Data and information
for this chapter are taken from available web resources (e.g. wikipedia)
or are personal notes.
A mineral is a naturally-occurring, homogeneous solid with
a definite, but generally not fixed, chemical composition
and an ordered atomic arrangement formed by inorganic processes.
1.) "Naturally occurring" means that synthetic compounds
not known to occur in nature cannot have a mineral name. However,
it may occur anywhere, other planets, deep in the earth, as
long as there exists a natural sample to describe.
2.) "Homogeneous solid" means that it must be chemically
and physically homogeneous down to the basic repeat unit of
the atoms. It will then have absolutely predictable physical
properties (density, compressibility, index of refraction,
etc.). This means that rocks such as granite or basalt are
not minerals because they contain more than one compound.
3.) "Definite, but generally not fixed, composition"
means that atoms, or groups of atoms must occur in specific
ratios. For ionic crystals (i.e. most minerals) ratios of
cations to anions will be constrained by charge balance, however,
atoms of similar charge and
radius may substitute freely for one another; hence definite,
but not fixed.4.) "Ordered atomic arrangement" means
crystalline. Crystalline materials are three-dimensional periodic
arrays of precise geometric arrangement of atoms. Glasses
such as obsidian, which are disordered solids, liquids (e.g.,
water, mercury), and gases (e.g., air) are not minerals.
5.) "Inorganic processes" means that crystalline
organic compounds formed by organisms are generally not considered
minerals. However, carbonate shells are minerals because they
are identical to compounds formed by purely inorganic processes.
An abbreviated definition of a mineral would be "a natural,
A phase is that part of a system which is physically and chemically
homogeneous within itself and is surrounded by a boundary
such that it is mechanically separable from the rest of the
Because minerals are crystals, dissimilar elements must occur
in fixed ratios to one another.
complete free substitution of very similar elements (e.g.,
Mg+2 and Fe+2 which are very similar in charge (valence) and
radius is very common and usually results in a crystalline
solution (solid solution). For example, the minerals forsterite
(Mg2SiO4) and fayalite (Fe2SiO4) are members of the olivine
group and have the same crystal structure, that is, the same
geometric arrangement of atoms. Mg and Fe substitute freely
for each other in this structure, and all compositions between
the two extremes, forsterite and fayalite, may occur. However,
Mg or Fe do not substitute for Si or O, so that the three
components, Mg/Fe, Si and O always maintain the same 2 to
1 to 4 ratio because the ratio is fixed by the crystalline
structure. These two minerals are called end-members of the
olivine series and represent extremes or "pure"
compositions. Because these two minerals have the same structure,
they are called isomorphs and the series, an isomorphous series.
contrast to the isomorphous series, it is also common for
a single compound (composition) to occur with different crystal
Each of these structures is then a different mineral and,
in general, will be stable under different conditions of temperature
and pressure. Different structural modifications of the same
compound are called polymorphs. An example
of polymorphism is the different minerals of SiO2 (silica);
alpha-quartz, beta-quartz, tridymite, cristobalite, coesite,
and stishovite. Although each of these has the same formula
and composition, they are different minerals because they
have different crystal structures. Each is stable under a
different set of temperature and pressure conditions, and
the presence of one of these in a rock may be used to infer
the conditions of formation of a rock. Another familiar example
of polymorphism is graphite and diamond, two different minerals
with the same formula, C (carbon).
Glasses (obsidian), liquids, and gases however, are not crystalline,
and the elements in them may occur in any ratios, so they
are not minerals. So in order for a natural compound to be
a mineral, it must have a unique composition and structure.
occurrence and formation
addition to physical properties, one of the most diagnostic features
of a mineral is the geological environment in which it is occurs.
Learning to recognize different types of geological environ ments
can be thus be very helpful in recognizing the common minerals. For
the purposes of aiding mineral identification, we can give the following
Minerals in igneous rocks must have high melting points and be able
to co-exist with, or crystallize from, silicate melts at temperatures
above 800 º C. Igneous rocks can be generally classed according
to their silica content with low-silica (<< 50 % SiO2) igneous
rocks being termed basic or mafic, and high-silica igneous rocks being
termed silicic or acidic. Basic igneous rocks include basalts, dolerites,
gabbros, kimberlites, and peridotites, and abundant minerals in such
rocks include olivine, pyroxenes, Ca-feldspar (plagioclase), amphiboles,
and biotite. The abundance of Fe in these rocks causes them to be
dark-colored. Silicic igneous rocks include granites, granodiorites,
and rhyolites, and abundant minerals include quartz, muscovite, and
alkali feldspars. These are commonly light-colored although color
is not always diagnostic.
| In addition
to basic and silicic igneous rocks, a third igneous mineral environment
representing the final stages of igneous fractionation is called a
pegmatite which is typically very coarse-grained and simi lar in composition
to silicic igneous rocks (i.e. high in silica). Elements that do not
readily substitute into the abundant minerals are called incompatible
elements, and these typically accumulate to form their own minerals
in pegmatites. Minerals containing the incompatible elements, Li,
Be, B, P, Rb, Sr, Y, Nb, rare earths, Cs, and Ta are typical and characteristic
of pegmatites. Metamorphic minerals.
