A mineral is a naturally occurring chemical compound, usually of
crystalline form and not produced by life processes. A mineral has one
specific chemical composition, whereas a rock can be an aggregate of
different minerals or mineraloids. The study of minerals is called
As of March 2018[update], there are more than 5,500 known mineral
species; 5,312 of these have been approved by the International
Mineralogical Association (IMA).
Minerals are distinguished by various chemical and physical
properties. Differences in chemical composition and crystal structure
distinguish the various species, which were determined by the
mineral's geological environment when formed. Changes in the
temperature, pressure, or bulk composition of a rock mass cause
changes in its minerals. Within a mineral species there may be
variation in physical properties or minor amounts of impurities that
are recognized by mineralogists or wider society as a mineral
variety, for example amethyst, a purple variety of the mineral
Minerals can be described by their various physical properties, which
are related to their chemical structure and composition. Common
distinguishing characteristics include crystal structure and habit,
hardness, lustre, diaphaneity, colour, streak, tenacity, cleavage,
fracture, parting, specific gravity, magnetism, taste or smell,
radioactivity, and reaction to acid.
Minerals are classified by key chemical constituents; the two dominant
systems are the Dana classification and the Strunz classification.
Silicon and oxygen constitute approximately 75% of the Earth's crust,
which translates directly into the predominance of silicate minerals.
The silicate minerals compose over 90% of the Earth's crust. 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 [SiO4]4− silica tetrahedron—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.
1.1 Basic definition
1.2 Recent advances
1.3 Rocks, ores, and gems
1.4 Nomenclature and classification
3 Physical properties
Crystal structure and habit
3.3 Lustre and diaphaneity
3.4 Colour and streak
3.5 Cleavage, parting, fracture, and tenacity
3.6 Specific gravity
3.7 Other properties
4.2.1 Native elements
4.2.8 Organic minerals
6 See also
9 General references
10 Further reading
11 External links
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.
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
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
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
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
The Nickel (1995)[clarification needed] 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
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
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
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
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
Schist is a metamorphic rock characterized by an abundance of platy
minerals. In this example, the rock has prominent sillimanite
porphyroblasts as large as 3 cm (1.2 in).
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
As of September 2017[update], 5,291 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. Linneaeus
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.
Hübnerite, the manganese-rich end-member of the wolframite series,
with minor quartz in the background
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
When minerals react, the products will sometimes assume the shape of
the reagent; the product mineral is termed a pseudomorph of (or after)
the reagent. Illustrated here is a pseudomorph of kaolinite after
orthoclase. Here, the pseudomorph preserved the Carlsbad twinning
common in orthoclase.
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
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 + H2O
As metamorphic grade increases, the pyrophyllite reacts to form
kyanite and quartz:
Al2Si4O10(OH)2 → Al2SiO5 + 3 SiO2 + H2O
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.
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,
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 system and
Topaz has a characteristic orthorhombic elongated crystal shape.
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):
Garnet, halite, pyrite
Rutile, zircon, andalusite
Olivine, aragonite, orthopyroxenes
Quartz, calcite, tourmaline
Clinopyroxenes, orthoclase, gypsum
Anorthite, albite, kyanite
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
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 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 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
Contact twins, as seen in spinel
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
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
Main article: Mohs scale of mineral hardness
Diamond is the hardest natural material, and has a Mohs hardness of
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
Main article: Lustre (mineralogy)
Pyrite has a metallic lustre.
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
Main article: Streak (mineralogy)
Colour is typically not a diagnostic property of minerals. Shown are
green uvarovite (left) and red-pink grossular (right), both garnets.
The diagnostic features would include dodecahedral crystals, resinous
lustre, and hardness around 7.
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
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
Cleavage (crystal) and Fracture (mineralogy)
Perfect basal cleavage as seen in biotite (black), and good cleavage
seen in the matrix (pink orthoclase).
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
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
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
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.
Galena, PbS, is a mineral with a high 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
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
Carnotite (yellow) is a radioactive uranium-bearing mineral.
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 rocks (limestone and dolostone
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
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
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
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
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
Main article: Silicate minerals
Aegirine, an iron-sodium clinopyroxene, is part of the inosilicate
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
Natrolite is a mineral series in the zeolite group; this sample has a
very prominent acicular crystal habit.
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
Forming 12% of the Earth's crust, quartz (SiO2) is the most abundant
mineral species. It is characterized by its high chemical and physical
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
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, 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
Muscovite, a mineral species in the mica group, within the
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
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
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.
Asbestiform tremolite, part of the amphibole group in the inosilicate
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
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
Earth's crust (about 10%) and are a key constituent of mafic
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.
An example of elbaite, a species of tourmaline, with distinctive
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
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
Epidote often has a distinctive pistachio-green colour.
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.
Black andradite, an end-member of the orthosilicate garnet group.
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
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.
Main article: Native element minerals
Native gold. Rare specimen of stout crystals growing off of a central
stalk, size 3.7 x 1.1 x 0.4 cm, from Venezuela.
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.
Main article: Sulfide minerals
Red cinnabar (HgS), a mercury ore, on dolomite.
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.
Main article: Oxide minerals
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.
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.
Main article: Halide minerals
Pink cubic halite (NaCl; halide class) crystals on a nahcolite matrix
(NaHCO3; a carbonate, and mineral form of sodium bicarbonate, used as
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
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
Main article: Carbonate minerals
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.
Main article: Sulfate minerals
Gypsum desert rose
The sulfate minerals all contain the sulfate anion, [SO4]2−. They
tend to be transparent to translucent, soft, and many are
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
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.
Main article: Phosphate minerals
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
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
Main article: Organic mineral
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.
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
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
Mars is now a primary
List of minerals
List of minerals
List of minerals (complete)
Polymorphism (materials science)
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Commons has media related to Minerals.
The Wikibook Historical Geology has a page on the topic of: Minerals
The Wikibook High School Earth Science has a page on the topic of:
Mindat mineralogical database, largest mineral database on the
Mineralogy Database" by David Barthelmy (2009)
Mineral Identification Key II" Mineralogical Society of America
Crystal Structure Database"
Minerals and the Origins of
Life (Robert Hazen, NASA) (video, 60m,
Pollution / quality
Ambient standards (USA)
Clean Air Act (USA)
Fossil fuels (peak oil)
Non-timber forest products
Types / location
storage and recovery
Earth Overshoot Day
Renewable / Non-renewable
Agriculture and agronomy