MINERAL CLASSIFICATION AND RELATED CONCEPTS Mineral Classification Mineral

MINERAL CLASSIFICATION AND RELATED CONCEPTS

• Mineral Classification • Mineral Classes • minerals are divided into classes based on chemical composition and primarily on the main anion, anionic complex or oxyacid anion, or lack of an anion present--the main mineral classes are: • native elements ( monoelemental composition--lack of anion) • sulfides, sulfarsenides, sulfosalts ( main anion is S-2 or As-3) • oxides ( main anion is O-2) • hydroxides ( main anion complex is OH-1) • halides (main anion is Cl-1, F-1, Br-1 or I-1) • carbonates ( main anion is the oxyacid anion, CO 3 -3)

• borates ( the oxyacid anion, Bx. Oy-z) • nitrates ( the oxyacid anion, NO 3 -1) • phosphates ( oxyacid anion, PO 4 -3) • sulfates ( the oxyacid anion, SO 4 -2) • tungstates ( the oxyacid anion, WO 4 -2) • silicates ( the oxyacid anion, Six. Oy-z) • Mineral subclasses • some classes are subdivided based on special chemistry or structural features • native element subclasses • native metals--minerals with metallic bonds • native semimetals--those with primarily semimetallic bonds

• silicate subclasses--based on linkage structure of the silica tetrahedra • neso-, soro-, cyclo-, ino-, phylo-, tectosilicates) • Mineral Groups • classes or subclasses can be divided based on atomic structure and similar chemistry--examples are isomorphic (isostructural) groups or polymorphic groups • 1. isomorphic group • an assemblage of minerals with the same atomic structure but different chemical formulas • the equivalent elements in different minerals in the isomorphic group have the same CN

• Fe. CO 3 (siderite) and Ca. CO 3(calcite) belong to the same isomorphic group in the carbonate class because according to Paulings rules, both have 6 O around each Fe and Ca, 3 O around each C and 1 C and 2 Fe or Ca around each O • often isomorphs have basically the same or similar chemical, physical and crystallographic properties • some examples are: 1. in the oxide class--hematite group, spinel group, and rutile group; 2. in the carbonate class--calcite group and aragonite group; 3. in the sulfate class--barite group; 4. in the silicate subclasses--(nesosilicates)--garnet group etc.

• isomorphism can exist between minerals outside the same class--these minerals are isomorphs (isostructural) but do not belong to the same isomorphic group--an example is Na. NO 3 nitratite which is isostructural with minerals in the calcite group • 2. polymorphic group • minerals in the same mineral class with the same chemical formula but different atomic structures • as mentioned previous, a different CN (different atomic structure) can result with atoms of the same elements under different temperatures or pressures of mineral formation--the same CN may exist between elements in polymorphs, but there is a different bond angle between elements

• the difference in atomic structures will often result in polymorphs forming in different crystal systems or crystal classes in the same crystal system • examples of some polymorphs are: • calcite and aragonite--Ca. CO 3 --calcite is hexagonal and aragonite is orthorhombic • pyrite and marcasite--Fe. S 2 --pyrite forms at a high temperature and is isometric while marcasite forms at a low temperature and is orthorhombic • quartz, tridymite , cristobalite, stishovite and coesite--Si. O 2 --quartz = low temp form and hexagonal, cristobalite = high temp form and tetragonal

• coesite = stable (forms) at very high pressures is monoclinic and is associated with meteor impact; stishovite = associated with rocks from Mars • kinds of polymorphism • 1. based on reversible vs irreversible changes • reversible (enantiotropic)--quartz = tridymite equilibrium at 867 degrees and 1 atm. Or graphite = diamond • irreversible (monotropy)--marcasite = pyrite but not pyrite = marcasite • 2. based on change or reconstitution nature of the atomic structure

• reconstructive change--during the change there is the breaking of atomic bonds and a reassembling of structural units--involves a lot of energy and change is not readily reversed and is sluggish--quartz = tridymite = cristobalite • displacive change--in the change, atomic bonds are not broken and the original structure is maintained--only a slight displacement of atoms results in different bond angles between atoms-instantaneous change involving little energy-high quartz = low quartz

• ordered-disordered change--in which the more disordered form will have more symmetry-microcline = ordered and orthoclase = disordered form • other groupings • minerals grouped based on the same general or empirical formula such as the pyroxenes (XYZ 2 O 6), amphiboles (W 0 -1 X 2 Y 5 Z 8 O 22(OH, F)2), and micas • Mineral Series • classes and groups can be subdivided into mineral series in which solid solution is most prominently displayed

• solid solution series • different specimens of the same mineral can vary in composition in which each specimen is comprised of a mixture of “end member minerals” caused by ionic substitution (proxying) between some cations in the “end members” during mineral formation • the degree of proxying of elements into a mineral structure depends largely on the temperature of formation of the mineral • Plagioclase series • end members are Ca. Al 2 Si 2 O 8 (anorthite)(An) and Na. Al. Si 3 O 8 (albite)(Ab) where there is proxying between Na and Ca, and Al and Si

