A mineral is, broadly speaking, a solid chemical
compound that occurs naturally in pure form.
Minerals are most commonly associated with
rocks due to the presence of minerals within
rocks. These rocks may consist of one type
of mineral, or may be an aggregate of two
or more different types of minerals, spacially
segregated into distinct phases. Compounds
that occur only in living beings are usually
excluded, but some minerals are often biogenic
(such as calcite) and/or are organic compounds
in the sense of chemistry (such as mellite).
Moreover, living beings often synthesize inorganic
minerals (such as hydroxylapatite) that also
occur in rocks.
In geology and mineralogy, the term "mineral"
is usually reserved for mineral species: crystalline
compounds with a fairly well-defined chemical
composition and a specific crystal structure.
Minerals without a definite crystalline structure,
such as opal or obsidian, are then more properly
called mineraloids. If a chemical compound
may occur naturally with different crystal
structures, each structure is considered different
mineral species. Thus, for example, quartz
and stishovite are two different minerals
consisting of the same compound, silicon dioxide.
The International Mineralogical Association
(IMA) is the world's premier standard body
for the definition and nomenclature of mineral
species. As of November 2018, the IMA recognizes
5,413 official mineral species. out of more
than 5,500 proposed or traditional ones.The
chemical composition of a named mineral species
may vary somewhat by the inclusion of small
amounts of impurities. Specific varieties
of a species sometimes have conventional or
official names of their own. For example,
amethyst is a purple variety of the mineral
species quartz. Some mineral species can have
variable proportions of two or more chemical
elements that occupy equivalent positions
in the mineral's structure; for example, the
formula of mackinawite is given as (Fe,Ni)9S8,
meaning FexNi9-xS8, where x is a variable
number between 0 and 9. Sometimes a mineral
with variable composition is split into separate
species, more or less arbitrarily, forming
a mineral group; that is the case of the silicates
CaxMgyFe2-x-ySiO4, the olivine group.
Besides the essential chemical composition
and crystal structure, the description of
a mineral species usually includes its common
physical properties such as habit, hardness,
lustre, diaphaneity, colour, streak, tenacity,
cleavage, fracture, parting, specific gravity,
magnetism, fluorescence, radioactivity, as
well as its taste or smell and its reaction
to acid.
Minerals are classified by key chemical constituents;
the two dominant systems are the Dana classification
and the Strunz classification. Silicate minerals
comprise approximately 90% of the Earth's
crust. Other important mineral groups include
the native elements, sulfides, oxides, halides,
carbonates, sulfates, and phosphates.
== Definition ==
=== 
Basic definition ===
One definition of a mineral encompasses the
following criteria:
Formed by a natural process (anthropogenic
compounds are excluded).
Stable or metastable at room temperature (25
°C). In the simplest sense, this means the
mineral must be solid. 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; because these two minerals were described
before 1959, they were grandfathered by the
International Mineralogical Association (IMA).
Modern advances have included extensive study
of liquid crystals, which also extensively
involve mineralogy.
Represented by a chemical formula. 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 composition,
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.
Ordered atomic arrangement. This generally
means crystalline; however, crystals are also
periodic, so the broader criterion is used
instead. An ordered atomic arrangement gives
rise to a variety of macroscopic physical
properties, such as crystal form, hardness,
and cleavage. There have been several recent
proposals to classify biogenic or amorphous
substances as minerals. The formal definition
of a mineral approved by the IMA in 1995:
"A mineral is an element or chemical compound
that is normally crystalline and that has
been formed as a result of geological processes."
Usually abiogenic (not resulting from the
activity of living organisms). Biogenic substances
are explicitly excluded by the IMA: "Biogenic
substances are chemical compounds produced
entirely by biological processes without a
geological component (e.g., urinary calculi,
oxalate crystals in plant tissues, shells
of marine molluscs, etc.) and are not regarded
as minerals. However, if geological processes
were involved in the genesis of the compound,
then the product can be accepted as a mineral."The
first three general characteristics are less
debated than the last two.
