Systematic Mineralogy The best way to remember this material is to make a table such as the one included at the end of these notes. Mineral photographs included below are from: http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm unless otherwise noted. Most of the other photographs are used courtesy of Amethyst Galleries (http://mineral.galleries.com/)and are copyrighted to them. I. Introduction A. Most commonly, and for our purposes, minerals are classified into large groupings based on the nature of the principal anion. 1. The exception is the first group – the Native Elements – which consist of only one element so do not have anions and cations. II. Native Elements A. Metals = Au (gold), Ag (silver), Cu (copper)

1. Used in jewelry, electrical wiring and as a basis for some currencies.

B. Non-metals = S (sulfur), C (diamond and graphite) 1. Sulfur is used to make sulfuric acid 2. Diamond is important for jewelry and as an abrasive 3. Graphite is used as a lubricant

gold http://mineral.galleries.com/ minerals/elements/gold/gold.htm

copper

graphite

diamond http://mineral.galleries.com/ minerals/elements/diamond/diamond.htm

sulfur

http://core.ecu.edu/geology/ harper/mineral/mineral_new.cfm

III. Halides A. The anion is a halide (F-, Cl-, Br-, or I-) B. The halogens most often bond with elements from the far left side of the Periodic Table. C. A fluoride and a chloride are the common mineral examples.

1. NaCl (halite is the mineral name) is the most commonly used industrial chemical and an important seasoning for food (table salt). CaF2, or fluorite, is used as a flux in making steel.

halite NaCl

fluorite CaF2

http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm IV. Oxides A. Oxide (O2-) is the dominant anion B. Oxides are the chief ores (economically profitable minerals) of the following metals: Al Fe Sn Cr Mn C. Ice (H2O) is an oxide mineral with a very low melting temperature (32oF or 0oC).

Hematite Fe2O3

Magnetite Fe3O4

bauxite (a rock more than a mineral)

Various Al-oxides

http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm V. Sulfides A. Sulfide (S- or S2-) is the dominant anion B. Sulfides are the chief ores (economically profitable minerals) of Cu, Pb, Zn, and Ag.

pyrite http://mineral.galleries.com/ minerals/sulfides/pyrite/pyrite.htm

FeS2

chalcopyrite CuFeS2

sphalerite ZnS

galena PbS

http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm VI. Sulfates A. Sulfate (SO4 2-) is the dominant anion B. Gypsum, the most common sulfate, is used as a building material to make sheet rock (AKA dry wall).

http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm VII. Carbonates A Carbonate (CO32-) is the dominant anion. B. Calcite CaCO3 LEARN THIS FORMULA

1. Extremely common component of sedimentary rocks --- dominates limestones 2. Important building and road construction material –

CEMENT MARBLE CRUSHED STONE 3. Many sea creatures construct their shells of calcite, although some use a polymorph of calcite called aragonite.

calcite CaCO3

dolomite CaMg(CO3)2

http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm VIII. Silicates silicon & oxygen A. Silicates make up the bulk of the rock-forming minerals

1. The non-silicates that are abundant enough to be considered rock-forming minerals are calcite, dolomite, pyrite, hematite, magnetite, gypsum, and fluorite.

2. Rock-forming minerals are those found most abundantly in the rocks of the Earth’s crust and are, therefore, made of the most common elements in Earth’s crust. 3. 90% of the rock-forming minerals are silicates, which are composed of varying combinations of silicon and oxygen along with metallic cations from the left side of the Periodic Table.

B. Basis of silicate structure 1. The basic structural unit of the silicates is the silicon-

oxygen tetrahedron (Tetra=4, hedra = sides) Diagram courtesy of the Mineralogical Society of America http://www.minsocam.org/MSA/K12/groups/silicates.html

a. Because of the relative sizes of the Si and O atoms the most stable configuration is the tetrahedron. b. Silicates are composed of these tetrahedra arranged and linked in different ways with cations such as Na, K, Ca, Mg, Fe, etc. in the interstices (gaps between tetrahedra). c. Let’s calculate the excess electrical charge associated with a silica tetrahedron:

ONE atom of Si4+ combined with FOUR atoms of O2-

1 X (4+) == + 4

4 X (2-) == - 8

TOTAL == - 4

d. Obviously, the silica tetrahedron is not ELECTRICALLY NEUTRAL by itself so positively-charged, metallic cations are necessary to neutralize the excess negative charge in a structure. e. How many Mg2+ or Fe2+ are necessary to neutralize this negative charge? (Two, because a total of 4 positive charges is required.)

