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Mineral classification is based primarily on the chemical
composition, atomic
structure, degree of ionic substitution,
and color and crystalline state of minerals
I. Mineral Classification
A.
Mineral Classes
- minerals
are classified primarily on the main anion ( O-2, S-2,
etc.), anionic complex (oxyacid
anion) (OH-1, SO4-2,
CO3-3, PO4-3, BxOy-Z,
SixOy-Z, etc), or
lack of an anion(native elements)
- some of the classes
are listed below with the chemical characterisic
used to classify them--find
more
mineral classes with the corresponding anion or complex anion in the text
Native elements ( comprised of atoms of only one element and no anion)----covalent
by
nature--- atomic structure cannot
be determined by Pauling’s Rule #1(radius ratio)
Sulfides, including Sulfarsenides, Arsenides, Sulfosalts (
main anion is S-2)---covalent by
nature—atomic structure
cannot be determined by Pauling’s Rule #1 (radius ratio)
Oxides ( main anion is O-2)----almost all are comprised of isodesmic bonds--atomic
can be determined by Pauling’s Rule #1 (radius ratio)
Hydroxides (main anion complex is OH-1)
Halides ( main anion is a halogen as Cl-1, F-1,
Br-1, I-1)
Carbonates ( the oxyacid anion, CO3-3)
Nitrates ( the oxyacid anion, NO3-1)
Borates ( the oxyacid anion, BxOy-Z)
Phosphates ( the oxyacid anion, PO4-3)
Sulfates ( the oxyacid anion, SO4-2)
Tungstates ( the oxyacid
anion, WO4-2)
Silicates ( the oxyacid anion, SixOy-Z)
B. Mineral
Subclasses
-some classes
can be subdivided based on chemical or structural grounds--examples are
the 1. Native
Element Class which is divided into
minerals with metallic bonding (metals),
those with mostly covalent
bonding ( nonmetals), and those with a mixture (semimetals);
and the 2.
the Silicate Class, with 6 subclasses (neso-, soro-, cyclo-, phylo-, tecto-silicates)
based on the linkage of the silica
tetrahedra--details concerning these
subclasses will be
treated later under a
discussion of the non silicates and silicates
C. Mineral
Groups
-classes or
subclasses can be further divided based on atomic structure and similar chem-
istry--examples
are isomorphic (isostructural) groups, polymorphic
groups and groups based
on a general empirical
formula with consistent properties
1.
isomorphic group is a group
of minerals with the same atomic structure but different
chemical formulas--atoms of different elements representing equivalents in
minerals
of this group have the same C.N.--FeCO3 (siderite) and CaCO3
(calcite) belong to
the same isomorphic group in the carbonate class because in both cases there
are 6
O around each Fe and Ca respectively, 3 O around each C, and one C and 2 Fe
or
Ca around each O--often the same atomic structure in different minerals
reflects
similar chemical and physical properties and similar crystallography
- some examples of isomorphic (isostructural)
groups are:
--in the oxide class-
hematite group, spinel group, rutile
group
--in the carbonate class-
calcite group, aragonite group
--in the sulfate class-
barite group
--in the silicate class-
(and nesosilcate subclass)--garnet group
(and inosilicate-pyroxenes subclass)--sodium
pyroxene group
(and inosilcate-amphibole subclass)--sodium
amphibole group
-isomorphism can exist with minerals which are not in the same mineral
class--since they
are not in the same mineral class they cannot be in the same isomorphic
group--NaNO3
(nitratite) is isomorphic or isostructural with the minerals in the calcite group of
the carbonate
class including siderite and calcite
2. polymorphic group is a mineral
group belonging to the same mineral class, all having the
same chemical formula but different atomic structures--these usually form or
are stable
under different temperatures
or pressures whereby the same cation forms a
different
C.N. with the same anion--or the same CN exists but there is a different bond
angle
between polyhedra--the difference in atomic
structures result in polymorphs
often forming in different crystal systems
-some examples of polymorphs are:
a. calcite and aragonite--CaCO3---calcite is hexagonal and
aragonite, orthorhombic
b. pyrite and marcasite--FeS2---pyrite
forms at a high temperature and is isometric
while
marcasite forms at a low temperature and is
orthorhombic
c. quartz, tridymite, cristobalite,
stishovite and coesite--SiO2---quartz
forms at a
low temperature and forms in the hexagonal system, cristobalite
forms at a high
temperature and forms in the tetragonal system, while tridymite
is an intermediate
temperature form which is orthorhombic---coesite
is stable at high pressures and
is associated with meteor impact and is a monoclinic mineral---stishovite is tet-
ragonal and is thought to be associated with rocks
from Mars
d. kyanite and andalusite--Al2SiO5---kyanite is triclinic and is formed at a high
temperature and andalusite is orthorhombic
and is the low temperature form
e. microcline, orthoclase, sanidine--KAlSi3O8---microcline,
a triclinic mineral is
the low temperature variety, sanidine, a
monoclinic mineral is the high temper-
ature variety and orthoclase is a monoclinic
mineral which forms at an inter-
temperature
kinds of polymorphism:
-two types of polymorphism are recognized according 1. to
whether a change from
one polymorph to another is reversible and takes place at a definite
temperature and
pressure, or 2. is irreversible and can
change in only one direction at a certain temperature
1. enantiotropy
-a reversible change as:
quartz >< tridymite
or
graphite >< diamond
2. monotropy
-a one way change between polymorphs as:
marcasite > pyrite marcasite to pyrite but not vice versa (irreversible)
-also, polymorphs can also be categorized as to the nature of their
change in respect to
the degree of reconstitution of the atomic structure
1. reconstructive change
- is the breaking of atomic bonds and a reassembly of structural units--this
type of
change involves alot of energy and the change is
not readily reversed and is
sluggish
quartz > tridymite > cristobalite
2. displacive change
-atomic bonds are not broken and the original structure is maintained--there
is
only a slight displacement of the atoms resulting in different bond angles--this
change is instantaneous and involves little energy
high quartz > low quartz
3. ordered-disordered change
-microcline (KAlSi3O8) has an ordered arrangement of
the Si and Al in its
structure while the same for orthoclase is disordered--the disordered form
will
have more symmetry since it forms at a higher temperature
3. Other
Groupings
-minerals grouped based on the same general or empirical formula such as the
pyroxene,
amphibole and mica groups
D. Mineral Series
-classes
and groups can be subdivided into mineral series in which solid
solution is most
prominently displayed
solid
solution is a homogeneous crystalline mineral of variable
composition comprised of
a
mixture of end members in which there is ionic substitution
between some cations of the
end
members--the principles of ionic substitution was treated earlier in the
semester
-the type
or quantity of cation(s) which can proxy for
locations in the atomic structure
of a
mineral during mineral formation to a large degree is a function of
temperature--in
most cases examples of proxying cations in a mineral series are Ca+2 and Na+1,
Al+3
and Si+4, and Fe+2 and Mg+2
-some examples of solid solution series are:
a. Plagioclase series (coupled
ionic substitution)
-end members are CaAl2Si2O8 (anorthite) (An) and NaAlSi3O8
(albite) (Ab)
in
which there is a proxying between both Na
and Ca, and Al and Si--a table below
expresses the different plagioclase minerals based on the degree of ionic
sub-
stitution of Na and Ca, and Al and Si in end
members:
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