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Rocks and Minerals

Gems and Minerals: Glossary of terms

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Satin Spar Gypsum

Twinned Aragonite


Glossary of Terms
Used in These Pages

allochromatic  (al-lo-chro-ma-tic)  The color of a mineral that results from mineral impurity, such as minor chemical substitutions or radiation damage.  Allochromatic is the opposite of idiochromatic.

amber  (am'-ber)  A fossil resin from coniferous trees.  It is transparent, usually yellow or brown, and may enclose insects or other organisms.

amorphous  (a-mor'-phous)  The state of a solid lacking a crystal structure, specifically lacking long-range order.

andesite  (an-de-site)  A dark-colored to light gray, fine-grained extrusive igneous rock that typically formed through the relatively rapid crystallization of a lava.  Andesites can also form through the accumulation of pyroclastic material explosively erupted from a volcano.  Andesites are typically porphyritic or composed of two sizes of crystalline material. The larger crystals or phenocrysts are composed primarily of zoned sodic plagioclase (especially andesine) and one or more of the following mafic minerals, biotite, hornblende, pyroxene, and are surrounded by a groundmass consisting of  finer-grained minerals of the same composition as the phenocrysts but too small to be seen without a microscope.  Andesites contain more silica, aluminum, sodium, and potassium and less magnesium, iron, and calcium than basalts. If an andesitic magmas undergoes magmatic differentiation typically there will be an increase in Si, Al, Na, and K, and a decrease in Mg, Fe, and Ca, dacite and ultimately rhyolite lavas could be produced.  Andesite was named after the Andes Mountains of South America. 

anion  (an'-i-on) A negatively charged ion.

anisotropic  (an'-i-so-trop'-ic)  Having physical properties that vary in different directions.  Characteristic of all crystalline substances, including minerals, except those belonging in the isometric system, which are isotropic.

apatite  (ap'-a-tite)  1. A group of hexagonal minerals consisting of calcium phosphate together with florine, chlorine, hydroxyl, or carbonate substituting for the phosphate radical having the general formula Ca5 (PO4)3 (F,Cl,OH).  Apatite occurs as accessory minerals in igneous and metamorphic rocks, ore deposits; and most commonly as fine-grained and often impure masses as the chief constituent of phosphate rock.  A material similar to apatite makes up bones and teeth.

aragonite  (a-rag'-o-nite)  An orthorhombic mineral with the same composition as calcite (CaCO3), but stable at higher pressures.  It occurs in hot-spring deposits, in coral reefs,  in the nacre of pearls, and in the shells of some molluscs and brachiopods formed at or near atmospheric pressure.

asthenosphere  (as-then'-o-sphere)  The layer or shell of the Earth below the lithosphere (typically 70 to 100 km down to a depth of 400 km) that deforms ductilely due to its low yield strength, permitting viscous or plastic flow under relatively small stresses.  The asthenosphere is that part of the upper mantle in which isostatic adjustments take place, magmas are generated, and seismic waves are strongly attenuated (shortened or damped). 

atom  (at'-om)  The smallest possible unit of a chemical element, consisting of one or more protons and (except for hydrogen) two or more neutrons located in the nucleus, and one or more electrons which revolve in orbits around the nucleus.  Because the nucleus of each atom contains the same number of positively charged protons as negatively charged electrons, it is electrically neutral, or uncharged.  Atoms of the various elements differ in mass (weight), also expressed as the number of neutrons and protons, as well as in the number of electrons.  Atoms of a given element are identical, but an element may have atoms of slightly different masses, called isotopes.  Isotopes vary only in their number of neutrons.  Atoms of the same or different elements combine to form molecules; when the atoms are of two or more different elements these molecules are called compounds.  Atoms remain essentially unchanged in chemical reactions, except that some of the outermost electrons may be removed, shared or transferred, as occurs in ionization and chemical bonding.

bacteria  (bac-te'-ri-a)  Single-celled microorganisms that lack chlorophyll (the green plant pigment essential to photosynthesis) and an evident nucleus.  Most bacteria are capable of decomposing or breaking down organic matter.

basalt  (ba-salt')  A dark-colored mafic (having a composition rich in iron and magnesium) igneous rock, commonly extrusive but locally intrusive in dikes, composed chiefly of calcic plagioclase and clinopyroxene, although nepheline, olivine, orthopyroxene, or quartz may be present.  Basalt is the fine-grained equivalent of gabbro.  Basalts typically have low viscosity and therefore occur in lava flows.

biogenic  (bi-o-gen'-ic)  Formed by the action of biological organisms.


birefringence  ()  The magnitude of the difference in the index of refraction of birefringent minerals i.e. that is all minerals that are nonisotropic (All minerals that crystallize in the tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic crystal systems).

