Minerals and Rocks

Chapter 2

Earth Materials – Minerals

• Some minerals, – such as gold, – have fascinated people for

thousands of years – and have been supposed – to have mystical or curative

powers• Minerals have many essential

uses – in industrial societies

• Minerals are the basic units – that make up most of Earth’s

materials

Earth Materials – Rocks

• Rocks also have many uses:– rocks crushed for aggregate in cement and for

roadbeds– sawed and polished rocks for tombstones,

monuments, mantle pieces and countertops– Even the soils we depend on

• for most of our food

• are formed by alteration of rocks

Rocks

• Mountains around Tenaya Lake in Yosemite National Park– Made of granite– Granite is

composed of quartz and feldspar minerals

Minerals

Minerals on display – at the

California Academy of Sciences in San Francisco

Earth Materials

• Some materials formed by the Earth – are interesting and

attractive

– such as this metamorphic rock

• from the shoreline of Lake Superior at Marquette, Michigan

Matter and Its Composition

• Matter– anything that has mass and occupies space– exists as solids, liquids, gases, and plasma– consists of elements and atoms

• Element– a chemical substance – composed of tiny particles called atoms

Atoms

• Atoms are the smallest units of matter – that retain the characteristics of the element

• Atoms have– a compact nucleus containing

• protons – particles with a positive electrical charge

• neutrons – electrically neutral particles

– particles outside the nucleus • electrons – negatively charged particles

Atoms

• Atomic number = the number of protons

• Atomic mass number = number of protons + number of neutrons

• The number of neutrons in nucleus of an element may vary

Carbon Isotopes

• Three isotopes of carbon (all with 6 protons) – 6 neutrons = Carbon 12 (12C)

– 7 neutrons = Carbon 13 (13C)– 8 neutrons = Carbon 14 (14C)

Electrons and Shells

• Electrons lie outside the nucleus in one or more shells• The outermost shells are involved

– in chemical bonding

– and contain up to 8 electrons

• Noble gas configuration of 8 electrons • or 2 for helium

– have complete outer shells

– and are stable

• Other atoms attain – a noble gas configuration

– through the process of bonding

Bonding and Compounds

• Bonding – the process whereby atoms join to other atoms

• Compound – a substance resulting from the bonding – of two or more elements

• Oxygen gas (O2) is an element• Ice (H2O) is a compound

– made up of hydrogen and oxygen atoms

• Most minerals are compounds

Ionic Bonding

• One way for atoms to attain the noble gas configuration – is by transferring electrons, producing ions

• Ion – an atom that has gained or lost one or more

electrons – and thus has a negative or positive charge

• Ionic bonding – attraction between two ions of opposite charge

Covalent Bonding

• Another way for atoms – to attain the noble gas configuration – is by sharing electrons

• Covalent bonding– results from

sharing electrons

shared electrons

Minerals

• Geological definition of a mineral:– naturally occurring– crystalline solid

• crystalline means that minerals • have atoms arranged in specific 3-dimensional

frameworks– inorganic– minerals have a narrowly

defined chemical composition– and characteristic physical

properties such as• density• hardness

Minerals—The Building Blocks of Rocks

• A mineral’s composition is shown by a chemical formula– a shorthand way of indicating how many atoms

of different kinds it contains

Quartz: SiO2

Ratio: 1: 2

– Quartz molecules consist of 1 silicon atom and 2 oxygen atoms

– Orthoclase molecules consists of 1 potassium, 1 aluminum, 3 silicon, and 8 oxygen atoms

KAlSi3O8

1: 1: 3: 8

Native Elements

• A few minerals consist of only one element.

• They are not compounds.

• They are known as native elements.

• Examples: – Gold: Au– Diamond: C

Crystalline Solids

• By definition, minerals are crystalline solids– with atoms arranged in a specific 3D framework

• If given enough room to grow freely, – minerals form perfect crystals with – planar surfaces, called crystal faces– sharp corners– straight edges

Narrowly Defined Chemical Composition

• Some minerals have very specific compositions– Examples: halite (NaCl), quartz (SiO2)

• Other minerals have a range of compositions – because one element can substitute for another – if the atoms of the two elements have

• the same electrical charge• and are about the same size

– Example: olivine • (Mg,Fe)2SiO4

• iron and magnesium substitution in any proportion

Mineral Properties

• Mineral properties are controlled by– Chemical composition

– Crystalline structure

• Mineral properties are particularly useful – for mineral identification and include:

• cleavage • fracture• hardness• specific gravity

• color• streak• luster• crystal form

How Many Minerals Are There?

