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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
definitions. 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 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. |
Minerals
characteristic of high grade metamorphic environments include sillimanite,
kyanite, staurolite, epidote, and amphiboles. Sedimentary minerals. 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. |
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, 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 at depth. |
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| The 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 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. 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.
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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 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. |
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. Tenacity 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. Density 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. |
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). |
Magnetite
is strongly magnetic and can be permanently magnetized to form a
lodestone; ilmenite is weakly magnetic; and pyrolusite is not magnetic
at all. |
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| 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. 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. 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. 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 elements (Na, K, and Ca). Familiar examples are halite (NaCl) (rock salt) and fluorite (CaF2). |
Oxides The oxide minerals contain various cations (not associated with a polyanion) and oxygen. Examples are hematite (Fe2O3) and magnetite (Fe3O4). Hydroxides The oxide minerals contain various cations (not associated with a polyanion) and oxygen. Examples are hematite (Fe2O3) and magnetite (Fe3O4). Carbonates 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. 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). 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. |
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). 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. 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). 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. Cyclosilicates These minerals contain typically six-membered rings of silicate tetrahedra with formula. (Si6O17)10-. An example is tourmaline. |
Chain
silicates
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