birefringent  ()  Having two or three refractive indices. Minerals that crystallize in the tetragonal and hexagonal crystal systems have two indices of refraction.  Minerals that crystallize in the orthorhombic, monoclinic, and triclinic crystal systems have three indices of refraction.

brachiopod  (bra'-chi-o-pod)  Any marine invertebrate belonging to the phylum Brachiopoda, and characterized by two bilaterally symmetrical (i.e.: both sides the same size and shape) valves.  Brachiopods most often attach themselves to a substrate, but may also be free.

breccia  (bre'-cchi-a)  A coarse-grained clastic rock (made of particals), composed of angular broken rock fragments held together by a mineral cement or in a fine-grained matrix.  Breccia differs from conglomerate in that the fragments have sharp edges and unworn corners. Breccia may originate as a result of talus accumulation, explosive igneous processes, collapse of rock material, or faulting. 

brilliance  (brilliance)  Degree to which a faceted mineral or gemstone sparkles and returns light from within.  Brilliance is dependent on the cup and refractive index of the mineral.

calcareous  NEED A DEFINITION.

calcite  (cal'-cite)  A common rock-forming hexagonal mineral (CaCO3) that has perfect rhombhedral cleavage and effervesces (bubbles) in cold, dilute HCl acid.


carbonatite  (car-bon'-a-tite)  A carbonate rock of of magmatic origin derived through partial melting of the upper mantle, generally associated with alkalic igneous rocks.  The carbonate material that makes up many carbonatites is in part recycled sedimentary rocks that have been returned into the mantle through subduction based on isotopic evidence.

cation  (cat'-i-on)  A positively charged ion. Si4+, Fe2+, Fe3+ and V4+ are all examples of cations.

cathodoluminescence  (ca-thod-o-lum-in-es-cence)  The emission of light by a mineral caused by electron bombardment (CL is a type of luminescence).  Cathodoluminescence can be observed directly using microscope-mounted systems or it can be recorded indirectly using electron microbeam instumentation.  Cathodoluminescence images of several minerals captured with a scanning electron microscope (SEM) are available at CL web

chatoyancy  (chatoyancy)  A cat's eye appearance to a polished mineral when illuminated.  Chatoyancy is caused by the parallel arrangement of tiny needles or prisms within a crystal.

cleavage  (cleav'-age)  The tendency of a mineral to break along specific crystallographic planes in all specimens because the presence of directions of weakness in the crystal due to fewer or weaker chemical bonds in those directions.  

coal  A combustible organic material containing more than 50% by weight and more than 70% by volume of carbonaceous (i.e.: rich in carbon) material, including inherent moisture.  Coal is formed through compaction and induration of plant remains similar to those in peat.

compaction  NEED A DEFINITION.

contact metamorphism  (con'-tact met'-a-mor-phism)  The metamorphic processes  by which rocks at or near their contact with a body of igneous rock are recrystallized or changed to other minerals due to increases in temperature and materials emanating from the magmas and by some deformation connected with the emplacement of the igneous magmas. 

consolidated  NEED A DEFINITION.


cryptocrystalline  (cryp'-to-crys-tal-line)  Composed of submicroscopic crystals.

crystal  (crys' tal)  A solid body having a regularly repeated long-range three dimensional internal arrangement of  atoms.  The external expression of a crystal may be bounded, although it does not have to be bounded, by natural planar surfaces called "faces".

crystalline  (cryst'-tal-line)  Having the properties of a crystal, specifically a regularly repeated long-range three dimensional internal arrangement of atoms.

crystallographic direction  NEED A DEFINITION.

cyclotron  (cy'-clo-tron)  An accelerator in which protons, deuterons, or ions are propelled by an alternating electric field within a constant magnetic field to study the nature of matter.

detrital sediments (de-tri'-tal sed'-i-ments) rock particles or mineral fragments derived through the mechanical disintegration of preexisting parent rocks either by erosion or by weathering.  Also called clastic sediments or detritus.  Detrital sediments are classified according to the size (boulder, cobble, sand, silt, and clay-sized) and nature of the material (lithic or rock fragments, quartz, feldspar, clay minerals, heavy oxide minerals, etc.). 

deuteron  (deu-te-ron')  The nucleus of the deuterium atom, consisting of one proton and one neutron, also referred to as heavy water.  The nucleus of the deuterium atom is the same as the isotope of hydrogen that has twice the mass of ordinary hydrogen.

diatom  (di'-a-tom)  A microscopic, unicellular (i.e.: single-celled) algae.  Diatoms are found almost everywhere, including marine, brackish, fresh water, soil, and ice environments - and even hot springs.  They secret siliceous frustules or exoskeletons in a great variety of forms.


