Mineral
A mineral is a naturally occurring substance
that is solid and stable at room temperature,
representable by a chemical formula, usually
abiogenic, and has an ordered atomic structure.
It is different from a rock, which can be
an aggregate of minerals or non-minerals and
does not have a specific chemical composition.
The exact definition of a mineral is under
debate, especially with respect to the requirement
a valid species be abiogenic, and to a lesser
extent with regards to it having an ordered
atomic structure. The study of minerals is
called mineralogy.
There are over 4,900 known mineral species;
over 4,660 of these have been approved by
the International Mineralogical Association
(IMA). The silicate minerals compose over
90% of the Earth's crust. The diversity and
abundance of mineral species is controlled
by the Earth's chemistry. Silicon and oxygen
constitute approximately 75% of the Earth's
crust, which translates directly into the
predominance of silicate minerals. Minerals
are distinguished by various chemical and
physical properties. Differences in chemical
composition and crystal structure distinguish
various species, and these properties in turn
are influenced by the mineral's geological
environment of formation. Changes in the temperature,
pressure, and bulk composition of a rock mass
cause changes in its mineralogy; however,
a rock can maintain its bulk composition,
but as long as temperature and pressure change,
its mineralogy can change as well.
Minerals can be described by various physical
properties which relate to their chemical
structure and composition. Common distinguishing
characteristics include crystal structure
and habit, hardness, lustre, diaphaneity,
colour, streak, tenacity, cleavage, fracture,
parting, and specific gravity. More specific
tests for minerals include reaction to acid,
magnetism, taste or smell, and radioactivity.
Minerals are classified by key chemical constituents;
the two dominant systems are the Dana classification
and the Strunz classification. The silicate
class of minerals is subdivided into six subclasses
by the degree of polymerization in the chemical
structure. All silicate minerals have a base
unit of a 4- silica tetrahedra—that is,
a silicon cation coordinated by four oxygen
anions, which gives the shape of a tetrahedron.
These tetrahedra can be polymerized to give
the subclasses: orthosilicates (no polymerization,
thus single tetrahedra), disilicates (two
tetrahedra bonded together), cyclosilicates
(rings of tetrahedra), inosilicates (chains
of tetrahedra), phyllosilicates (sheets of
tetrahedra), and tectosilicates (three-dimensional
network of tetrahedra). Other important mineral
groups include the native elements, sulfides,
oxides, halides, carbonates, sulfates, and
phosphates.
Definition
Basic definition
The general definition of a mineral encompasses
the following criteria:
Naturally occurring
Stable at room temperature
Represented by a chemical formula
Usually abiogenic (not resulting from the
activity of living organisms)
Ordered atomic arrangement
The first three general characteristics are
less debated than the last two. The first
criterion means that a mineral has to form
by a natural process, which excludes anthropogenic
compounds. Stability at room temperature,
in the simplest sense, is synonymous to the
mineral being solid. More specifically, a
compound has to be stable or metastable at
25°C. Classical examples of exceptions to
this rule include native mercury, which crystallizes
at −39°C, and water ice, which is solid
only below 0°C; as these two minerals were
described prior to 1959, they were grandfathered
by the International Mineralogical Association
(IMA). Modern advances have included extensive
study of liquid crystals, which also extensively
involve mineralogy. Minerals are chemical
compounds, and as such they can be described
by fixed or a variable formula. Many mineral
groups and species are composed of a solid
solution; pure substances are not usually
found because of contamination or chemical
substitution. For example, the olivine group
is described by the variable formula (Mg,
Fe)2SiO4, which is a solid solution of two
end-member species, magnesium-rich forsterite
and iron-rich fayalite, which are described
by a fixed chemical formula. Mineral species
themselves could have a variable compositions,
such as the sulfide mackinawite, (Fe, Ni)9S8,
which is mostly a ferrous sulfide, but has
a very significant nickel impurity that is
reflected in its formula.
The requirement of a valid mineral species
to be abiogenic has also been described as
similar to have to be inorganic; however,
this criterion is imprecise and organic compounds
have been assigned a separate classification
branch. Finally, the requirement of an ordered
atomic arrangement is usually synonymous to
being crystalline; however, crystals are periodic
in addition to being ordered, so the broader
criterion is used instead. The presence of
an ordered atomic arrangement translates to
a variety of macroscopic physical properties,
such as crystal form, hardness, and cleavage.
There have been several recent proposals to
amend the definition to consider biogenic
or amorphous substances as minerals. The formal
definition of a mineral approved by the IMA
in 1995:
In addition, biogenic substances were explicitly
excluded:
Recent advances
Mineral classification schemes and their definitions
are evolving to match recent advances in mineral
science. Recent changes have included the
addition of an organic class, in both the
new Dana and the Strunz classification schemes.
The organic class includes a very rare group
of minerals with hydrocarbons. The IMA Commission
on New Minerals and Mineral Names recently
adopted (in 2009) a hierarchical scheme for
the naming and classification of mineral groups
and group names and established seven commissions
and four working groups to review and classify
minerals into an official listing of their
published names. According to these new rules,
"mineral species can be grouped in a number
of different ways, on the basis of chemistry,
crystal structure, occurrence, association,
genetic history, or resource, for example,
depending on the purpose to be served by the
classification."
The Nickel (1995) exclusion of biogenic substances
was not universally adhered to. For example,
Lowenstam (1981) stated that "organisms are
capable of forming a diverse array of minerals,
some of which cannot be formed inorganically
in the biosphere." The distinction is a matter
of classification and less to do with the
constituents of the minerals themselves. Skinner
(2005) views all solids as potential minerals
and includes biominerals in the mineral kingdom,
which are those that are created by the metabolic
activities of organisms. Skinner expanded
the previous definition of a mineral to classify
"element or compound, amorphous or crystalline,
formed through biogeochemical processes,"
as a mineral.
Recent advances in high-resolution genetic
and x-ray absorption spectroscopy is opening
new revelations on the biogeochemical relations
between microorganisms and minerals that may
make Nickel's (1995) biogenic mineral exclusion
obsolete and Skinner's (2005) biogenic mineral
inclusion a necessity. For example, the IMA
commissioned 'Environmental Mineralogy and
Geochemistry Working Group' deals with minerals
in the hydrosphere, atmosphere, and biosphere.
Mineral forming microorganisms inhabit the
areas that this working group deals with.
These organisms exist on nearly every rock,
soil, and particle surface spanning the globe
reaching depths at 1600 metres below the sea
floor (possibly further) and 70 kilometres
into the stratosphere (possibly entering the
mesosphere). Biologists and geologists have
recently started to research and appreciate
the magnitude of mineral geoengineering that
these creatures are capable of. Bacteria have
contributed to the formation of minerals for
billions of years and critically define the
biogeochemical cycles on this planet. Microorganisms
can precipitate metals from solution contributing
to the formation of ore deposits in addition
to their ability to catalyze mineral dissolution,
to respire, precipitate, and form minerals.
Prior to the International Mineralogical Association's
listing, over 60 biominerals had been discovered,
named, and published. These minerals (a sub-set
tabulated in Lowenstam (1981)) are considered
minerals proper according to the Skinner (2005)
definition. These biominerals are not listed
in the International Mineral Association official
list of mineral names, however, many of these
biomineral representatives are distributed
amongst the 78 mineral classes listed in the
Dana classification scheme. Another rare class
of minerals (primarily biological in origin)
include the mineral liquid crystals that are
crystalline and liquid at the same time. To
date over 80,000 liquid crystalline compounds
have been identified.
