between burrow and surrounding sediments. A color mottling can be produced by burrowing organisms.

F. Slump folds: formed by down slope mass movement of sediment up on glide plane involving significant bending of sediment layers.

Depositional Environments of Sedimentary rocks• Sediments are formed and accumulated in different places under different conditions. • The sedimentary depositional environment describes the combination of physical, chemical and biological processes took place during the deposition of a particular type of sediment. • Determination of depositional environment of sediments is important to construct the paleogeography and paleoclimatic condition in the study of the geologic history of an area, and to infer economically potential parts of the basin in the exploration of hydrocarbons and minerals. • In most cases, ancient depositional environments are found to be analogous to the existing sedimentation areas.

Important parameters to determine the depositional environment of sediment are:

lithologic composition and rock associationtexturesedimentary structuresfossil contentthe geometry of the sediment

TYPES OF DEPOSITIONAL ENVIRONMENTS

Continental Environments• Alluvial Environment• Aeolian Environment• Fluvial Environment• Lacustrine/ lake/ Environment• Glacial Environment

Transitional Environments• Deltaic Environment• Tidal Environment• Lagoonal Environment• Beach Environment

Marine Environment• Shallow water marine• Deep water Marine

Reef Environment

Rock type Continental environments

Marine environments

Clastic sediments

Mudstones& shale lakes and river flood-plains deep ocean and submarine fans

Sandstone rivers, deserts (eolian) deltas, submarine fans

Conglomerate rivers, glaciers submarine fans

Breccia talus slopes

coal-bearing rock river-bank or near-shore swamps

Chemical sediments

Limestone shallow to moderately deep ocean

Chert deep ocean

evaporite deposits salt lakes and inland seas

METAMORPHIC ROCKS

1. Definitions of MetamorphismThe word "Metamorphism" comes from the Greek: meta = change, morph = form,

Therefore, Metamorphism is defined as :The mineralogical and structural adjustment of solid rocks to physical and

chemical conditions that have been imposed at depths below the near surface zones of weathering and diagenesis and which differ from conditions under

which the rocks in question originated.

So, Metamorphism, In geology refers to the changes in mineral assemblage and texture that result from subjecting a rock to conditions such pressures,

temperatures, and chemical environments different from those under which the rock originally formed.

Metamorphism is characterized by:Phase changes: - growth of new physically discrete, separable components (minerals), either with or without (isochemical) addition of new material; and/or

Textural changes: - recrystallization, alignment and/or grain size, usually as a result of unequal application of stress.

• Note that Diagenesis is also a change in form that occurs in sedimentary rocks. In geology, however, we restrict diagenetic processes to those which occur at temperatures below 200oC and pressures below about 300 MPa (MPa stands for Mega Pascals), this is equivalent to about 3 kilobars of pressure (1kb = 100 MPa).

• Metamorphism therefore occurs at temperatures and pressures higher than 200oC and 300 MPa. Rocks can be subjected to these higher temperatures and pressures as they are buried deeper in the Earth. Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.

• The upper limit of metamorphism occurs at the pressure and temperature where melting of the rock in question begins. Once melting begins the process changes to an igneous process rather than a metamorphic process.

LIMITS OF METAMORPHISM

Factors that Control Metamorphism

TEMPERATURE Temperature increases with depth in the Earth along the Geothermal Gradient. Thus

higher temperature can occur by burial of rock. Temperature can also increase due to igneous intrusion.

PRESSURE Pressure increases with depth of burial, thus, both pressure and temperature will vary

with depth in the Earth. Pressure is defined as force acting equally from all directions. It is a type of stress, called hydrostatic stress, or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress.

FLUID PHASE Any existing open space between mineral grains in rocks can potentially contain a fluid.

This fluid is mostly H2O, but contains dissolved mineral matter. The fluid phase is important because chemical reactions that involve one solid mineral changing into another solid mineral can be greatly speeded up by having dissolved ions transported by the fluid. Within increasing pressure of metamorphism, the pore spaces in which the fluid resides is reduced, and thus the fluid is driven off. Thus, no fluid will be present when pressure and temperature decrease and, as discussed earlier, retrograde metamorphism will be inhibited.