• More than 3500 minerals are known• Only about 2 dozen are particularly common• Many others are important resources• Mineral groups:

– minerals with the same negatively charged ion or ion group

– belong to the same mineral group

• Most minerals in the crust – belong to the group called silicates

Silicates

• Silicates are minerals containing silica – Si and O

• These minerals make up almost 95% of Earth’s crust– and account for about 1/3 of all known minerals

• The basic building block of silicates – is the silica tetrahedron

• which consists of one silicon atom• surrounded by four oxygen atoms

Types of Silicates

• Silica tetrahedra can be – isolated units bonded to

other elements– arranged in chains (single or

double)– arranged in sheets– arranged in complex

3D networks

Types of Silicates

• Ferromagnesian silicates – contain iron (Fe), magnesium (Mg), or both

• Nonferromagnesian silicates – do not contain iron or magnesium

Ferromagnesian Silicates• Common ferromagnesian silicates include

Nonferromagnesian Silicates

Other Mineral Groups

• Carbonates – minerals with carbonate ion (CO3)-2

• calcite (CaCO3), – constituent of limestone

• dolomite [CaMg(CO3)2], – constituent of dolostone

• Other mineral groups are important, – but more as resources – than as constituents of rocks

Rock-Forming Minerals

• Most rocks are solid aggregates – of one or more minerals

• Hundreds of minerals occur in rocks, – but only a few are common – and called rock-forming minerals

• Most rock-forming minerals are silicates, – but carbonates are also important

• Accessory minerals are present in small amounts – and are ignored in classifying rocks

Rock Cycle

• The rock cycle is a pictorial representation – of events leading to – the origin, destruction, change – and reformation of rocks

• Rocks belong to 3 major families– igneous– sedimentary– metamorphic

• The rock cycle shows – how these rock families are interrelated – and can be derived from one another

Rock Cycle

Igneous Rocks

• All igneous rocks – cool and crystallize from magma, – solidify from lava, – or consolidate from pyroclastic materials

• Magma is molten material – below the surface

• Lava is molten material on the surface

• Pyroclastic materials – are particles such as volcanic ash

Pyroclastic material

Lava

Igneous Part of the Rock Cycle

Categories of Igneous Rocks

• Extrusive or volcanic rocks – formed at the surface – from lava or pyroclastic materials

• Intrusive or plutonic rocks – formed from magma injected into the crust– or formed in place in the crust– Plutons are intrusive bodies

Plutons

Igneous Rock Textures

• Texture – is the size, shape, and arrangement– of crystals, grains, and other constituents of a

rock

• Igneous rocks have several textures – that relate to cooling rate of magma or lava

Igneous Rock Textures

Cooling-Rate Textures• phaneritic,

– with visible grains • cooled slowly

• aphanitic, – with grains too small to see without magnification

• cooled quickly

• porphyritic, – with larger grains (phenocrysts) surrounded by a

finer-grained groundmass • cooled slowly intrusively, then expelled onto the surface

• glassy, – with no grains

• cooled too quickly for minerals to grow

Igneous Rock Textures• Other textures reveal further details

– of the formation of the rock• Vesicular texture, with holes (vesicles),

– indicates the rock formed– as water vapor and other gases– became trapped during cooling of lava

• Pyroclastic or fragmental texture, – containing fragments,– formed by consolidation of volcanic ash– or other pyroclastic material

Igneous Rock Textures

Classifying Igneous Rocks

• Texture and composition are the criteria – used to classify most igneous rocks

• Composition categories are based on mineral composition – FELSIC, light colored, >65% silica– INTERMEDIATE, 53-65% silica– MAFIC, dark colored, 45-52% silica– ULTRAMAFIC, <45% silica

Classifying Igneous Rocks

Common Igneous Rocks

Basalt Gabbro

Andesite Diorite

Common Igneous Rocks

Rhyolite Granite

Classifying Igneous Rocks with Special Textures

Igneous Rocks with Special Textures

Outcrop with basalt underlain by tuff

Pumice is glassy and extremely vesicular.

Sedimentary Rocks

• Sedimentary rocks form – by the lithification of sediment

• In the rock cycle, sediment originates when– mechanical and chemical weathering

• disintegrate and decompose rocks at the surface

– Transport removes sediment • from its source area and carries it elsewhere

– Sediments accumulate in deposits, • or as minerals that precipitate from solution

• or that organisms extract from solution.