C. Basic Subdivisions of the silicates Again, I suggest you make a table to organize and study this information, you will find examples of such tables in your textbook and at the end of these notes.

1. Isolated tetrahedra – individual tetrahedra of silica are bonded to metallic cations to eliminate the excess negative charge. The tetrahedra are not bonded directly to each other, that is neither the Si nor the O atoms of any given tetrahedron are bonded directly to those of another tetrahedron.

a. The Mg-rich and Fe-rich olivine minerals are an example of a complete solid solution. A solid solution is a group of minerals which all have the same structure (i.e., arrangement of atoms), but have different chemical compositions. The two end-member compositions are shown below and all mixtures of different proportions of the Mg and Fe cations are found in nature in the oli o pvine gr u .

Mg2SiO4 Fe2SiO4

b. Olivine is an example of a ferromagnesian mineral (mineral rich in Fe and Mg), which are typically dark-colored (green or black) and have high densities. c. Olivine is believed to be the predominant silicate at depth in Earth’s mantle.

http://wrgis.wr.usgs.gov/docs/parks/rxmin/mineral.html 2. Pairs of tetrahedra - individual tetrahedra are linked to

one another by sharing one of their oxygens to form pairs. a. No very common

3. minerals with this structure exist.

Rings of tetrahedra – individual tetrahedra are linked to one another by sharing two of their oxygens to form six- membered rings. a. Only one mineral you are likely to have heard of crystallizes with this structure == BERYL = EMERALD (BeAl2Si6O18) 4. Chains of tetrahedra - individual tetrahedra are linked to one another by sharing 2 or 2.5 of their oxygens to form

single or double chains a. Includes two very important groups of silicates b. Single chains = Pyroxenes = Augite Tetrahedra share 2 of their oxygens c. Double chains = Amphiboles = Hornblende Tetrahedra share 2.5 of their oxygens

http://wrgis.wr.usgs.gov/docs/parks/rxmin/mineral.html

d. Common cations in the chain silicates are Ca, Mg, and Fe and both groups are dominated by ferromagnesian minerals.

e. The differing cleavages of the two groups are a function of the different chain structures.

5. Sheets of tetrahedra - individual tetrahedra linked to one another by sharing 3 oxygens to form sheets.

a. The fourth oxygen stands above the sheet and is called the “apical” oxygen.

b. Two very important groups of sheet silicates 1) Micas and clays

Kaolinite is one of the most common clay minerals http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm

Biotite is the dark-colored and muscovite the light-colored mica http://wrgis.wr.usgs.gov/docs/parks/rxmin/mineral.html

c. Sheets of internally linked silica tetrahedra are bonded to layers containing cations and oxygens. Aluminum substitutes for some of the silicon atoms in the tetrahedra. d. Within sheets bonding is covalent and much stronger than between sheets, which are attached to one another

by weak bonds. This accounts for the excellent cleavage as the mineral breaks easily between sheets.

6. Frameworks of tetrahedra a. All the oxygens of every tetrahedron are shared with other tetrahedra to make a three-dimensional framework. b. Non-ferromagnesian minerals domina

c. Quartz SiOte.

2 - know formula 1) No cations besides Si in tetrahedra 2) Most abundant rock-forming mineral 3) Very hard mineral due to strong bonds 4) Fractures -- NO CLEAVAGE

Quartz crystals http://mineral.galleries.com/ minerals/silicate/quartz/quartz.htm

Massive quartz Photo by TL Woods

d. Finally we come to the most abundant GROUP of rock-forming silicates -- FELDSPARS

1) Tetrahedra are linked with Na Ca and K 2) Not very dense because contain light elements such as Na instead of Fe 3) Very hard minerals because of strong bonds 4) Differ from quartz because in the center of at least ¼ of the tetrahedra, Al substitutes for Si.