Minerals in metamorphic rocks have crystallized from other minerals
rather than from melts and need not be stable to such high temperatures
as igneous minerals. In
a very general way, metamorphic environments may be classified as
low-grade metamorphic (temperatures of 60 º to 400 º C and
pressures << .5 GPa (=15km depth) and high-grade meta morphic
(temperatures > 400 º and/or pressures > .5GPa)
.Minerals characteristic of low- grade metamorphic environments include
the zeolites, chlorites, and andalusite.
characteristic of high grade metamorphic environments include sillimanite,
kyanite, staurolite, epidote, and amphiboles.
Minerals in sedimentary rocks are either stable in low-temperature
hydrous environments (e.g. clays) or are high temperature minerals
that are extremely resistant to chemical weathering (e.g. quartz).
One can think of sedimentary minerals as exhibiting a range of solubilities
so that the most insoluble minerals such as quartz gold, and diamond
accumulate in the coarsest detrital sedimentary rocks, less resistant
minerals such as feldspars, which weather to clays, accumulate in
finer grained siltstones and mudstones, and the most soluble minerals
such as calcite and halite (rock-salt) are chemically precipitated
in evaporite deposits. Accordingly, sedimentary minerals can be classified
into detrital sediments and evaporites. Detrital sedimentary minerals
include quartz, gold, diamond, apatite and other phosphates, calcite,
and clays.Evaporite sedimentary minerals include calcite, gypsum,
anhydrite, halite and sylvite, plus some of the borate minerals.
The fourth major mineral environment is hydrothermal, minerals precipitated
from hot aqueous solutions associated with emplacement of intrusive
igneous rocks. This environment is commonly grouped with metamorphic
environments, but the minerals that form by this process and the elements
that they contain are so distinct from contact or regional metamorphic
rocks that it us useful to consider them as a separate group. These
may be sub-classi fied as high temperature hydrothermal, low temperature
hydrothermal, and oxydized hydrothermal. Metals of the center and
right-hand side of the periodic table (e.g. Cu, Zn, Sb, As, Pb, Sn,
Cd, Hg, Ag) most commonly occur in sulfide minerals and are termed
the chalcophile elements. Sulfides may occur in igneous and metamorphic
rocks, but are most typically hydrothermal. High temperature hydrothermal
minerals include gold, silver, tungstate minerals, chalcopyrite, bornite,
the tellurides, and molybdenite. Low temperature hydrothermal minerals
include barite, gold, cinnabar, pyrite, and cassiterite. Sulfide minerals
are not stable in atmospheric oxygen and will weather by oxidation
to form oxides, sulfates and carbonates of the chalcophile metals,
and these minerals are characteristic of oxidized hydrothermal deposits.
Such deposits are called gossans and are marked by yellow-red iron
oxide stains on rock surfaces. These usually mark mineralized zones
crystal faces that can be observed in a mineral specimen may arise
either as a result of growth or of cleavage. In either case, they
reflect the internal symmetry of the crystal structure that makes
the mineral unique. The crystal faces commonly seen on quartz are
growth faces and represent the slow est growing directions in the
grows rapidly along its c-axis (three-fold or trigonal symmetry axis)
direction and so never shows faces perpendicular to this direction.
On the other hand, calcite rhomb faces and mica plates are cleavages
and represent the weakest chemical bonds in the structure. There is
a complex terminology for crystal faces, but some obvious names for
faces are prisms and pyramids.
| A prism
is a face that is perpendicular to a major axis of the crystal, whereas
a pyramid is one that is not perpendicular to any major axis. Crystals
that commonly develop prism faces are said to have a prismatic or
columnar habit. Crystals that grow in fine needles are acicular; crystals
growing flat plates are tabular. Crystals forming radiating sprays
of needles or fibers are stellate.
forming parallel fibers are fibrous, and crystals forming branching,
tree-like growths are dendritic.