The Skinner (2005) definition of a mineral
takes this matter into account by stating
that a mineral can be crystalline or amorphous.
Liquid mineral crystals are amorphous. Biominerals
and liquid mineral crystals, however, are
not the primary form of minerals, most are
geological in origin, but these groups do
help to identify at the margins of what constitutes
a mineral proper. The formal Nickel (1995)
definition explicitly mentioned crystalline
nature as a key to defining a substance as
a mineral. A 2011 article defined icosahedrite,
an aluminium-iron-copper alloy as mineral;
named for its unique natural icosahedral symmetry,
it is also a quasicrystal. Unlike a true crystal,
quasicrystals are ordered but not periodic.
Rocks, ores, and gems
Minerals are not equivalent to rocks. Whereas
a mineral is a naturally occurring usually
solid substance, stable at room temperature,
representable by a chemical formula, usually
abiogenic, and has an ordered atomic structure,
a rock is either an aggregate of one or more
minerals, or not composed of minerals at all.
Rocks like limestone or quartzite are composed
primarily of one mineral—calcite or aragonite
in the case of limestone, and quartz in the
latter case. Other rocks can be defined by
relative abundances of key (essential) minerals;
a granite is defined by proportions of quartz,
alkali feldspar, and plagioclase feldspar.
The other minerals in the rock are termed
accessory, and do not greatly affect the bulk
composition of the rock. Rocks can also be
composed entirely of non-mineral material;
coal is a sedimentary rock composed primarily
of organically derived carbon.
In rocks, some mineral species and groups
are much more abundant than others; these
are termed the rock-forming minerals. The
major examples of these are quartz, the feldspars,
the micas, the amphiboles, the pyroxenes,
the olivines, and calcite; except the last
one, all of the minerals are silicates. Overall,
around 150 minerals are considered particularly
important, whether in terms of their abundance
or aesthetic value in terms of collecting.
Commercially valuable minerals and rocks are
referred to as industrial minerals. For example,
muscovite, a white mica, can be used for windows
(sometimes referred to as isinglass), as a
filler, or as an insulator. Ores are minerals
that have a high concentration of a certain
element, typically a metal. Examples are cinnabar
(HgS), an ore of mercury, sphalerite (ZnS),
an ore of zinc, or cassiterite (SnO2), an
ore of tin. Gems are minerals with an ornamental
value, and are distinguished from non-gems
by their beauty, durability, and usually,
rarity. There are about 20 mineral species
that qualify as gem minerals, which constitute
about 35 of the most common gemstones. Gem
minerals are often present in several varieties,
and so one mineral can account for several
different gemstones; for example, ruby and
sapphire are both corundum, Al2O3.
Nomenclature and classification
In general, a mineral is defined as naturally
occurring solid, that is stable at room temperature,
representable by a chemical formula, usually
abiogenic, and has an ordered atomic structure.
However, a mineral can be also narrowed down
in terms of a mineral group, series, species,
or variety, in order from most broad to least
broad. The basic level of definition is that
of mineral species, which is distinguished
from other species by specific and unique
chemical and physical properties. For example,
quartz is defined by its formula, SiO2, and
a specific crystalline structure that distinguishes
it from other minerals with the same chemical
formula (termed polymorphs). When there exists
a range of composition between two minerals
species, a mineral series is defined. For
example, the biotite series is represented
by variable amounts of the endmembers phlogopite,
siderophyllite, annite, and eastonite. In
contrast, a mineral group is a grouping of
mineral species with some common chemical
properties that share a crystal structure.
The pyroxene group has a common formula of
XY(Si,Al)2O6, where X and Y are both cations,
with X typically bigger than Y; the pyroxenes
are single-chain silicates that crystallize
in either the orthorhombic or monoclinic crystal
systems. Finally, a mineral variety is a specific
type of mineral species that differs by some
physical characteristic, such as colour or
crystal habit. An example is amethyst, which
is a purple variety of quartz.
Two common classifications are used for minerals;
both the Dana and Strunz classifications rely
on the composition of the mineral, specifically
with regards to important chemical groups,
and its structure. The Dana System of Mineralogy
was first published in 1837 by James Dwight
Dana, a leading geologist of his time; it
is presently in its eighth edition (1997 ed.).
The Dana classification, assigns a four-part
number to a mineral species. First is its
class, based on important compositional groups;
next, the type gives the ratio of cations
to anions in the mineral; finally, the last
two numbers group minerals by structural similarity
with a given type or class. The less commonly
used Strunz classification, named for German
mineralogist Karl Hugo Strunz, is based on
the Dana system, but combines both chemical
and structural criteria, the latter with regards
to distribution of chemical bonds.
There are presently over 4,660 approved mineral
species. They are most commonly named after
a person (45%), followed by discovery location
(23%); names based on chemical composition
(14%) and physical properties (8%) are the
two other major groups of mineral name etymologies.
The common suffix -ite of mineral names descends
from the ancient Greek suffix - ί τ η ς
(-ites), meaning "connected with or belonging
to".
Mineral chemistry
The abundance and diversity of minerals is
controlled directly by their chemistry, in
turn dependent on elemental abundances in
the Earth. The majority of minerals observed
are derived from the Earth's crust. Eight
elements account for most of the key components
of minerals, due to their abundance in the
crust. These eight elements, summing to over
98% of the crust by weight, are, in order
of decreasing abundance: oxygen, silicon,
aluminium, iron, magnesium, calcium, sodium
and potassium. Oxygen and silicon are by far
the two most important — oxygen composes
46.6% of the crust by weight, and silicon
accounts for 27.7%.
The minerals that form are directly controlled
by the bulk chemistry of the parent body.
For example, a magma rich in iron and magnesium
will form mafic minerals, such as olivine
and the pyroxenes; in contrast, a more silica-rich
magma will crystallize to form minerals than
incorporate more SiO2, such as the feldspars
and quartz. In a limestone, calcite or aragonite
(both CaCO3) form because the rock is rich
in calcium and carbonate. A corollary is that
a mineral will not be found in a rock whose
bulk chemistry does not resemble the bulk
chemistry of a given mineral with the exception
of trace minerals. For example, kyanite, Al2SiO5
forms from the metamorphism of aluminium-rich
shales; it would not likely occur in aluminium-poor
rock, such quartzite.
The chemical composition may vary between
end member species of a mineral series. For
example, the plagioclase feldspars comprise
a continuous series from sodium-rich end member
albite (NaAlSi3O8) to calcium-rich anorthite
(CaAl2Si2O8) with four recognized intermediate
varieties between them (given in order from
sodium- to calcium-rich): oligoclase, andesine,
labradorite, and bytownite. Other examples
of series include the olivine series of magnesium-rich
forsterite and iron-rich fayalite, and the
wolframite series of manganese-rich hübnerite
and iron-rich ferberite.
Chemical substitution and coordination polyhedra
explain this common feature of minerals. In
nature, minerals are not pure substances,
and are contaminated by whatever other elements
are present in the given chemical system.