Sedimentary Part of the Rock Cycle

Lithification

• Lithification means – converting sediment into sedimentary rock

• Lithification occurs by – compaction

• Pressure exerted by overlying sediments

• reduction of the amount of pore space between particles

– cementation • precipitation of minerals within pores • effectively binds sediment together

– calcium carbonate (CaCO3) cement is common– silica (SiO2) cement is common– iron oxide or iron hydroxide (Ex: Fe2O3) cement is less common

Categories of Sedimentary Rocks

• Detrital sedimentary rocks – consist of solid particles – derived from preexisting rocks (detritus)

• Chemical sedimentary rocks – consist of minerals derived from materials in

solution and – extracted by either

• inorganic chemical processes • or by the activities of organisms

– subcategory biochemical sedimentary rocks, in which

• the activities of organisms are important

Detrital Sedimentary Rocks• are composed of fragments or particles

– known as clasts = Clastic texture• These rocks are defined primarily by size of

clasts• conglomerate

– composed of gravel (>2mm)– with rounded clasts

• sedimentary breccia– also composed of gravel (>2mm)– but clasts are angular

• sandstone– composed of sand

Detrital Sedimentary Rocks

• Mudrocks consist of particles < 1/16 mm– mudstone

• composed of particles less than 1/16 mm particles

• consists of both silt- and clay-size particles

– siltstone• composed of silt-sized particles 1/16-1/256 mm

– claystone• composed of clay-sized particles <1/256 mm

– shale• mudstone or claystone that

• breaks along closely spaced parallel planes (fissile)

Chemical Sedimentary Rocks

• Recall that these rocks result – when inorganic chemical processes – or organisms extract minerals from solution

• This can result in different textures– Crystalline texture

• has an interlocking mosaic of mineral crystals

– Clastic texture • has an accumulation of broken pieces of shells

Chemical Sedimentary Rocks• Limestone – carbonate rock made of calcite

precipitated chemically or (most commonly) by organisms

• Dolostone – carbonate rock made of dolomite usually altered from limestone

• Evaporites formed by – inorganic chemical precipitation during evaporation– Rock salt and rock gypsum – evaporites made of

sodium chloride and gypsum– Chert – compact, hard, fine grained silica, formed by

chemical or biological precipitation

– Coal – made of partially altered, compressed remains of land plants accumulated in swamps

Common Sedimentary Rocks

Conglomerate Sedimentary breccia

Quartz sandstone Shale

Common Sedimentary Rocks

Fossiliferous limestone

Rock salt

Chert Coal

Rock gypsum

Metamorphic Rocks

• Metamorphic rocks – result from transformation of other rocks – in the solid state, without melting

• Changes from metamorphism include– compositional

• new minerals form

– textural• minerals become aligned

– or both

Metamorphic Part of the Rock Cycle

Agents of Metamorphism• Heat

– Increases the rate of chemical reactions– Yields different minerals from parent rock

• Pressure – Lithostatic pressure

• Weight of overlying rocks• Forms smaller, denser minerals

– Differential pressure • exerts force more intensely from one direction

• Fluid activity is an important metamorphic agent as well

Types of Metamorphism• Contact metamorphism

– heat and chemical fluids– from an igneous body– alter adjacent rocks

• Regional metamorphism– most common– large, elongated areas– tremendous pressure, elevated temperatures, and fluid

activity– occurs at convergent and divergent plate boundaries

• Dynamic metamorphism– Confined to zones adjacent to faults– Differential pressure

Metamorphic Textures

• Foliated texture– platy and elongate minerals aligned parallel to

one another– caused by differential pressure

• Nonfoliated texture– mosaic of roughly equidimensional minerals

Formation of Foliation

• When rocks are subjected to differential pressure the minerals typically rearrange in a parallel fashion

Formation of Foliation

• Microscopic view of a metamorphic rock with foliation showing the parallel arrangement of minerals

Foliated Metamorphic Rocks

• Slate – very fine-grained, low-grade metamorphism

• Phyllite – fine-grained (coarser than slate but grains are

still too small to see without magnification)

• Schist – clearly visible platy and/or elongate minerals

• Gneiss – alternating dark and light bands of minerals

Nonfoliated Metamorphic Rocks

• Marble – Composed of calcite or dolomite metamorphosed from

limestone or dolostone

• Quartzite – Composed of quartz metamorphosed from quartz

sandstone

• Greenstone– Green, altered mafic igneous rock

• Hornfels– Clay-rich, results from contact metamorphism

• Anthracite– Black, lustrous, hard coal

Common Metamorphic Rocks

Slate Schist

Gneiss Marble Quartzite

Plate Tectonics and the Rock Cycle

• The atmosphere, hydrosphere and biosphere – act on earth materials

– and cause weathering, erosion, and deposition

• Earth’s internal heat – aids melting and metamorphism

• Plate tectonics recycles Earth materials– heat and pressure at convergent plate boundaries

• lead to metamorphism and igneous activity

– Some rocks in a subducted plate are deformed and incorporated into an evolving mountain system

• that in turn weather and erode to form sediment