The complete study of crystal
structure is quite complex and a summary is given in separate page:
recognition of a mineral is based on seven easily examined properties
plus a few unique properties such as magnetism or radioactivity that
are strong clues to a mineral's identity. These seven properties are:
Luster and transparency.
The way a mineral transmits or reflects light is a diagnostic property.
The transparency may be either opaque, translucent, or transparent.
This reflectance property is called luster.
Native metals and many sulfides are opaque and reflect most of the
light hitting their surfaces and have a metallic luster. Other opaque
or nearly opaque oxides may appear dull, or resinous. Transparent
minerals with a high index of refraction such as diamond appear brilliant
and are said to have an adamantine luster, whereas those with a lower
index of refraction such as quartz or calcite appear glassy and are
said to have a vitreous luster.
Color is fairly self-explanatory property describing the reflectance.
Metallic minerals are either white, gray, or yellow.
The presence of transition metals with unfilled electron shells (e.g.
V, Cr, Mn, Fe, Co, Ni, and Cu) in oxide and silicate minerals causes
them to be opaque or strongly colored so that the streak, the mark
that they leave when scratched on a white ceramic tile, will also
be strongly colored.
fracture, and parting
Because bonding is not of equal strength in all directions in most
crystals, they will tend to break along crystallographic directions
giving them a fracture property that reflects the underlying structure
and is frequently diagnostic. A perfect cleavage results in regular
flat faces resembling growth faces such as in mica, or calcite. A
less well developed cleavage is said to be imperfect, or if very weak,
a parting. If a fracture is irregular and results in a rough surface,
it is hackly. If the irregular fracture propagates as a single surface
resulting in a shiny surface as in glass, the fracture is said to
It is the ability of a mineral to deform plastically under stress.
Minerals may be brittle, that is, they do not deform, but rather fracture,
under stress as do most silicates and oxides. They may be sectile,
or be able to deform so that they can be cut with a knife. Or, they
may be ductile and deform readily under stress as does gold.
It is a well-defined physical property measured in g/cm3. Most silicates
of light element have densities in the range 2.6 to 3.5. Sulfides
are typically 5 to 6. Iron metal about 8, lead about 13, gold about
19, and osmium, the densest substance, and a native element mineral,
is 22. Density may be measured by measuring the volume, usually by
displacing water in a graduated cylinder, and the mass. Specific gravity
is very similar to density, but is a dimensionless quantity and is
measured in a slightly different way.
gravity is measured by determining the weight in air (Wa) and the
weight in water (Ww) and computing specific gravity from SG = Wa
It is usually tested by seeing if some standard minerals are able
to scratch others. A standard scale was developed by Friedrich Mohs
The standard minerals making up the Mohs scale of hardness are:
This scale is approximately linear up to corundum, but diamond is
approximately 5 times harder than corundum.
A few minerals may have easily tested unique properties that may
greatly aid identification. For example, halite (NaCl) (common table
salt) and sylvite (KCl) are very similar in most of their physical
properties, but have a distinctly different taste on the tongue,
with sylvite having a more bitter taste.
Another unique property that can be used to distinguish between
otherwise similar back opaque minerals is magnetism. For example,
magnetite (Fe3O4), ilmenite (FeTiO3), and pyrolusite (MnO2) are
all dense, black, opaque minerals which can easily be distinguished
by testing the magnetism with a magnet.
is strongly magnetic and can be permanently magnetized to form a
lodestone; ilmenite is weakly magnetic; and pyrolusite is not magnetic
There are numerous other properties that are diagnostic of minerals,
but which generally require more sophisticated devices to measure
or detect. For example, minerals containing the elements U or Th
are radioactive (although generally not dangerously so), and this
radioactivity can be easily detected with a Geiger counter. Examples
of radioactive minerals are uraninite (UO2), thorite (ThSiO4), and
carnotite (K2(UO2)(VO4)2 rH2O). Some minerals may also be fluorescent
under ultraviolet light, that is they absorb UV light and emit in
Other optical properties such as index of refraction and pleochroism
(differential light absorption) require an optical microscope to
measure. Electrical conductivity is an important physical property
but requires an impedance bridge to measure. In general native metals
are good conductors, sulfides of transition metals are semi-conductors,
whereas most oxygen-bearing min erals (i.e., silicates, carbonates,
oxides, etc.) are insulators. Additionally, quartz (SiO2) is piezoelectric
(develops an electrical charge at opposite end under an applied
mechanical stress); and tourmaline is pyroelectric (develops an
electrical charge at opposite end under an applied thermal gradient).
are classified on their chemistry, particularly on the anionic element
or polyanionic group of elements that occur in the mineral. An anion
is a negatively charge atom, and a polyanion is a strongly bound group
of atoms consisting of a cation plus several anions (typically oxygen)
that has a net negative charge. For example carbonate, (CO3) 2-, silicate,
(SiO4)4- are common poly anions. This classification has been successful
because minerals rarely contain more than one anion or polyanion,
whereas they typically contain several different cations.