As a result, it is possible for one element
to be substituted for another. Chemical substitution
will occur between ions of a similar size
and charge; for example, K+ will not substitute
for Si4+ because of chemical and structural
incompatibilities caused by a big difference
in size and charge. A common example of chemical
substitution is that of Si4+ by Al3+, which
are close in charge, size, and abundance in
the crust. In the example of plagioclase,
there are three cases of substitution. Feldspars
are all framework silicates, which have a
silicon-oxygen ratio of 2:1, and the space
for other elements is given by the substitution
of Si4+ by Al3+ to give a base unit of -; without
the substitution, the formula would be charge-balanced
as SiO2, giving quartz. The significance of
this structural property will be explained
further by coordination polyhedra. The second
substitution occurs between Na+ and Ca2+;
however, the difference in charge has to accounted
for by making a second substitution of Si4+
by Al3+.
Coordination polyhedra are geometric representation
of how a cation is surrounded by an anion.
In mineralogy, due its abundance in the crust,
coordination polyhedra are usually considered
in terms of oxygen. The base unit of silicate
minerals is the silica tetrahedron — one
Si4+ surrounded by four O2-. An alternate
way of describing the coordination of the
silicate is by a number: in the case of the
silica tetrahedron, the silicon is said to
have a coordination number of 4. Various cations
have a specific range of possible coordination
numbers; for silicon, it is almost always
4, except for very high-pressure minerals
where compound is compressed such that silicon
is in six-fold (octahedral) coordination by
oxygen. Bigger cations have a bigger coordination
number because of the increase in relative
size as compared to oxygen (the last orbital
subshell of heavier atoms is different too).
Changes in coordination numbers between leads
to physical and mineralogical differences;
for example, at high pressure such as in the
mantle, many minerals, especially silicates
such as olivine and garnet will change to
a perovskite structure, where silicon is in
octahedral coordination. Another example are
the aluminosilicates kyanite, andalusite,
and sillimanite (polymorphs, as they share
the formula Al2SiO5), which differ by the
coordination number of the Al3+; these minerals
transition from one another as a response
to changes in pressure and temperature. In
the case of silicate materials, the substitution
of Si4+ by Al3+ allows for a variety of minerals
because of the need to balance charges.
Changes in temperature and pressure, and composition
alter the mineralogy of a rock sample. Changes
in composition can be caused by processes
such as weathering or metasomatism (hydrothermal
alteration). Changes in temperature and pressure
occur when the host rock undergoes tectonic
or magmatic movement into differing physical
regimes. Changes in thermodynamic conditions
make it favourable for mineral assemblages
to react with each other to produce new minerals;
as such, it is possible for two rocks to have
an identical or a very similar bulk rock chemistry
without having a similar mineralogy. This
process of mineralogical alteration is related
to the rock cycle. An example of a series
of mineral reactions is illustrated as follows.
Orthoclase feldspar (KAlSi3O8) is a mineral
commonly found in granite, a plutonic igneous
rock. When exposed to weathering, it reacts
to form kaolinite (Al2Si2O5(OH)4, a sedimentary
mineral, and silicic acid):
Under low-grade metamorphic conditions, kaolinite
reacts with quartz to form pyrophyllite (Al2Si4O10(OH)2):
As metamorphic grade increases, the pyrophyllite
reacts to form kyanite and quartz:
Alternatively, a mineral may change its crystal
structure as a consequence of changes in temperature
and pressure without reacting. For example,
quartz will change into a variety of its SiO2
polymorphs, such as tridymite and cristobalite
at high temperatures, and coesite at high
pressures.
Physical properties of minerals
Classifying minerals ranges from simple to
difficult. A mineral can be identified by
several physical properties, some of them
being sufficient for full identification without
equivocation. In other cases, minerals can
only be classified by more complex optical,
chemical or X-ray diffraction analysis; these
methods, however, can be costly and time-consuming.
Physical properties applied for classification
include crystal structure and habit, hardness,
lustre, diaphaneity, colour, streak, cleavage
and fracture, and specific gravity. Other
less general tests include fluorescence, phosphorescence,
magnetism, radioactivity, tenacity (response
to mechanical induced changes of shape or
form), piezoelectricity and reactivity to
dilute acids.
Crystal structure and habit
Crystal structure results from the orderly
geometric spatial arrangement of atoms in
the internal structure of a mineral. This
crystal structure is based on regular internal
atomic or ionic arrangement that is often
expressed in the geometric form that the crystal
takes. Even when the mineral grains are too
small to see or are irregularly shaped, the
underlying crystal structure is always periodic
and can be determined by X-ray diffraction.
Minerals are typically described by their
symmetry content. Crystals are restricted
to 32 point groups, which differ by their
symmetry. These groups are classified in turn
into more broad categories, the most encompassing
of these being the six crystal families.
These families can be described by the relative
lengths of the three crystallographic axes,
and the angles between them; these relationships
correspond to the symmetry operations that
define the narrower point groups. They are
summarized below; a, b, and c represent the
axes, and α, β, γ represent the angle opposite
the respective crystallographic axis (e.g.
α is the angle opposite the a-axis, viz.
the angle between the b and c axes):
The hexagonal crystal family is also split
into two crystal systems — the trigonal,
which has a three-fold axis of symmetry, and
the hexagonal, which has a six-fold axis of
symmetry.
Chemistry and crystal structure together define
a mineral. With a restriction to 32 point
groups, minerals of different chemistry may
have identical crystal structure. For example,
halite (NaCl), galena (PbS), and periclase
(MgO) all belong to the hexaoctahedral point
group (isometric family), as they have a similar
stoichiometry between their different constituent
elements. In contrast, polymorphs are groupings
of minerals that share a chemical formula
but have a different structure. For example,
pyrite and marcasite, both iron sulfides,
have the formula FeS2; however, the former
is isometric while the latter is orthorhombic.
This polymorphism extends to other sulfides
with the generic AX2 formula; these two groups
are collectively known as the pyrite and marcasite
groups.
Polymorphism can extend beyond pure symmetry
content. The aluminosilicates are a group
of three minerals — kyanite, andalusite,
and sillimanite — which share the chemical
formula Al2SiO5. Kyanite is triclinic, while
andalusite and sillimanite are both orthorhombic
and belong to the dipyramidal point group.
These difference arise correspond to how aluminium
is coordinated within the crystal structure.
In all minerals, one aluminium ion is always
in six-fold coordination by oxygen; the silicon,
as a general rule is in four-fold coordination
in all minerals; an exception is a case like
stishovite (SiO2, an ultra-high pressure quartz
polymorph with rutile structure). In kyanite,
the second aluminium is in six-fold coordination;
its chemical formula can be expressed as AlAlSiO5,
to reflect its crystal structure. Andalusite
has the second aluminium in five-fold coordination
(AlAlSiO5) and sillimanite has it in four-fold
coordination (AlAlSiO5).
Differences in crystal structure and chemistry
greatly influence other physical properties
of the mineral. The carbon allotropes diamond
and graphite have vastly different properties;
diamond is the hardest natural substance,
has an adamantine lustre, and belongs to the
isometric crystal family, whereas as graphite
is very soft, has a greasy lustre, and crystallises
in the hexagonal family. This difference is
accounted by differences in bonding. In diamond,
the carbons are in sp3 hybrid orbitals, which
means they form a framework where each carbon
is covalently bonded to three neighbours in
a tetrahedral fashion; on the other hand,
graphite is composed of sheets of carbons
in sp2 hybrid orbitals, where each carbon
is bonded covalently to only two others. These
sheets are held together by much weaker van
der Waals forces, and this discrepancy translates
to big macroscopic differences.