=== 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 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
genetics and X-ray absorption spectroscopy
are providing 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 "Working Group on Environmental
Mineralogy and Geochemistry " deals with minerals
in the hydrosphere, atmosphere, and biosphere.
The group's scope includes mineral-forming
microorganisms, which exist on nearly every
rock, soil, and particle surface spanning
the globe to depths of at least 1600 metres
below the sea floor and 70 kilometres into
the stratosphere (possibly entering the mesosphere).
Biogeochemical cycles have contributed to
the formation of minerals for billions of
years. Microorganisms can precipitate metals
from solution, contributing to the formation
of ore deposits. They can also catalyze the
dissolution of 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 have properties of both
liquids and crystals. 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, the
latter group including liquid crystals. Although
biominerals and liquid mineral crystals, are
not the most common form of minerals, they
help to define the limits of what constitutes
a mineral proper. The formal Nickel (1995)
definition explicitly mentioned crystallinity
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 a quasicrystal.
Unlike a true crystal, quasicrystals are ordered
but not periodic.
=== Rocks, ores, and gems ===
Minerals are not equivalent to rocks. A rock
is an aggregate of one or more minerals or
mineraloids. Some rocks, such as 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
for the last one, all of these 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 ===
Minerals are classified by variety, species,
series and group, in order of increasing generality.
The basic level of definition is that of mineral
species, each of which is distinguished from
the others by 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, Dana and
Strunz, are used for minerals; both rely on
composition, specifically with regards to
important chemical groups, and structure.
James Dwight Dana, a leading geologist of
his time, first published his System of Mineralogy
in 1837; as of 1997, it is in its eighth edition.
The Dana classification assigns a four-part
number to a mineral species. Its class number
is based on important compositional groups;
the type gives the ratio of cations to anions
in the mineral, and the last two numbers group
minerals by structural similarity within 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.As of November 2018, 5,413
mineral species are approved by the IMA. 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 word "species"
(from the Latin species, "a particular sort,
kind, or type with distinct look, or appearance")
comes from the classification scheme in Systema
Naturae by Carl Linnaeus. He divided the natural
world into three kingdoms – plants, animals,
and minerals – and classified each with
the same hierarchy. In descending order, these
were Phylum, Class, Order, Family, Tribe,
Genus, and Species.
== 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
47% of the crust by weight, and silicon accounts
for 28%.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 that 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 as quartzite.
The chemical composition may vary between
end member species of a solid solution 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 [AlSi3O8]−;
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 representations of
how a cation is surrounded by an anion. In
mineralogy, coordination polyhedra are usually
considered in terms of oxygen, due its abundance
in the crust. 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 the compound is compressed
such that silicon is in six-fold (octahedral)
coordination with oxygen. Bigger cations have
a bigger coordination numbers 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 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. Other examples
are the aluminosilicates kyanite, andalusite,
and sillimanite (polymorphs, since 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):
2 KAlSi3O8 + 5 H2O + 2 H+ → Al2Si2O5(OH)4
+ 4 H2SiO3 + 2 K+Under low-grade metamorphic
conditions, kaolinite reacts with quartz to
form pyrophyllite (Al2Si4O10(OH)2):
Al2Si2O5(OH)4 + SiO2 → Al2Si4O10(OH)2 +
H2OAs metamorphic grade increases, the pyrophyllite
reacts to form kyanite and quartz:
Al2Si4O10(OH)2 → Al2SiO5 + 3 SiO2 + H2OAlternatively,
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 ==
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 differences arise corresponding to how
aluminium is coordinated within the crystal
structure. In all minerals, one aluminium
ion is always in six-fold coordination with
oxygen. 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 Al[6]Al[6]SiO5,
to reflect its crystal structure. Andalusite
has the second aluminium in five-fold coordination
(Al[6]Al[5]SiO5) and sillimanite has it in
four-fold coordination (Al[6]Al[4]SiO5).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 graphite
is very soft, has a greasy lustre, and crystallises
in the hexagonal family. This difference is
accounted for 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 four 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 three others.