TIME The chemical reactions involved in metamorphism, along with recrystallization, and

growth of new minerals are extremely slow processes. Laboratory experiments suggest that the longer the time available for metamorphism, the larger are the sizes of the mineral grains produced. Thus, coarse grained metamorphic rocks involve long times of metamorphism. Experiments suggest that the time involved is millions of years.

Mineral Asseemblage/ParagenesisMinerals those possessing the lowest chemical potential energy under the conditions of metamorphism are said to be in equilibrium.

These equilibrium minerals referred to simply as Mineral assemblage or Mineral paragenesis.

Mineral assemblages will be written as lists of mineral names separated by plus signs, thus: A+B+C If a rock is to be regarded as having an equilibrium mineral assemblage are as follows:

Each mineral in the assemblage list must have a boundary somewhere in the

rock with all the other members.

The texture must be of a type thought to have formed by metamorphic

recrystallization, not by fragmentation during dynamic metamorphism or igneous

crystallization from a melt.

The minerals must not show compositional zoning.

The minerals must not show obvious replacement textures such as reaction

rims or alteration along cracks.

Reflection!

• How do you distinguish Sedimentary process from Metamorphic process?

• What is the critical factor in determining the upper limit of metamorphism?

116

Sandstonetexture

Quartzitetexture

The difference between diagenetic (sandstone),and metamorphic (quartzite) texture is exemplifiesBy the development of grain boundaries at the latter.

117

The critical factor in determining the upper limit of metamorphism is the presence of water.

If water present (hydrous condition) the metamorphism ceased at low temperature (600-800c), Where melting will start.

If water absent (anhydrous condition), the metamorphism exceeds up to 1000c.

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2. Types of Metamorphism

Contact Metamorphism

Regional Metamorphism

Cataclastic Metamorphism

Hydrothermal Metamorphism

Burial Metamorphism

Shock (impact) Metamorphism

Contact Metamorphism• occurs adjacent to igneous intrusions and results

from high temperatures associated with the igneous intrusion.

• Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole.

• The grade of metamorphism increases in all directions toward the intrusion. Because the temperature contrast between the surrounding rock and the intruded magma is larger at shallow levels in the crust where pressure is low.

• contact metamorphism is often referred to as high temperature, low pressure metamorphism.

• The rock produced is often a fine-grained rock that shows no foliation, called a hornfels.

Regional Metamorphism Occurs over large areas and generally does not show any relationship to igneous

bodies.

Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions.

Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schist, and gneisses.

Regionally metamorphosed rocks occur in the cores of fold/thrust mountain belts or in eroded mountain ranges.

Compressive stresses result in folding of rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to higher temperatures and pressures.

Occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone.

Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing.

is not very common and is restricted to a narrow zone along which the shearing occurred.

Cataclastic Metamorphism

Rocks that are altered at high temperatures and moderate pressures by hydrothermal fluids.

This is common in basaltic rocks that generally lack

hydrous minerals.

The hydrothermal metamorphism results in alteration to Mg-Fe rich hydrous minerals such as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals.

Rich ore deposits are often formed as a result of hydrothermal metamorphism.

Hydrothermal Metamorphism

When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300oC may develop in the absence of differential stress. New minerals grow, but the rock does not appear to be metamorphosed.

The main minerals produced are often the Zeolites. Overlaps, to some extent, with diagenesis, and

grades into regional metamorphism as temperature and pressure increase.

Burial Metamorphism

When an extraterrestrial body, such as a meteorite or comet impacts with the Earth or if there is a very large volcanic explosion,

Ultrahigh pressures can be generated in the impacted rock. These ultrahigh pressures can produce minerals that are only stable at very high pressure, such as the SiO2 polymorphs coesite and stishovite,

Produce textures known as shock lamellae in

mineral grains, and such textures as shatter cones in the impacted rock.

Shock Metamorphism (Impact Metamorphism)

3. Grade of Metamorphism

Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.

• Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals. With increasing grade of metamorphism, the hydrous minerals begin to react with other minerals and/or break down to less hydrous minerals.

• High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O, and non-hydrous minerals become more common.

4. Metamorphic facies The mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to. This pressure and temperature environment is referred to as metamorphic Facies. (This is similar to the concept of sedimentary facies, in that a sedimentary facies is also a set of environmental conditions present during deposition). The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism (see fig. below).

A high geothermal gradient might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies (Low P, High T). Under a normal geothermal gradient, would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism (or depth of burial) increased (Medium P, Medium T). If a low geothermal gradient was present, then rocks would progress from zeolite facies to blueschist facies to eclogite facies (High P, Low T). Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.

Facies Typical mineral assemblages in basic igneous rocks

Typical mineral assemblages in pelitic rocks

Prehnite-pumpellyite

(with relict igneous plagioclase and clinopyroxene) not defined

Medium pressure and Medium temperature

Zeolite smectite + zeolite (with relict igneous plagioclase) not defined

Greenschist chlorite + actinolite + albite + epidote + quartz chlorite + muscovite + chloritoid + quartz

Epidote-amphibolite

hornblende + epidote albite + almandine garnet + quartz almandine garnet + chlorite + muscovite+ biotite + quartz

Amphibolitehornblende + andesine garnet + quartz garnet + biotite + muscovite + sillimanite + quartz

Granuliteclinopyroxene + labradorite + orthopyroxene + quartz garnet + cordierite + biotite + sillimanite + quartz

Pyroxene hornfelsclinopyroxene + labradorite + quartz cordierite + andalusite + biotite + quartz Low pressure and High temperatureSanidiniteclinopyroxene + labradorite + Quartz sanidine + sillimanite + hypersthene + cordierite + quartz

Glaucophane schistglaucophane + lawsonite + quartz muscovite + chlorite + spessartine garnet + quartz High pressure and Low temperatureEclogitepyrope-garnet + omphacite and clinopyroxene) not known

Metamorphism and Plate Tectonics

At present, the geothermal gradients observed are strongly affected by plate tectonics. Along zones where subduction is occurring, magmas are generated near the subduction zone and

intrude into shallow levels of the crust. Because high temperature is brought near the surface, the geothermal gradient (region A in Fig. below), in these regions becomes high and contact metamorphism (hornfels facies) results.

Because compression occurs along a subduction margin (the oceanic crust moves toward the volcanic arc) rocks may be pushed down to depths along either a normal or slightly higher than normal geothermal gradient. Actually the geothermal gradient is likely to be slightly higher, because the passage of magma through the crust will tend to heat the crust somewhat. In these regions (region B in Fig. below), we expect to see greenschist, amphibolite, and granulite facies metamorphic rocks.

Along a subduction zone, relatively cool oceanic lithosphere is pushed down to great depths. This results in producing a low geothermal gradient (temperature increases slowly with depth). This low geothermal gradient results in metamorphism into the blueschist and eclogite facies (region C in Fig. below).

5. Classification of Metamorphic rocks

• Metamorphic rock names are commonly derived utilizing any one, or a combination of the following criterion (Yardley, 1989):

1) The nature of the parent material (bulk composition)

2) The metamorphic mineralogy 3) The rock's texture (grain size and fabric

development) 4) Any appropriate special name

I. Classification of metamorphic rocks based on the nature of the parent material (bulk composition)

Parent Material Rock type

Argillaceous/clay-rich sediments (lutites)

Pelites

Arenaceous (predominately sand-size) sediments

Psammites

Clay-sand mixtures Semi-pelite

Quartz-sand (quartz arenite) Quartzite

(lime muds) Calc-silicate/calcareous

Limestone or dolostone Marble

Volcanics (basalt, andsite, rhyolite, etc.)

Metavolcanics (metabasite (metandesite….etc.)

Ultramafics Metaultramafics

Cont’d• Pelitic. These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks. Because

of their high concentrations of alumina they are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.