Twinning is the intergrowth of two or more
crystal of a single mineral species. The geometry
of the twinning is controlled by the mineral's
symmetry. As a result, there are several types
of twins, including contact twins, reticulated
twins, geniculated twins, penetration twins,
cyclic twins, and polysynthetic twins. Contact,
or simple twins, consist of two crystals joined
at a plane; this type of twinning is common
in spinel. Reticulated twins, common in rutile,
are interlocking crystals resembling netting.
Geniculated twins have a bend in the middle
that is caused by start of the twin. Penetration
twins consist of two single crystals that
have grown into each other; examples of this
twinning include cross-shaped staurolite twins
and Carlsbad twinning in orthoclase. Cyclic
twins are caused by repeated twinning around
a rotation axis. It occurs around three, four,
five, six, or eight-fold axes, and the corresponding
patterns are called threelings, fourlings,
fivelings, sixlings, and eightlings. Sixlings
are common in aragonite. Polysynthetic twins
are similar to cyclic twinning by the presence
of repetitive twinning; however, instead of
occurring around a rotational axis, it occurs
along parallel planes, usually on a microscopic
scale.
Crystal habit refers to the overall shape
of crystal. Several terms are used to describe
this property. Common habits include acicular,
which described needlelike crystals like in
natrolite, bladed, dendritic (tree-pattern,
common in native copper), equant, which is
typical of garnet, prismatic (elongated in
one direction), and tabular, which differs
from bladed habit in that the former is platy
whereas the latter has a defined elongation.
Related to crystal form, the quality of crystal
faces is diagnostic of some minerals, especially
with a petrographic microscope. Euhedral crystals
have a defined external shape, while anhedral
crystals do not; those intermediate forms
are termed subhedral.
Hardness
The hardness of a mineral defines how much
it can resist scratching. This physical property
is controlled by the chemical composition
and crystalline structure of a mineral. A
mineral's hardness is not necessarily constant
for all sides, which is a function of its
structure; crystallographic weakness renders
some directions softer than others. An example
of this property exists in kyanite, which
has a Mohs hardness of 5½ parallel to but
7 parallel to.
The most common scale of measurement is the
ordinal Mohs hardness scale. Defined by ten
indicators, a mineral with a higher index
scratches those below it. The scale ranges
from talc, a phyllosilicate, to diamond, a
carbon polymorph that is the hardest natural
material. The scale is provided below:
Lustre and diaphaneity
Lustre indicates how light reflects from the
mineral's surface, with regards to its quality
and intensity. There are numerous qualitative
terms used to describe this property, which
are split into metallic and non-metallic categories.
Metallic and sub-metallic minerals have high
reflectivity like metal; examples of minerals
with this lustre are galena and pyrite. Non-metallic
lustres include: adamantine, such as in diamond;
vitreous, which is a glassy lustre very common
in silicate minerals; pearly, such as in talc
and apophyllite, resinous, such as members
of the garnet group, silky which common in
fibrous minerals such as asbestiform chrysotile.
The diaphaneity of a mineral describes the
ability of light to pass through it. Transparent
minerals do not diminish the intensity of
light passing through it. An example of such
a mineral is muscovite (potassium mica); some
varieties are sufficiently clear to have been
used for windows. Translucent minerals allow
some light to pass, but less than those that
are transparent. Jadeite and nephrite (mineral
forms of jade are examples of minerals with
this property). Minerals that do not allow
light to pass are called opaque.
The diaphaneity of a mineral depends on thickness
of the sample. When a mineral is sufficiently
thin (e.g., in a thin section for petrography),
it may become transparent even if that property
is not seen in hand sample. In contrast, some
minerals, such as hematite or pyrite are opaque
even in thin-section.
Colour and streak
Colour is the most obvious property of a mineral,
but it is often non-diagnostic. It is caused
by electromagnetic radiation interacting with
electrons (except in the case of incandescence,
which does not apply to minerals). Two broad
classes of elements are defined with regards
to their contribution to a mineral's colour.
Idiochromatic elements are essential to a
mineral's composition; their contribution
to a mineral's colour is diagnostic. Examples
of such minerals are malachite (green) and
azurite (blue). In contrast, allochromatic
elements in minerals are present in trace
amounts as impurities. An example of such
a mineral would be the ruby and sapphire varieties
of the mineral corundum. The colours of pseudochromatic
minerals are the result of interference of
light waves. Examples include opal, labradorite,
ammolite and bornite.
In addition to simple body colour, minerals
can have various other distinctive optical
properties, such as play of colours, asterism,
chatoyancy, iridescence, tarnish, and pleochroism.
Several of these properties involve variability
in colour. Play of colour, such as in opal,
results in the sample reflecting different
colours as it is turned, while pleochroism
describes the change in colour as light passes
through a mineral in a different orientation.
Iridescence is a variety of the play of colours
where light scatters off a coating on the
surface of crystal, cleavage planes, or off
layers having minor gradations in chemistry.
In contrast, the play of colours in opal is
caused by light refracting from ordered microscopic
silica spheres within its physical structure.
Chatoyancy ("cat's eye") is the wavy banding
of colour that is observed as the sample is
rotated; asterism, a variety of chatoyancy,
gives the appearance of a star on the mineral
grain. The latter property is particularly
common in gem-quality corundum.
The streak of a mineral refers to the colour
of a mineral in powdered form, which may or
may not be identical to its body colour. The
most common way of testing this property is
done with a streak plate, which is made out
of porcelain and coloured either white or
black. The streak of a mineral is independent
of trace elements or any weathering surface.
A common example of this property is illustrated
with hematite, which is coloured black, silver,
or red in hand sample, but has a cherry-red
to reddish-brown streak. Streak is more often
distinctive for metallic minerals, in contrast
to non-metallic minerals whose body colour
is created by allochromatic elements. Streak
testing is constrained by the hardness of
the mineral, as those harder than 7 powder
the streak plate instead.
Cleavage, parting, fracture, and tenacity
By definition, minerals have a characteristic
atomic arrangement. Weakness in this crystalline
structure causes planes of weakness, and the
breakage of a mineral along such planes is
termed cleavage. The quality of cleavage can
be described based on how cleanly and easily
the mineral breaks; common descriptors, in
order of decreasing quality, are "perfect",
"good", "distinct", and "poor". In particularly
transparent mineral, or in thin-section, cleavage
can be seen a series of parallel lines marking
the planar surfaces when viewed at a side.
Cleavage is not a universal property among
minerals; for example, quartz, consisting
of extensively interconnected silica tetrahedra,
does not have a crystallographic weakness
which would allow it to cleave. In contrast,
micas, which have perfect basal cleavage,
consist of sheets of silica tetrahedra which
are very weakly held together.