These sheets are held together by much weaker
van der Waals forces, and this discrepancy
translates to large macroscopic differences.
Twinning is the intergrowth of two or more
crystals 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.
This type of twinning 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 twins through
the presence of repetitive twinning; however,
instead of occurring around a rotational axis,
polysynthetic twinning 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 describes
needlelike crystals as 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 [001]
but 7 parallel to [100].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 is 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 them. An example of a transparent
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 the 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 a 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 (idiochromatic and allochromatic)
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 labradorite 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 minerals, or in thin-section,
cleavage can be seen as a series of parallel
lines marking the planar surfaces when viewed
from the 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 directions,
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) onto
a mineral aids in distinguishing carbonates
from other mineral classes. The acid reacts
with the carbonate ([CO3]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 form. An example
of this test is done when distinguishing calcite
from dolomite, especially within the rocks
(limestone and dolomite 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 magnetism
is also present, albeit not as strongly, in
pyrrhotite and ilmenite. Some minerals exhibit
electrical properties – for example, quartz
is piezoelectric – but electrical properties
are rarely used as diagnostic criteria for
minerals because of incomplete data and natural
variation.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 with various techniques, such
as thin-section petrography.
== Classification ==
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 includes native elements,
sulfides, halides, oxides and hydroxides,
carbonates and nitrates, borates, sulfates,
phosphates, and organic compounds. Most non-silicate
mineral species are rare (constituting in
total 8% of the Earth's crust), although some
are relatively 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.
close-packed structures 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 as 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 unit of a silicate mineral is the
[SiO4]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 in 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 cases, the [AlO4]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 [AlSi3O8]− or [Al2Si2O8]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.Feldspathoids 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,
feldspathoids cannot be associated with quartz.
A common example of a feldspathoid 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 balance out the basic tetrahedra, which
have a negative charge (e.g. [Si4O10]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. [Si2O6]4−), whereas the
double-chain variety has a ratio of 4:11,
e.g. [Si8O22]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 [Si8O22]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
([OH]−), 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 [Si6O18]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 [Si2O7]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 [(SiO4)(Si2O7)]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 –bkyanite, andalusite,
and sillimanite, all Al2SiO5 – are structurally
composed of one [SiO4]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 traits; 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 in which
a halogen (fluorine, chlorine, iodine, or
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 in which
the main anionic group is carbonate, [CO3]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, which 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, in
which 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 mineral species
are members of eponymous mineral groups: the
calcite group includes carbonates with the
general formula XCO3, and the dolomite group
constitutes minerals with the general formula
XY(CO3)2.
==== Sulfates ====
The 
sulfate minerals all contain the sulfate anion,
[SO4]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. Sulfates can also be found in hydrothermal
vein systems associated with sulfides, or
as oxidation products of sulfides. Sulfates
can be subdivided into anhydrous and hydrous
minerals. The most common hydrous sulfate
by far is gypsum, CaSO4⋅2H2O. It forms as
an evaporite, and is associated with 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 [PO4]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 in monazite geochronology 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.
== Astrobiology ==
It has been suggested that biominerals could
be important indicators of extraterrestrial
life and thus could play an important role
in the search for past or present life on
the planet Mars. Furthermore, organic components
(biosignatures) that are often associated
with biominerals are believed to play crucial
roles in both pre-biotic and biotic reactions.On
January 24, 2014, NASA reported that current
studies by the Curiosity and Opportunity rovers
on Mars will now be searching for evidence
of ancient life, including a biosphere based
on autotrophic, chemotrophic and/or chemolithoautotrophic
microorganisms, as well as ancient water,
including fluvio-lacustrine environments (plains
related to ancient rivers or lakes) that may
have been habitable. The search for evidence
of habitability, taphonomy (related to fossils),
and organic carbon on the planet Mars is now
a primary NASA objective.
== See also ==
== Notes
