As the gas of these primitive elements expanded, gravitational instabilities caused parts to co alesce into huge clouds that eventually became galaxies and clusters of galaxies. Further, gravitational instabilities within each galaxy caused further collapse of the gas into primitive stars where the very high temperatures caused by gravitational collapse ignited the fires of thermonuclear fusion in which the nuclei of the primitive light elements combined to form heavier elements. In the largest of these stars, the fusion reactions proceeded in stages producing successively heavier elements. The final reaction in which Si combines to form Fe proceeds so rapidly once ignited that the star explodes in what we call a supernova. Our own solar system coalesced from the remnants of one or more of these supernova explosions.
As the gas that formed our solar system collapsed from gravitation it formed a rotating disk of gas that first heated and then cooled. As the gas cooled the heavy elements began to precipitate solid particles of dust. The first precipitates were crystals (minerals) of platinum-group metals, Os, Ru, and Ir, followed by aluminum oxides, metallic nickel-iron, and Mg silicates. This was followed by more complex silicates, then various sulfides of heavy metals. We can see this precipitation sequence preserved in primitive meteorites. Before temperatures cooled sufficiently in the inner solar system so that the most volatile elements (H, C, N, and the noble gases) could condense, H fusion in the sun ignited and blew these elements to the outer solar system where they are enriched in the outer planets, Jupiter, Saturn, Uranus, and Neptune.
The Earth accreted from these early solid particles called chondrules, and these refractory elements are enriched in the Earth relative to their abundance in the Sun. As the proto Earth grew from the influx of solid particles it got hot enough to melt so that the dense Ni-Fe metal together with elements soluble in the metal sank to the center and formed the core, and the lighter oxygen-bearing minerals (mostly silicates) formed the mantle. Today the mantle is entirely solid and has been throughout most of Earth's history, whereas the core comprises a liquid metal outer core and a solid metal inner core.
So, except for the oceans and atmosphere, the Earth today is made up of solid minerals to a depth of about 2900 km. The physics and chemistry of the solid phases of the Earth control much of the physics and chemistry of our environment. Unlike fluids, minerals preserve the records of Earth's history. Further minerals contain the wealth of natural resources of the planet. Therefore understand ing the physics and chemistry of the solid materials of the planet (mineralogy) is central to much of the Earth Sciences.
Let's look at the five parts of this definition:
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 ionic 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, crystalline phase".
Chemists have a precise definition of a phase:
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 system.1.2. Definition of a Mineral
A mineral is a naturally-occurring, homogeneous solid with a definite, but generally
not fixed, chemical composition and an ordered atomic arrangement. It is usually
formed by inorganic processes.
In contrast to the isomorphous series, it is also common for a single compound (composition) to occur with different crystal structures. 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. We will return in a few weeks to further discussion of stoichiometry and stability. The fourth part of our definition of a mineral, the part about the ordered atomic arrangement, leads us to a discussion of symmetry which will occupy our first few weeks.
1. Crystal form and habit (shape).
2. Luster and transparency
3. Color and streak.
4. Cleavage, fracture, and parting.
5. Tenacity
6. Density
7. Hardness
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.
Crystals forming parallel fibers are fibrous, and crystals forming branching,
tree-like growths are dendritic.
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.
Magnetite is strongly magnetic and can be permanently magnetized to form a
lodestone; ilmenite is weakly magnetic; and pyrolusite is not magnetic at all.
1.3. Mineral Properties in Hand Specimen
Learning to recognize hand specimens of approximately 100 of the most common rock-forming
minerals is an important part of this course. This recognition 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:1.3.1. Crystal form and habit.
Recognizing crystal
forms (a crystal face plus its symmetry equivalents) in the various crystal systems
is one of the reasons we spend some time in lab studying block models. The crystal
faces developed on a 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 structure.
Quartz 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.1.3.2. 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. 1.3.3. Color and streak.
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. 1.3.4. Cleavage, 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 be
conchoidal. 1.3.5. Tenacity
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. 1.3.6. Density
is a well-defined physical property measured in g/cm3.a
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. Specific gravity is measured by determining
the weight in air (Wa) and the weight in water (Ww) and computing specific gravity
from SG = Wa / (Wa-Ww). In practice this is done using a Jolly balance as we will see
in lab.1.3.7. Hardness
is usually tested by seeing
if some standard minerals are able to scratch others. A
standard scale was developed by Friedrich Mohs in 1812 The standard
minerals making up the Mohs
scale of hardness are:
1. Talc 6. Orthoclase
2. Gypsum 7. Quartz
3. Calcite 8. Topaz
4. Fluorite 9. Corundum
5. Apatite 10 Diamond
This scale is approximately linear up to corundum, but diamond is
approximately 5 times harder than corundum.1.3.8. Unique Properties.
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.