As cleavage is a function of crystallography,
there are a variety of cleavage types. Cleavage
occurs typically in either one, two, three,
four, or six directions. Basal cleavage in
one direction is a distinctive property of
the micas. Two-directional cleavage is described
as prismatic, and occurs in minerals such
as the amphiboles and pyroxenes. Minerals
such as galena or halite have cubic (or isometric)
cleavage in three directions, at 90°; when
three directions of cleavage are present,
but not at 90°, such as in calcite or rhodochrosite,
it is termed rhombohedral cleavage. Octahedral
cleavage (four directions) is present in fluorite
and diamond, and sphalerite has six-directional
dodecahedral cleavage.
Minerals with many cleavages might not break
equally well in all of the directions; for
example, calcite has good cleavage in three
direction, but gypsum has perfect cleavage
in one direction, and poor cleavage in two
other directions. Angles between cleavage
planes vary between minerals. For example,
as the amphiboles are double-chain silicates
and the pyroxenes are single-chain silicates,
the angle between their cleavage planes is
different. The pyroxenes cleave in two directions
at approximately 90°, whereas the amphiboles
distinctively cleave in two directions separated
by approximately 120° and 60°. The cleavage
angles can be measured with a contact goniometer,
which is similar to a protractor.
Parting, sometimes called "false cleavage",
is similar in appearance to cleavage but is
instead produced by structural defects in
the mineral as opposed to systematic weakness.
Parting varies from crystal to crystal of
a mineral, whereas all crystals of a given
mineral will cleave if the atomic structure
allows for that property. In general, parting
is caused by some stress applied to a crystal.
The sources of the stresses include deformation
(e.g. an increase in pressure), exsolution,
or twinning. Minerals that often display parting
include the pyroxenes, hematite, magnetite,
and corundum.
When a mineral is broken in a direction that
does not correspond to a plane of cleavage,
it is termed to have been fractured. There
are several types of uneven fracture. The
classic example is conchoidal fracture, like
that of quartz; rounded surfaces are created,
which are marked by smooth curved lines. This
type of fracture occurs only in very homogeneous
minerals. Other types of fracture are fibrous,
splintery, and hackly. The latter describes
a break along a rough, jagged surface; an
example of this property is found in native
copper.
Tenacity is related to both cleavage and fracture.
Whereas fracture and cleavage describes the
surfaces that are created when a mineral is
broken, tenacity describes how resistant a
mineral is to such breaking. Minerals can
be described as brittle, ductile, malleable,
sectile, flexible, or elastic.
Specific gravity
Specific gravity numerically describes the
density of a mineral. The dimensions of density
are mass divided by volume with units: kg/m3
or g/cm3. Specific gravity measures how much
water a mineral sample displaces. Defined
as the quotient of the mass of the sample
and difference between the weight of the sample
in air and its corresponding weight in water,
specific gravity is a unitless ratio. Among
most minerals, this property is not diagnostic.
Rock forming minerals — typically silicates
or occasionally carbonates — have a specific
gravity of 2.5–3.5.
High specific gravity is a diagnostic property
of a mineral. A variation in chemistry (and
consequently, mineral class) correlates to
a change in specific gravity. Among more common
minerals, oxides and sulfides tend to have
a higher specific gravity as they include
elements with higher atomic mass. A generalization
is that minerals with metallic or adamantine
lustre tend to have higher specific gravities
than those having a non-metallic to dull lustre.
For example, hematite, Fe2O3, has a specific
gravity of 5.26 while galena, PbS, has a specific
gravity of 7.2–7.6, which is a result of
their high iron and lead content, respectively.
A very high specific gravity becomes very
pronounced in native metals; kamacite, an
iron-nickel alloy common in iron meteorites
has a specific gravity of 7.9, and gold has
an observed specific gravity between 15 and
19.3.
Other properties
Other properties can be used to diagnose minerals.
These are less general, and apply to specific
minerals.
Dropping dilute acid (often 10% HCl) aids
in distinguishing carbonates from other mineral
classes. The acid reacts with the carbonate
(2-) group, which causes the affected area
to effervesce, giving off carbon dioxide gas.
This test can be further expanded to test
the mineral in its original crystal form or
powdered. An example of this test is done
when distinguish calcite from dolomite, especially
within rocks (limestone and dolostone respectively).
Calcite immediately effervesces in acid, whereas
acid must be applied to powdered dolomite
(often to a scratched surface in a rock),
for it to effervesce. Zeolite minerals will
not effervesce in acid; instead, they become
frosted after 5–10 minutes, and if left
in acid for a day, they dissolve or become
a silica gel.
When tested, magnetism is a very conspicuous
property of minerals. Among common minerals,
magnetite exhibits this property strongly,
and it is also present, albeit not as strongly,
in pyrrhotite and ilmenite.
Minerals can also be tested for taste or smell.
Halite, NaCl, is table salt; its potassium-bearing
counterpart, sylvite, has a pronounced bitter
taste. Sulfides have a characteristic smell,
especially as samples are fractured, reacting,
or powdered.
Radioactivity is a rare property; minerals
may be composed of radioactive elements. They
could be a defining constituent, such as uranium
in uraninite, autunite, and carnotite, or
as trace impurities. In the latter case, the
decay of a radioactive element damages the
mineral crystal; the result, termed a radioactive
halo or pleochroic halo, is observable by
various techniques, such as thin-section petrography.
Mineral classes
As the composition of the Earth's crust is
dominated by silicon and oxygen, silicate
elements are by far the most important class
of minerals in terms of rock formation and
diversity. However, non-silicate minerals
are of great economic importance, especially
as ores.
Non-silicate minerals are subdivided into
several other classes by their dominant chemistry,
which included native elements, sulfides,
halides, oxides and hydroxides, carbonates
and nitrates, borates, sulfates, phosphates,
and organic compounds. The majority of non-silicate
mineral species are extremely rare (constituting
in total 8% of the Earth's crust), although
some are relative common, such as calcite,
pyrite, magnetite, and hematite. There are
two major structural styles observed in non-silicates:
close-packing and silicate-like linked tetrahedra.
The close-packed structures, which is a way
to densely pack atoms while minimizing interstitial
space. Hexagonal close-packing involves stacking
layers where every other layer is the same
("ababab"), whereas cubic close-packing involves
stacking groups of three layers ("abcabcabc").
Analogues to linked silica tetrahedra include
SO4 (sulfate), PO4 (phosphate), AsO4 (arsenate),
and VO4 (vanadate). The non-silicates have
great economic importance, as they concentrate
elements more than the silicate minerals do.
The largest grouping of minerals by far are
the silicates; most rocks are composed of
greater than 95% silicate minerals, and over
90% of the Earth's crust is composed of these
minerals. The two main constituents of silicates
are silicon and oxygen, which are the two
most abundant elements in the Earth's crust.
Other common elements in silicate minerals
correspond to other common elements in the
Earth's crust, such aluminium, magnesium,
iron, calcium, sodium, and potassium. Some
important rock-forming silicates include the
feldspars, quartz, olivines, pyroxenes, amphiboles,
garnets, and micas.
Silicates
The base of unit of a silicate mineral is
the 4- tetrahedron. In the vast majority of
cases, silicon is in four-fold or tetrahedral
coordination with oxygen. In very high-pressure
situations, silicon will be six-fold or octahedral
coordination, such as in the perovskite structure
or the quartz polymorph stishovite (SiO2).