Al Si Ionic radius(angstroms) 0.5 0.4

Charge +3 +4 5) An interesting situation arises because of the differing charges of Al3+ and Si4+. When an Al

substitutes for an Si – excess negative charge results. This excess negative charge must be neutralized by the incorporation of K, Na, or Ca in the interstices of the silica framework.

a) Notice also that the charge on Ca is 2+ and that of K and Na is 1+, so one Ca does the

work of two Na’s or K’s, which is good because the structure won’t accommodate two of the singly charged cations.

e. Two different groups of feldspars 1) Alkalai or potassium feldspars where one Si out of every 4 is replaced by an Al. The charge deficit is made-up by the addition of a K+ and Na+.

a) This is not a SOLID SOLUTION because an Al must replace every fourth Si. This ratio is not variable in the alkalai feldspars. b) Various polymorphs of K-fspr exist and structures reflect conditions of formation.

Formula Total Charge Si O 0 4

4+ 2-8

Al 3+ Si3 4+ O8 -1

K+ Al 3+ Si3 4+ O8 0

Potassium feldspar = K-spr 2) Plagioclase feldspar a) These do represent a SOLID SOLUTION with the Al for Si substitution ranging from 1 to 2 out of every 4 Si being replaced by Al. b) The result is variation in the amount of Na versus Ca in the mineral. Increasing amounts of Al substituting for Si require increasing amounts of Ca instead of Na.

Anorthite Albite CaAl2Si2O8 NaAlSi3O8

Formula Total Charge Si4 4+ O2-

8 0

Al2 3+ Si2 4+ O8 -2

Ca2+ Al2 3+ Si2 4+ O8 0

Plagioclase feldspar = anorthite

Potassium Feldspar Na - plagioclase Ca - plagioclase http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm D. Certain groups of minerals are associated with specific geologic settings and specific pressure and temperature conditions because they only form under a restricted range of conditions. For

this reason some minerals are useful indicators of the physical conditions that existed when the rocks containing them formed. E. The minerals we see today have not, for the most part, existed since the Earth was formed. Because the Earth is such a dynamic system, from one year to the next over the course of millions of years the elements making up the Earth undergo rearrangements in terms of their combinations, their relative proportions and their associations.

1. New minerals are formed from old. 2. New concentrations of elements and minerals develop where they did not exist before and old minerals are broken down into their constituent elements.

Group Defining Atom or Anion Mineral Economic

Importance

Native Elements Au, Ag, Cu, S, C

graphite,sulfur, diamond, silver

Jewelry, abrasive, chemicals, “lead”

Halides Cl-, F- halite, fluorite

industrial chemical

flux in steel product.

Oxides O2- magnetite, bauxite, hematite

Fe and Al ores

Sulfides S2- & S-galena,

sphalerite, chalcopyrite,

pyrite

Cu, Pb, Zn ores

Sulfates SO42- gypsum dry wall

Carbonates CO32- calcite construction,

fill, concrete

Structure Shared 0xygens Si:O Group Mineral Other Cations

Isolated 0 1:4 Olivine Olivine Mg, Fe

Pairs 1 2:7 Epidote Epidote Ca, Al, Fe

Rings 2 6:18 Beryl Tourmaline

Be, Al Na, Al, Mg, Li, B, F

Single chain 2 1:3 Pyroxene Augite Ca, Mg, Fe

Double Chain 2 or 3 4:11 AmphiboleHornblende Na,K,Ca,Fe,Mg,

etc.

Sheets 3 2:5 Micas Biotite Muscovite

K, Mg, Fe, Al K, Al

Sheets 3 2:5 Clays Kaolinite K, Mg, Al, Na, etc.

Frameworks 4 1:2 Quartz Quartz None

Feldspar

Potassium feldsparPlagioclase feldspar: Albite, Anorthite

K, Al Ca, Na, Al