The first group of minerals is the native elements, and as pure elements,
these minerals contain no anion or polyanion. Native elements such
as gold (Au), silver (Ag), copper (Cu), and platinum (Pt) are metals,
graphite is a semi-metal, and diamond (C) is an insulator.
The sulfides contain sulfur (S) as the major "anion". Although
sulfides should not be considered ionic, the sulfide minerals rarely
contain oxygen, so these minerals form a chemically distinct group.
Examples are pyrite (FeS2), sphalerite (ZnS), and galena (PbS). Minerals
containing the elements As, Se, and Te as "anions" are also
included in this group.
The halides contain the halogen elements (F, Cl, Br, and I) as the
dominant anion. These minerals are ionically bonded and typically
contain cations of alkali and alkaline earth elements (Na, K, and
Ca). Familiar examples are halite (NaCl) (rock salt) and fluorite
The oxide minerals contain various cations (not associated with a
polyanion) and oxygen. Examples are hematite (Fe2O3) and magnetite
The oxide minerals contain various cations (not associated with a
polyanion) and oxygen. Examples are hematite (Fe2O3) and magnetite
The carbonates contain CO32- as the dominant polyanion in which C4+
is surrounded by three O2- anions in a planar triangular arrangement.
A familiar example is calcite (CaCO3). Because NO3- shares this geometry,
the nitrate minerals such as soda niter (nitratite) (NaNO3) are included
in this group.
These minerals contain SO42- as the major polyanion in which S6+ is
surrounded by four oxygen atoms in a tetrahedron. Note that this group
is distinct from sulfides which contain no O. A familiar example is
gypsum (CaSO4 2H2O).
The phosphates contain tetrahedral PO43- groups as the dominant polyanion.
A common example is apatite (Ca5(PO4)3(OH)) a principal component
of bones and teeth. The other trivalent tetrahedral polyanions, arsenate
AsO43-, and vanadate VO43- are structurally and chemically similar
and are included in this group.
The borates contain triangular BO33- or tetrahedral BO45-, and commonly
both coordinations may occur in the same mineral. A common example
is borax, (Na2BIII2BIV2O5(OH)4 8H2O).
This group of minerals contains SiO44- as the dominant polyanion.
In these minerals the Si4+ cation is always surrounded by 4 oxygens
in the form of a tetrahedron. Because Si and O are the most abundant
elements in the Earth, this is the largest group of minerals and is
divided into subgroups based on the degree of polymerization of the
These minerals contain isolated SiO44- polyanionic groups in which
the oxygens of the polyanion are bound to one Si atom only, i.e.,
they are not polymerized. Examples are forsterite (Mg-olivine, Mg2SiO4),
and pyrope (Mg-garnet, Mg3Al2Si3O12).
These minerals contain double silicate tetrahedra in which one of
the oxygens is shared with an adjacent tetrahedron, so that the polyanion
has formula (Si2O7)6-. An example is epidote (Ca2Al2FeO(OH)SiO4 Si2O7),
a mineral common in metamorphic rocks.
These minerals contain typically six-membered rings of silicate tetrahedra
with formula. (Si6O17)10-. An example is tourmaline.
These minerals contain SiO4 polyhedra that are polymerized in one
direction to form chains. They may be single chains, so that of
the four oxygen coordinating the Si atom, two are shared with adjacent
tetrahedra to form an infinite chain with formula (SiO3)2-. The
single chain silicates include the pyroxene and pyroxenoid minerals
which are common constituents of igneous rocks. Or they may form
double chains with formula (Si4O11)8-, as in the amphibole minerals,
which are common in metamorphic rocks.
These minerals contain SiO 4 polyhedra that are polymerized in two
dimensions to form sheets with formula (Si4O10)4-. Common examples
are the micas in which the cleavage reflects the sheet structure
of the mineral.
These minerals contain SiO4 polyhedra that are polymerized in three
dimensions to form a framework with formula (SiO2) 0. Common examples
are quartz (SiO2) and the feldspars (NaAlSi3O8) which are the most
abundant minerals in the Earth's crust. In the feldspars Al3+ may
substitute for Si4+ in the tetrahedra, and the resulting charge
imbalance is compensated by an alkali cation (Na or K) in interstices
in the framework.