In the latter case, the mineral no longer
has a silicate structure, but that of rutile
(TiO2), and its associated group, which are
simple oxides. These silica tetrahedra are
then polymerized to some degree to create
various structures, such as one-dimensional
chains, two-dimensional sheets, and three-dimensional
frameworks. The basic silicate mineral where
no polymerization of the tetrahedra has occurred
requires other elements to balance out the
base 4- charge. In other silicate structures,
different combinations of elements are required
to balance out the resultant negative charge.
It is common for the Si4+ to be substituted
by Al3+ because of similarity in ionic radius
and charge; in those case, the 5- tetrahedra
form the same structures as do the unsubstituted
tetrahedra, but their charge-balancing requirements
are different.
The degree of polymerization can be described
by both the structure formed and how many
tetrahedral corners (or coordinating oxygens)
are shared (for aluminium and silicon in tetrahedral
sites). Orthosilicates (or nesosilicates)
have no linking of polyhedra, thus tetrahedra
share no corners. Disilicates (or sorosilicates)
have two tetrahedra sharing one oxygen atom.
Inosilicates are chain silicates; single-chain
silicates have two shared corners, whereas
double-chain silicates have two or three shared
corners. In phyllosilicates, a sheet structure
is formed which requires three shared oxygens;
in the case of double-chain silicates, some
tetrahedra must share two corners instead
of three as otherwise a sheet structure would
result. Framework silicates, or tectosilicates,
have tetrahedra that share all four corners.
The ring silicates, or cyclosilicates, only
need tetrahedra to share two corners to form
the cyclical structure.
The silicate subclasses are described below
in order of decreasing polymerization.
Tectosilicates
Tectosilicates, also known as framework silicates,
have the highest degree of polymerization.
With all corners of a tetrahedra shared, the
silicon:oxygen ratio becomes 1:2. Examples
are quartz, the feldspars, feldspathoids,
and the zeolites. Framework silicates tend
to be particularly chemically stable as a
result of strong covalent bonds.
Forming 12% of the Earth's crust, quartz (SiO2)
is the most abundant mineral species. It is
characterized by its high chemical and physical
resistivity. Quartz has several polymorphs,
including tridymite and cristobalite at high
temperatures, high-pressure coesite, and ultra-high
pressure stishovite. The latter mineral can
only be formed on Earth by meteorite impacts,
and its structure has been composed so much
that it had changed from a silicate structure
to that of rutile (TiO2). The silica polymorph
that is most stable at the Earth's surface
is α-quartz. Its counterpart, β-quartz,
is present only at high temperatures and pressures
(changes to α-quartz below 573°C at 1 bar).
These two polymorphs differ by a "kinking"
of bonds; this change in structure gives β-quartz
greater symmetry than α-quartz, and they
are thus also called high quartz (β) and
low quartz (α).
Feldspars are the most abundant group in the
Earth's crust, at about 50%. In the feldspars,
Al3+ substitutes for Si4+, which creates a
charge imbalance that must be accounted for
by the addition of cations. The base structure
becomes either - or 2- There are 22 mineral
species of feldspars, subdivided into two
major subgroups—alkali and plagioclase—and
two less common groups—celsian and banalsite.
The alkali feldspars are most commonly in
a series between potassium-rich orthoclase
and sodium-rich albite; in the case of plagioclase,
the most common series ranges from albite
to calcium-rich anorthite. Crystal twinning
is common in feldspars, especially polysynthetic
twins in plagioclase and Carlsbad twins in
alkali feldspars. If the latter subgroup cools
slowly from a melt, it forms exsolution lamellae
because the two components—orthoclase and
albite—are unstable in solid solution. Exsolution
can be on a scale from microscopic to readily
observable in hand-sample; perthitic texture
forms when Na-rich feldspar exsolve in a K-rich
host. The opposite texture (antiperthitic),
where K-rich feldspar exsolves in a Na-rich
host, is very rare.
Feldsapthoids are structurally similar to
feldspar, but differ in that they form in
Si-deficient conditions which allows for further
substitution by Al3+. As a result, feldsapthoids
cannot be associated with quartz. A common
example of a feldsapthoid is nepheline ((Na,
K)AlSiO4); compared to alkali feldspar, nepheline
has an Al2O3:SiO2 ratio of 1:2, as opposed
to 1:6 in the feldspar. Zeolites often have
distinctive crystal habits, occurring in needles,
plates, or blocky masses. They form in the
presence of water at low temperatures and
pressures, and have channels and voids in
their structure. Zeolites have several industrial
applications, especially in waste water treatment.
Phyllosilicates
Phyllosilicates consist of sheets of polymerized
tetrahedra. They are bound at three oxygen
sites, which gives a characteristic silicon:oxygen
ratio of 2:5. Important examples include the
mica, chlorite, and the kaolinite-serpentine
groups. The sheets are weakly bound by van
der Waals forces or hydrogen bonds, which
causes a crystallographic weakness, in turn
leading to a prominent basal cleavage among
the phyllosilicates. In addition to the tetrahedra,
phyllosilicates have a sheet of octahedra
(elements in six-fold coordination by oxygen)
that balanced out the basic tetrahedra, which
have a negative charge (e.g. 4-) These tetrahedra
(T) and octahedra (O) sheets are stacked in
a variety of combinations to create phyllosilicate
groups. Within an octahedral sheet, there
are three octahedral sites in a unit structure;
however, not all of the sites may be occupied.
In that case, the mineral is termed dioctahedral,
whereas in other case it is termed trioctahedral.
The kaolinite-serpentine group consists of
T-O stacks (the 1:1 clay minerals); their
hardness ranges from 2 to 4, as the sheets
are held by hydrogen bonds. The 2:1 clay minerals
(pyrophyllite-talc) consist of T-O-T stacks,
but they are softer (hardness from 1 to 2),
as they are instead held together by van der
Waals forces. These two groups of minerals
are subgrouped by octahedral occupation; specifically,
kaolinite and pyrophyllite are dioctahedral
whereas serpentine and talc trioctahedral.
Micas are also T-O-T-stacked phyllosilicates,
but differ from the other T-O-T and T-O-stacked
subclass members in that they incorporate
aluminium into the tetrahedral sheets (clay
minerals have Al3+ in octahedral sites). Common
examples of micas are muscovite, and the biotite
series. The chlorite group is related to mica
group, but a brucite-like (Mg(OH)2) layer
between the T-O-T stacks.
Because of their chemical structure, phyllosilicates
typically have flexible, elastic, transparent
layers that are electrical insulators and
can be split into very thin flakes. Micas
can be used in electronics as insulators,
in construction, as optical filler, or even
cosmetics. Chrysotile, a species of serpentine,
is the most common mineral species in industrial
asbestos, as it is less dangerous in terms
of health than the amphibole asbestos.
Inosilicates
Inosilicates consist of tetrahedra repeatedly
bonded in chains. These chains can be single,
where a tetrahedron is bound to two others
to form a continuous chain; alternatively,
two chains can be merged to create double-chain
silicates. Single-chain silicates have a silicon:oxygen
ratio of 1:3 (e.g. 4-), whereas the double-chain
variety has a ratio of 4:11 (e.g. 12-. Inosilicates
contain two important rock-forming mineral
groups; single-chain silicates are most commonly
pyroxenes, while double-chain silicates are
often amphiboles. Higher-order chains exist
(e.g. three-member, four-member, five-member
chains, etc.) but they are rare.