dispersion  (dis-per'-sion)  The systematic variation of the index of refraction with color within a substance, due to the fact that white light is composed of a range of wavelengths.  Minerals such as diamond with high dispersion are said to have "fire" or the ability to split white light into its respective colors.

dolomite  (do'-lo-mite)  1:  A common rock forming hexagonal mineral (CaMg(CO3)2) with perfect rhombohedral cleavage.  Powdered dolomite effervesces (bubbles) in cold dilute HCl acid.   2:  A sedimentary rock, composed of more than 50% by weight of the mineral dolomite.  A dolomitic limestone (do-lo-mit'-ic lime-stone) is a limestone in which the mineral dolomite makes up 10-50% of the rock.

dolomitization  (do'-lo-mit'-i-za'-tion)  The process by which limestone is wholly or partly converted to dolomite rock, or dolomitic limestone, by the replacement of the original calcite by the mineral dolomite, usually through the action of magnesium-bearing water (either sea water or percolating meteoric water).

effervesce (ef-fer-vesce)  The bubbling produced when carbon dioxide (CO2)is liberated by the reaction of dilute HCl (hydrochloric acid) and calcite (CaCO3) and powdered dolomite (CaMg(CO3)2) or other carbonate minerals.

element  (el'-e-ment)  One of the 116 to 118 presently known fundamental substances that consist of atoms of only one kind, and that singly or in combination constitute all matter.  The most recently discovered elements are artificially produced by atomic collisions of atomic particles in cyclotrons, and exist for only fractions of a second before decaying to other elements.

electron  (e-lec'-tron)  A fundamental particle of matter that can exist either as part of an atom or in the free state.  It has a negative electric charge (4.8 x 10-10 e.s.u.) and a mass 1/1837th that of a proton (about 9.1 x 10-28 gram).  The number of electrons in an atom is equal to the number of protons in the nucleus (also known as the atomic number).  Depending on the position of an element in the Periodic Table, the electrons are arranged in from 1 to 7 orbits (called shells), in which they revolve around the nucleus.  The maximum number of electrons in any shell is precisely limited by the laws of physics.

evaporite  (e-vap'-o-rite)  A mineral or rock chemically precipitated directly from evaporating sea or lake water in arid and semiarid regions.  Gypsum, halite, and sylvite are evaporite minerals. 

feldspar  (feld'-spar)  The  monoclinic or triclinic minerals with the general formulae KAlSi3O8 - K,NaAlSi3 O8 and NaAlSi3 O8 - CaAl2Si2O8.  The feldspar mineral group contains two high-temperature series, the plagioclase series (albite, oligoclase, andesine, labradorite, bytownite, and anorthite) and the alkali feldspar  series (sanidine, anorthoclase, orthoclase, and microcline).  Barium feldspars such as celsian and hyalophane are realatively rare.  The feldspar minerals are colorless, white, pink, salmon-pink, light or dark gray and clear to translucent  and commonly twinned by one or more twin laws.  Feldspar group minerals have two directions of cleavage that intersect at or near 90o and a Mohs hardness of 6.  Feldspars group minerals compose 60% of the Earth's crust.   Feldspar occurs in all rock types and alters at atmospheric conditions to form clay minerals.

fire  Division of colors in a colorless, transparent gemstone as a result of dispersion.

fluorite  (fluor'-ite)  An isometric mineral, CaF2 with perfect octahedral cleavage that is typically
transparent to translucent and defines 4 on the Mohs hardness scale.  Fluorite occurs in hydrothermal veins 
as a gangue mineral, in carbonate rock, and is an accessory in igneous rocks. 

gangue (gangue)  A mineral without economic value that is part of an ore deposit.  Quartz, calcite, and fluorite are common gangue minerals.

glass  A state of matter intermediate between the close-packed, highly ordered array of atoms in a crystal and the poorly packed, highly disordered array of atoms in a gas.  Most glasses are supercooled liquids but there is no clear dividing point in the range of properties between the metastable and stable states.  The distinction between glass and liquid is on the basis of viscosity, or how much internal resistance the material offers to flow. 

granite  (gran'-ite)  1. A course-grained plutonic rock in which quartz constitutes 10% to 50% of the felsic components and in which the alkali feldspar/total feldspar ratio is generally restricted to the range of 65% to 90%.  Rocks in this range of composition are scarce, and sentiment has been growing to expand the definition to include rocks designated as adamellite or quartz monzonite, which are abundant in the United States. 2. Broadly applied, any holocrystalline (totally crystalline), quartz-bearing plutonic rock.  For more information on granite, see Rob's Granite Page

gypsum  (gyp'-sum)  A monoclinic mineral (CaSO4 .2H2O) that is colorless to white in crystals but gypsum in massive beds may range from red to yellow to brown, gray, or black.  It is the most common natural sulfate mineral.  Gypsum defines 2 on the Mohs hardness scale.  It is commonly associated with rock salt (halite) and anhydrite and forms beds and lenses interstratified with limestone, shale, and clay, especially in rocks of Permian to Triassic age.  Gypsum also occurs in volcanic fumarolic deposits and as an accessory mineral in metalliferous veins.

habit  (ha'-bit)  The characteristic shape of a mineral, either the shape of an individual crystal or the shape and style of polycrystalline intergrowths of the same mineral species.