• Quartzo-Feldspathic. Rocks that originally contained mostly quartz and feldspar like granitic rocks and arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since these minerals are stable over a wide range of temperature and pressure. Those that exhibit mostly quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic.

• Calcareous. Calcareous rocks are calcium rich. They are usually derivatives of carbonate rocks, although they contain other minerals that result from reaction of the carbonates with associated siliceous detrital minerals that were present in the rock. At low grades of metamorphism calcareous rocks are recognized by their abundance of carbonate minerals like calcite and dolomite. With increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich biotite), chlorite, and tremolite. At even higher grades anhydrous minerals like diopside, forsterite, wollastonite, grossularite, and calcic plagioclase.

• Basic. Just like in igneous rocks, the general term basic refers to low silica content. Basic metamorphic rocks are generally derivatives of basic igneous rocks like basalts and gabbros. They have an abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like plagioclase and epidote.

• Magnesian. Rocks that are rich in Mg with relatively less Fe, are termed magnesian. Such rocks would contain Mg-rich minerals like serpentine, brucite, talc, dolomite, and tremolite. In general, such rocks usually have an ultrabasic protolith, like peridotite, dunite, or pyroxenite.

• Ferruginous. Rocks that are rich in Fe with little Mg are termed ferruginous. Such rocks could be derivatives of Fe-rich cherts or ironstones. They are characterized by an abundance of Fe-rich minerals like greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades.

• Manganiferrous. Rocks that are characterized by the presence of Mn-rich minerals are termed manganiferrous. They are characterized by such minerals as Stilpnomelane and spessartine.

II. Textural and Fabric development

TEXTURAL CLASSIFICATIONThe textures are used to differentiate the relative timing of crystal growth and deformation. Below is a list of common textures. Note: Many of the terms for metamorphic textures contain

the suffix "-blastic ". Terms related to crystals: shape, orientation, and content:

Porphyroblast: a mineral that is larger than its neighbors which grew in the solid state.

– Idioblast: euhedral (well developed crystal faces) porphyroblast

– Xenoblast: anhedral (poorly developed crystal faces) porphyroblast

Porphyroblast

Granoblastic polygonal: a texture in which all grains are about the same size and have planar boundaries intersecting at approximately 120 degrees.

Poikiloblastic: a texture produced when a growing crystal face has enveloped inclusions from its surroundings.

Granoblastic

Poikiloblastic

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Replacement textures:

Corona: a ring of one or more minerals around another mineral or structure, formed by reaction with its surroundings. Pseudomorph: produced when one or more minerals replaces another mineral while retaining its crystal shape.

Symplecitite: intergrowth between mineralsTerms related to deformation and timing of recrystallization:

Relict: A texture of mineral (porphyroclasts) that is inherited from unmetamorphosed rock or from another metamorphic grade (e.g., bedding).

Helicitic: applies to porphyroblasts or porphyroclasts possessing internal foliations (Si) that are curved.

Porphyroclast

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METAMORPHIC FABRIC

 

Grain Size

Fine Medium Coarse

Poorly Foliated hornfels

granofelsmarble

quartziteamphibolite

granofelsmarble

quartziteamphibolite

Well Foliated slate, phyllite schist gneiss

Well Foliated and Sheared mylonite mylonite schist augen gneiss

Marble

Quartzite

Hornfels

Poorly to non-foliated rocks

Hornfels: contact metamorphic rock; tough rock with a random fabric of interlocking grains

Geological Survey of Ethiopia 137

Name Description Slate strongly cleaved rock

-cleavage planes pervasively developed through rock due to orientation of fine phyllosilcate grains-individual grains too fine to be visible with naked eye-overall dull appearance

Phyllite similar to slate, but slightly coarser phyllosilicate grains-grains sometimes discernible in hand specimen, giving silk appearance to cleavage surfaces-often cleavage planes less perfectly planar than slates

Schist -parallel alignment of moderately coarse grains (fabric=schistosity)-grains usually cle4arly visible by eye-grains composed of phyllosilicates and other minerals such as hornblende, actinolite, kyanite