The pyroxene group consists of 21 mineral
species. Pyroxenes have a general structure
formula of XY(Si2O6), where X is an octahedral
site, while Y can vary in coordination number
from six to eight. Most varieties of pyroxene
consist of permutations of Ca2+, Fe2+ and
Mg2+ to balance the negative charge on the
backbone. Pyroxenes are common in the Earth's
crust (about 10%) and are a key constituent
of mafic igneous rocks.
Amphiboles have great variability in chemistry,
described variously as a "mineralogical garbage
can" or a "mineralogical shark swimming a
sea of elements". The backbone of the amphiboles
is the 12-; it is balanced by cations in three
possible positions, although the third position
is not always used, and one element can occupy
both remaining ones. Finally, the amphiboles
are usually hydrated, that is, they have a
hydroxyl group (-), although it can be replaced
by a fluoride, a chloride, or an oxide ion.
Because of the variable chemistry, there are
over 80 species of amphibole, although variations,
as in the pyroxenes, most commonly involve
mixtures of Ca2+, Fe2+ and Mg2+. Several amphibole
mineral species can have an asbestiform crystal
habit. These asbestos minerals form long,
thin, flexible, and strong fibres, which are
electrical insulators, chemically inert and
heat-resistant; as such, they have several
applications, especially in construction materials.
However, asbestos are known carcinogens, and
cause various other illnesses, such as asbestosis;
amphibole asbestos (anthophyllite, tremolite,
actinolite, grunerite, and riebeckite) are
considered more dangerous than chrysotile
serpentine asbestos.
Cyclosilicates
Cyclosilicates, or ring silicates, have a
ratio of silicon to oxygen of 1:3. Six-member
rings are most common, with a base structure
of 12-; examples include the tourmaline group
and beryl. Other ring structures exist, with
3, 4, 8, 9, 12 having been described. Cyclosilicates
tend to be strong, with elongated, striated
crystals.
Tourmalines have a very complex chemistry
that can be described by a general formula
XY3Z6(BO3)3T6O18V3W. The T6O18 is the basic
ring structure, where T is usually Si4+, but
substitutable by Al3+ or B3+. Tourmalines
can be subgrouped by the occupancy of the
X site, and from there further subdivided
by the chemistry of the W site. The Y and
Z sites can accommodate a variety of cations,
especially various transition metals; this
variability in structural transition metal
content gives the tourmaline group greater
variability in colour. Other cyclosilicates
include beryl, Al2Be3Si6O18, whose varieties
include the gemstones emerald (green) and
aquamarine (bluish). Cordierite is structurally
similar to beryl, and is a common metamorphic
mineral.
Sorosilicates
Sorosilicates, also termed disilicates, have
tetrahedron-tetrahedron bonding at one oxygen,
which results in a 2:7 ratio of silicon to
oxygen. The resultant common structural element
is the 6- group. The most common disilicates
by far are members of the epidote group. Epidotes
are found in variety of geologic settings,
ranging from mid-ocean ridge to granites to
metapelites. Epidotes are built around the
structure 10- structure; for example, the
mineral species epidote has calcium, aluminium,
and ferric iron to charge balance: Ca2Al2(Fe3+,
Al)(SiO4)(Si2O7)O(OH). The presence of iron
as Fe3+ and Fe2+ helps understand oxygen fugacity,
which in turn is a significant factor in petrogenesis.
Other examples of sorosilicates include lawsonite,
a metamorphic mineral forming in the blueschist
facies (subduction zone setting with low temperature
and high pressure), vesuvianite, which takes
up a significant amount of calcium in its
chemical structure.
Orthosilicates
Orthosilicates consist of isolated tetrahedra
that are charge-balanced by other cations.
Also termed nesosilicates, this type of silicate
has a silicon:oxygen ratio of 1:4 (e.g. SiO4).
Typical orthosilicates tend to form blocky
equant crystals, and are fairly hard. Several
rock-forming minerals are part of this subclass,
such as the aluminosilicates, the olivine
group, and the garnet group.
The aluminosilicates—kyanite, andalusite,
and sillimanite, all Al2SiO5—are structurally
composed of one 4- tetrahedron, and one Al3+
in octahedral coordination. The remaining
Al3+ can be in six-fold coordination (kyanite),
five-fold (andalusite) or four-fold (sillimanite);
which mineral forms in a given environment
is depend on pressure and temperature conditions.
In the olivine structure, the main olivine
series of (Mg, Fe)2SiO4 consist of magnesium-rich
forsterite and iron-rich fayalite. Both iron
and magnesium are in octahedral by oxygen.
Other mineral species having this structure
exist, such as tephroite, Mn2SiO4. The garnet
group has a general formula of X3Y2(SiO4)3,
where X is a large eight-fold coordinated
cation, and Y is a smaller six-fold coordinated
cation. There are six ideal endmembers of
garnet, split into two group. The pyralspite
garnets have Al3+ in the Y position: pyrope
(Mg3Al2(SiO4)3, almandine (Fe3Al2(SiO4)3),
and spessartine (Mn3Al2(SiO4)3). The ugrandite
garnets have Ca2+ in the X position: uvarovite
(Ca3Cr2(SiO4)3), grossular (Ca3Al2(SiO4)3)
and andradite (Ca3Fe2(SiO4)3). While there
are two subgroups of garnet, solid solutions
exist between all six end-members.
Other orthosilicates include zircon, staurolite,
and topaz. Zircon (ZrSiO4) is useful in geochronology
as the Zr4+ can be substituted by U6+; furthermore,
because of its very resistant structure, it
is difficult to reset it as a chronometer.
Staurolite is a common metamorphic intermediate-grade
index mineral. It has a particularly complicated
crystal structure that was only fully described
in 1986. Topaz (Al2SiO4(F, OH)2, often found
in granitic pegmatites associated with tourmaline,
is a common gemstone mineral.
Non-silicates
Native elements
Native elements are those that are not chemically
bonded to other elements. This mineral group
includes native metals, semi-metals, and non-metals,
and various alloys and solid solutions. The
metals are held together by metallic bonding,
which confers distinctive physical properties
such as their shiny metallic lustre, ductility
and malleability, and electrical conductivity.
Native elements are subdivided into groups
by their structure or chemical attributes.
The gold group, with a cubic close-packed
structure, includes metals such as gold, silver,
and copper. The platinum group is similar
in structure to the gold group. The iron-nickel
group is characterized by several iron-nickel
alloy species. Two examples are kamacite and
taenite, which are found in iron meteorites;
these species differ by the amount of Ni in
the alloy; kamacite has less than 5–7% nickel
and is a variety of native iron, whereas the
nickel content of taenite ranges from 7–37%.
Arsenic group minerals consist of semi-metals,
which have only some metallic; for example,
they lack the malleability of metals. Native
carbon occurs in two allotropes, graphite
and diamond; the latter forms at very high
pressure in the mantle, which gives it a much
stronger structure than graphite.
