mineral classif

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, arsenides, 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, CO3-3)


• borates ( the oxyacid anion, BxOy-z)
• nitrates ( the oxyacid anion, NO3-1)
• phosphates ( oxyacid anion, PO4-3)
• sulfates ( the oxyacid anion, SO4-2)
• tungstates ( the oxyacid anion, WO4-2)
• silicates ( the oxyacid anion, SixOy-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


• FeCO3 (siderite) and CaCO3(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
NaNO3 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--CaCO3--calcite is
hexagonal and aragonite is orthorhombic
• pyrite and marcasite--FeS2--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--SiO2--quartz = low temp form and
hexagonal, cristobalite = high temp form and


• 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 (XYZ2O6),
amphiboles (W0-1X2Y5Z8O22(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 CaAl2Si2O8 (anorthite)(An) and
NaAlSi3O8 (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. 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)2SiO4
• end members are fayalite, Fe2SiO4 and forsterite,
Mg2SiO4 in which there is proxying between Fe and
Mg concentrations as a function of temperature
• Enstatite-ferosilite series--(Fe, Mg) SiO3
• end members are enstatite, MgSiO3 and ferosilite,
FeSiO3 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, NaAlSi3O8) stringers in
orthoclase (Or, KAlSi3O8)
• 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, CaSO4 to
gypsum, CaSO4•H2O; galena, PbS to anglesite,
PbSO4; pyrite, FeS2 or siderite, FeCO3 to limonite,
FeO(OH)•nH2O
• 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


• 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