• Plagioclase series of minerals--formed from a magma • • Ab% An% name type temp. of form. 100 -90 0 -10 albite sodic plag. low 90 -70 10 -30 oligioclase “ low 70 -50 30 -50 andesine intermed. 50 -30 50 -70 labradorite “ “ 30 -10 70 -90 bytownite Ca plag. high 10 -0 90 -100 anorthite “ “ • determination of the specific plagioclase mineral in a rock can be made by use of the petrographic microscope and is very important in the naming of igneous rocks

• Olivine series--(Fe, Mg)2 Si. O 4 • end members are fayalite, Fe 2 Si. O 4 and forsterite, Mg 2 Si. O 4 in which there is proxying between Fe and Mg concentrations as a function of temperature • Enstatite-ferosilite series--(Fe, Mg) Si. O 3 • end members are enstatite, Mg. Si. O 3 and ferosilite, Fe. Si. O 3 in which Fe and Mg concentrations proxy as a function of temperature of mineral formation

• Mineral Varieties • 1. Chemical rich minerals • are those which can be expressed by an adjective denoting an unusual large amounts of chemical constituents

• some examples of modifiers are: • aluminian = Al-rich • calcian = Ca-rich • chromian = Cr-rich • ferroan = Fe+2 -rich, ferrian = Fe+3 -rich • magnesian = Mg-rich • manganoan = Mn-rich • examples of specific mineral variety names are: manganoan aegerine, ferrian diopside or magnesian augite • 2. Crystalline variety types • based on crystal size, color or appearance as in quartz

Coarsely crystalline quartz varieties rock crystals--colorless amethyst--purple rose quartz--rose

Coarsely crystalline quartz varieties smoky quartz--smoky yellow to brown citrine-- clear yellow rutilated quartz-with rutile inclusions

Microcrystalline quartz--fibrous varieties chrysoprase--apple green heliotrope (bloodstone)-greenish with small red spots agate--parallel or curved color bands

Microcrystalline quartz--granular varieties flint or chert jasper--red color caused by hematite inclusions

Related Topics • Exsolution • the association of similar composition minerals formed instantaneously from a cooling magma resulting in stringers of one mineral appearing in the other more abundant mineral • this occurs if both minerals have equivalent cations close to ionic substitution at high temperature but not at the actual solidification temperature of both minerals • examples are perthites and antiperthites ( macro-, microand crypto-) • perthite is albite (Ab, Na. Al. Si 3 O 8) stringers in orthoclase (Or, KAl. Si 3 O 8) • antiperthite is Or stringers in Ab

• Pseudomorphism • the shape of mineral B displayed by mineral A--the existance of a mineral with the outward form of another • a false form • kinds of pseudomorphism • 1. encrustation • one mineral is deposited over crystals of another as quartz (a hexagonal mineral) encrusting cubes of fluorite (isometric mineral)--the more solubilized fluorite can be dissolved leaving a cast of its form in the quartz • 2. alteration • a mineral can be chemically altered (weathered)

• to result in an external layer of another mineral on the weathered mineral • the weathering or alteration of anhydrite, Ca. SO 4 to gypsum, Ca. SO 4 • H 2 O; galena, Pb. S to anglesite, Pb. SO 4; pyrite, Fe. S 2 or siderite, Fe. CO 3 to limonite, Fe. O(OH) • n. H 2 O • in the above examples gypsum is said to be a pseudomorph after anhydrite, anglesite the same after galena and limonite the same after pyrite or siderite • 3. substitution (replacement) • involves a gradual removal of the original mineral and a simultaneous molecule for molecule replacement by another • silica replacing fluorite; silica replacement wood

• Mineraloids (Gels) • substances resembling minerals but have no ordered atomic arrangement • cannot belong to any of the crystal classes of the crystal systems because they are amorphous • are usually products of chemical weathering and are often in mammillary, botryoidal, or stalactitic masses as opal or limonite • Metamict Structure • minerals whose atomic structure has been altered by radiation • the original ordered atomic arrangement may be restored by exposure to elevated temperatures with an emission of light causing the mineral to appear incandescent

• examples are some specimens of allanite and zircon • Geologic Thermometry ( Geothermometry) • is the determination of the temperature of formation of a mineral and associated minerals by knowing the concentration of proxyed element in a mineral through ionic substitution • the temperature of formation of sphalerite and associated minerals occurring with it can be established by determining the concentration of ionically substituted Fe in the mineral, (Fe, Zn)S • the darker or higher red color of sphalerite indicates a higher Fe concentration while a clear or yellow color represents a lower Fe and higher Zn concentration • the following table explains the above

Concentration of Fe in Sphalerite as a Function of Temperature of Formation • the concentration of Ca ionically substituted in plagioclase is a function of temperature of formation but plagioclase has not been traditionally used a geothermal mineral