Sulfides
The sulfide minerals are chemical compounds
of one or more metals or semimetals with a
sulfur; tellurium, arsenic, or selenium can
substitute for the sulfur. Sulfides tend to
be soft, brittle minerals with a high specific
gravity. Many powdered sulfides, such as pyrite,
have a sulfurous smell when powdered. Sulfides
are susceptible to weathering, and many readily
dissolve in water; these dissolved minerals
can be later redeposited, which creates enriched
secondary ore deposits. Sulfides are classified
by the ratio of the metal or semimetal to
the sulfur, such as M:S equal to 2:1, or 1:1.
Many sulfide minerals are economically important
as metal ores; examples include sphalerite
(ZnS), an ore of zinc, galena (PbS), an ore
of lead, cinnabar (HgS), an ore of mercury,
and molybdenite (MoS2, an ore of molybdenum.
Pyrite (FeS2), is the most commonly occurring
sulfide, and can be found in most geological
environments. It is not, however, an ore of
iron, but can be instead oxidized to produce
sulfuric acid. Related to the sulfides are
the rare sulfosalts, in which a metallic element
is bonded to sulfur and a semimetal such as
antimony, arsenic, or bismuth. Like the sulfides,
sulfosalts are typically soft, heavy, and
brittle minerals.
Oxides
Oxide minerals are divided into three categories:
simple oxides, hydroxides, and multiple oxides.
Simple oxides are characterized by O2- as
the main anion and primarily ionic bonding.
They can be further subdivided by the ratio
of oxygen to the cations. The periclase group
consists of minerals with a 1:1 ratio. Oxides
with a 2:1 ratio include cuprite (Cu2O) and
water ice. Corundum group minerals have a
2:3 ratio, and includes minerals such as corundum
(Al2O3), and hematite (Fe2O3). Rutile group
minerals have a ratio of 1:2; the eponymous
species, rutile (TiO2) is the chief ore of
titanium; other examples include cassiterite
(SnO2; ore of tin), and pyrolusite (MnO2;
ore of manganese). In hydroxides, the dominant
anion is the hydroxyl ion, OH-. Bauxites are
the chief aluminium ore, and are a heterogeneous
mixture of the hydroxide minerals diaspore,
gibbsite, and bohmite; they form in areas
with a very high rate of chemical weathering
(mainly tropical conditions). Finally, multiple
oxides are compounds of two metals with oxygen.
A major group within this class are the spinels,
with a general formula of X2+Y3+2O4. Examples
of species include spinel (MgAl2O4), chromite
(FeCr2O4), and magnetite (Fe3O4). The latter
is readily distinguishable by its strong magnetism,
which occurs as it has iron in two oxidation
states (Fe2+Fe3+2O4), which makes it a multiple
oxide instead of a single oxide.
Halides
The halide minerals are compounds where a
halogen (fluorine, chlorine, iodine, and bromine)
is the main anion. These minerals tend to
be soft, weak, brittle, and water-soluble.
Common examples of halides include halite
(NaCl, table salt), sylvite (KCl), fluorite
(CaF2). Halite and sylvite commonly form as
evaporites, and can be dominant minerals in
chemical sedimentary rocks. Cryolite, Na3AlF6,
is a key mineral in the extraction of aluminium
from bauxites; however, as the only significant
occurrence at Ivittuut, Greenland, in a granitic
pegmatite, was depleted, synthetic cryolite
can be made from fluorite.
Carbonates
The carbonate minerals are those were the
main anionic group is carbonate, 2−. Carbonates
tend to be brittle, many have rhombohedral
cleavage, and all react with acid. Due to
the last characteristic, field geologists
often carry dilute hydrochloric acid to distinguish
carbonates from non-carbonates. The reaction
of acid with carbonates, most commonly found
as the polymorph calcite and aragonite (CaCO3),
relates to the dissolution and precipitation
of the mineral, which is a key in the formation
of limestone caves, features within them such
as stalactite and stalagmites, and karst landforms.
Carbonates are most often formed as biogenic
or chemical sediments in marine environments.
The carbonate group is structurally a triangle,
where a central C4+ cation is surrounded by
three O2- anions; different groups of minerals
form from different arrangements of these
triangles. The most common carbonate mineral
is calcite, and is the primary constituent
of sedimentary limestone and metamorphic marble.
Calcite, CaCO3, can have a high magnesium
impurity; under high-Mg conditions, its polymorph
aragonite will form instead; the marine geochemistry
in this regard can be described as an aragonite
or calcite sea, depending on which mineral
preferentially forms. Dolomite is a double
carbonate, with the formula CaMg(CO3)2. Secondary
dolomitization of limestone is common, where
calcite or aragonite are converted to dolomite;
this reaction increases pore space (the unit
cell volume of dolomite is 88% that of calcite),
which can create a reservoir for oil and gas.
These two minerals species are members of
eponymous mineral groups: the calcite group
includes carbonates with the general formula
XCO3, and the dolomite group constitutes minerals
with general formula XY(CO3)2.
Sulfates
The sulfate minerals all contain the sulfate
anion, 2−. They tend to be transparent to
translucent, soft, and many are fragile. Sulfate
minerals commonly form as evaporites, where
they precipitate out of evaporating saline
waters; alternative, sulfates can also be
found in hydrothermal vein systems associated
with sulfides, or as oxidation products of
sulfides. Sulfates can be subdivded into anhydrous
and hydrous minerals. The most common hydrous
sulfate by far is gypsum, CaSO4⋅2H2O. It
forms as an evaporite, and is associated other
evaporites such as calcite and halite; if
it incorporates sand grains as it crystallizes,
gypsum can form desert roses. Gypsum has very
low thermal conductivity and maintains a low
temperature when heated as it loses that heat
by dehydrating; as such, gypsum is used as
an insulator in materials such as plaster
and drywall. The anhydrous equivalent of gypsum
is anhydrite; it can form directly from seawater
in highly arid conditions. The barite group
has the general formula XSO4, where the X
is a large 12-coordinated cation. Examples
include barite (BaSO4), celestine (SrSO4),
and anglesite (PbSO4); anhydrite is not part
of the barite group, as the smaller Ca2+ is
only in eight-fold coordination.
Phosphates
The phosphate minerals are characterized by
the tetrahedral 3- unit, although the structure
can be generalized, and phosphorus is replaced
by antimony, arsenic, or vanadium. The most
common phosphate is the apatite group; common
species within this group are fluorapatite
(Ca5(PO4)3F), chlorapatite (Ca5(PO4)3Cl) and
hydroxylapatite (Ca5(PO4)3(OH)). Minerals
in this group are the main crystalline constituents
of teeth and bones in vertebrates. The relatively
abundant monazite group has a general structure
of ATO4, where T is phosphorus or arsenic,
and A is often a rare-earth element (REE).
Monazite is important in two ways: first,
as a REE "sink", it can sufficiently concentrate
these elements to become an ore; secondly,
monazite group elements can incorporate relatively
large amounts of uranium and thorium, which
can be used to date the rock based on the
decay of the U and Th to lead.
Organic minerals
The Strunz classification includes a class
for organic minerals. These rare compounds
contain organic carbon, but can be formed
by a geologic process. For example, whewellite,
CaC2O4⋅H2O is an oxalate that can be deposited
in hydrothermal ore veins. While hydrated
calcium oxalate can be found in coal seams
and other sedimentary deposits involving organic
matter, the hydrothermal occurrence is not
considered to be related to biological activity.