Whereas it is not recommended that students routinely taste mineral specimens (some
are toxic), taste can be used to distinguish between these two common minerals.1.3.9. Other Properties
. 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 lighta
and emit in the visible. (There is a display of fluorescent mineral on the first floor
of the (old)Geology Building.) Other optical properties such as index of refraction and
pleochroism (differential light absorption) require an optical microscope to measure and
are the subject of a major section of this course. 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).
1.4. Mineral Occurrences and Environments
In addition to physical properties, one of the most diagnostic features of a mineral
is the geologi cal 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 have developed
a very rough classification of geological environments,
most of which can be visited locally. 1.4.1. Igneous Minerals.
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
(BIR) 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 (SIR) 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 (PEG) 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.1.4.2. 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 (LGM) (temperatures of 60 º to 400 º C
and pressures << .5 GPa (=15km depth) and high-grade meta morphic (HGM)
(temperatures > 400 º and/or pressures > .5GPa). Minerals characteristic of low-
grade metamorphic environments include the zeolites, chlorites, and andalusite.
Minerals character istic of high grade metamorphic environments include sillimanite,
kyanite, staurolite, epidote, and amphiboles. 1.4.3. Sedimentary minerals.
Minerals in sedimentary rocks are either stable
in low-tempera ture 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, I would classify sedimentary minerals into detrital sediments (DSD) and
evaporites (EVP). 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.1.4.4. Hydrothermal 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 (HTH), low temperature hydrothermal (LTH), and oxydized
hydrothermal (OXH). 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 at depth and are very common
in Colorado.
1.5. Classification of Minerals
Minerals 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. 1.5.1. Native elements.
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. 1.5.2. Sulfides.
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.1.5.3. Halides.
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 ele ments (Na, K, and Ca). Familiar examples are halite
(NaCl) (rock salt) and fluorite (CaF2). 1.5.4. Oxides.
The oxide minerals contain various cations (not associated
with a polyanion) and oxygen. Examples are hematite (Fe2O3)
and magnetite (Fe3O4).1.5.5. Hydroxides.
These minerals contain the polyanion OH- as the dominant
anionic species. Examples include brucite (Mg(OH)2) and gibbsite
(Al(OH)3).1.5.6. Carbonates.
The carbonates contain CO32- as the
dominant polyanion in which C4+ is sur rounded 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.1.5.7. Sulfates.
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).1.5.8. Phosphates.
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.1.5.9. Borates.
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). 1.5.6. Silicates.
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 SiO4 tetrahedra.1.5.6.1. Orthosilicates.
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).1.5.6.2. Sorosilcates.
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.1.5.6.3. Cyclosilicates.
These minerals contain typically six-membered rings
of silicate tetrahedra with formula. (Si6O17)10-.
An example is tourmaline.1.5.6.4. Chain silicates.
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.1.5.6.5. Sheet silicates.
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.1.5.6.6. Framework silicates.
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.
M. Fleischer (1986) Glossary of Mineral Species (5th edition). (Mineralogical Record, Tucson) 202p.
E.H. Nickel and M.C. Nichols (1991) Mineral Reference Manual. Van Nostrand, Reinhold, New York, 250p.
W.L. Roberts, T.J. Campbell and G.R. Rapp, Jr. (1990) Encyclopedia of Minerals (Second edition) Van Nostrand, Reinhold, New York, 979p.
W.A. Deer, R.A. Howie and J. Zussman (1962, 1974, 1980) Rock Forming Minerals (Seven Volumes) Longmans, London.
R.W.G. Wyckoff (1964) Crystal Structures. Wiley (New York) (Eight Volumes) (Vols 1, 2, 3 and 4 contain mineral structures).
T. Zoltai and J. M. Stout (1985) Mineralogy: Concepts and Principles. Burgess (Minneapolis). This text has good cover age of crystal structures, optical properties X-ray diffraction, and physical properties of minerals, but is somewhat difficult to read and understand at an introductory level.
C. Klein and C.S. Hurlbut, Jr. (1993) Manual of Mineralogy (After J.D. Dana, 21st edition). Wiley, (New York). This is a well written and clear presentation of most of the material presented in this course. It has recently been updated and thoroughly revised.
L.G. Berry, B. Mason, and R.V. Dietrich (1983) Mineralogy (Second Edition) Freeman (San Francisco). This is a revision of an older standard text and belabors morphological crystallography at the expense of structural crystallography and is not well integrated with modern geology.
W.H. Balckburn and W.H. Dennen (1988) Principles of Mineralogy . W.C. Brown
K. Frye (1974) Modern Mineralogy. Prentice-Hall.
Deer, Howie, Zussman (1992) An Introduction to the Rock Forming Minerals (Second Edition) London Longmans ISBN 0-582-30094-0. 696pp. This is a standard reference covering structure, chemistry, occurrences, and optical properties for the more common minerals.
Mineral Structures and Properties Data Base