Gneiss coarse grained rock (grain size several millimeters) and-foliated (planar fabric: either schistosity or compositional layering)-tendency for different minerals to segregate into layers parallel to foliation (gneissic layering):typically quartz and feldspar rich layers tend to separate from micaceous layers.Varieties: --Orthogneiss: igneous parentage-- paragneiss: metasedimentary gneisses

Mylonite fine grained rock produced in intense ductile deformation-pre-existing grains are deformed and re-crystallized as finer grains

Foliated texture

Geological Survey of Ethiopia

Slate

Phyllite

Schist

GneissMylonite

Geological Survey of Ethiopia

III. Mineralogical classification

• The most distinguishing minerals are used as a prefix to a textural term.

• Thus, a schist containing biotite, garnet, quartz, and feldspar, would be called biotite-garnet schist.

• A gneiss containing hornblende, pyroxene, quartz, and feldspar would be called hornblende-pyroxene gneiss. A schist containing porphyroblasts of garnet would be called garnet porphyroblastic schist.

• If a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite, metagranite.

IV. Special metamorphic rocks

• Amphibolites: These are medium to coarse grained, dark colored rocks whose principal minerals are hornblende and plagioclase. They result from metamorphism of basic igneous rocks. Foliation is highly variable, but when present the term schist can be appended to the name (i.e. amphibolite schist).

• Marbles: These are rocks composed mostly of calcite, and less commonly of dolomite. They result from metamorphism of limestones and dolostones. Some foliation may be present if the marble contains micas.

• Eclogites: These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphacite, that result from high grade metamorphism of basic igneous rocks. Eclogites usually do not show foliation.

• Quartzites: Quartz arenites and chert both are composed mostly of SiO2. Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz. Such a rock is called a quartzite.

• Serpentinites: Serpentinites are rocks that consist mostly of serpentine. These form by hydrothermal metamorphism of ultrabasic igneous rocks.

• Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock a greasy feel, similar to that of soap. Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.

• Skarns: Skarns are rocks that originate from contact metamorphism of limestones or dolostones, and show evidence of having exchanged constituents with the intruding magma. Thus, skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant calcium and magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma. The chemical exchange is that takes place is called “Metasomatism”.

Cont’d

• Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust. They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear.

• Migmatites: a mixed rock of schistose or gneissic portion intimately mixed with veins of apparently quartzo-feldspathic material (known as leucosomes). Migmatites and its related terms are best reserved for regional field studies and should not be used in hand specimen descriptions.

Migmatitic structures on outcrop

At high-temperatures, rocks undergo partial melting, and the yielded rock is known as Migmatites.

The leucocratic (granitic) materials are well known as leucosomes, while the metamorphic parts are known as mesosome (resistite) and melansomes.

Stromatic

Agmatitic

Vein-type

Nebulitic Philibitic

Folded

Schilleren

6. Structure of Metamorphic rocks

If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. Rounded grains can become flattened in the direction of maximum compressional stress. Minerals that crystallize or grow in the differential stress field may develop a preferred orientation. Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress.

This is because growth of such minerals is easier along directions parallel to sheets or along the direction of elongation and thus will grow along 3 or 2, perpendicular to 1.

The type of structures formed during metamorphism is represented as follows:•Slates/phyllites form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a slatey cleavage. •Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism.  Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates (mainly biotite and muscovite).  Quartz and feldspar grains however show no preferred orientation.  The irregular planar foliation at this stage is called schistosity •Gneiss As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow. These dark colored minerals tend to become segregated into distinct bands through the rock (this process is called metamorphic differentiation), giving the rock a gneissic banding.  Because the dark colored minerals tend to form elongated crystals, rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress. •Granulite - At the highest grades of metamorphism most of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.  The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks. In general, the grain size of metamorphic rocks tends to increase with increasing grade of metamorphism, as seen in the progression form fine grained shales to coarser (but still fine) grained slates, to coarser grained schists and gneisses.

Structural development in metamorphic rocks.