HCl  Abbreviation for hydrochloric acid.

hydrothermal  (hy-dro-ther-mal) Of or pertaining to hot water, to the action of hot water, or to the 
products of this action, such as a mineral deposit precipitated from a hot aqueous solution, with or without demonstrable association with igneous processes; also, said of the solution itself.

hypothermal deposit  (hy-dro-ther-mal de-pos-it)  A mineral deposit that formed through the crystallization of minerals from the hot, ascending aqueous solutions typically but not always derived from or heated by a magma. 

hydrothermal solution  (hy-dro-ther-mal so-lu-tion)  Heated or hot magmatic emanations rich in water as well as heated aqueous solutions that are not demonstratively produced by or related to igneous magmas or activity.  Acidic or salt-carrying hydrothermal solutions are capable of dissolving silicate and ore minerals and carrying the dissolved metals and ions in solution.

hypothesis  (hy-poth'-e-sis)  A formulation of a natural principle based on inference from observed data that is tentatively assumed, and then tested for validity by comparison with observed facts and by experimentation.  A theory is less firmly founded and accepted by the scientific community than a theory.  Once a hypothesis has been tested and is generally accepted by the scientific community, it becomes a theory.

idiochromatic  (id'-i-o-chro-ma-tic)  Color is inherent and due to some aspect of the chemical composition and crystal structure.  Idiochromatic is the opposite of allochromatic.

igneous (ig'-ne-ous)  A rock or mineral that formed through the crystallization or solidified of molten or partly molten magma (molten rock).  Igneous is applied to processes leading to, related to, or resulting from the formation of such rocks.  Igneous rocks are one of the three main classes of rocks, the others being metamorphic and sedimentary.  

index of refraction  (in'-dex of re-frac'-tion)  The ratio of the velocity of light in a vacuum to the velocity of light within a substance.  The index of refraction is a function of temperature, pressure and wavelength of light.  Birefringent substances, that is, all substances that are anisotropic, have more than one index of refraction.  Isometric minerals and amorphous mineraloids are isotropic, that is, their properties are the same in all directions.  All other minerals and materials have optical properties that vary with crystallographic direction.

induration  (in'-dur-a-tion)  The hardening or lithifiation of a rock or rock material by heat, pressure, or the introduction of cementing material, especially the process by which relatively consolidated rock is made harder or more compact. 

inorganic  (in-or-gan'-ic)  Any chemical compound that does not contain the element carbon, with the exception of carbon dioxide, and compounds containing a carbonate radical (i.e. carbonate minerals with the CO3 radical).

invertebrate ()

ion  (i'-on)  An atom or radical that has lost or gained one or more electrons, and thus acquired an electric charge.  Cations are positively charged ions.  Anions are negatively charged ions.

island arc  A group of islands located on an oceanic plate that have a curving, arc-shaped pattern of distribution.  The islands in an island arc are the result of convergent plate volcanism over a subduction zone.

isometric  (i'-so-me-tric)  The crystal system characterized by three orthogonal axes of equal length.  Also known as cubic.

isotope (i'-so-tope) An atom of any particular element that has the same number of protons but differing numbers of neutrons in their nucleus.  Carbon isotopes, for example, all have six protons in their nuclei, but carbon-12 has six neutrons, carbon-13 has seven neutrons, and carbon-14 has eighteen neutrons.

jet  A dense, compact, black, low-grade coal or lignite that can be polished to a high shine.

lava  (la'-va)  An extrusive volcanic rock with sufficiently low viscosity that it can flow under the influence of gravity.  

limestone  (lime'-stone)  A sedimentary rock consisting primarily of the mineral calcite (hexagonal CaCO3) with or without some magnesium carbonate (MgCO3) or dolomite (CaMg(CO3)2).  Limestone is the most widely distributed of the carbonate rocks, and is the consolidated equivalent of limy mud, calcareous sand, and/or shell fragments.  Clay minerals, quartz, and chalcedony are common impurities.

lithification  (lith'-i-fic-ca-tion)  The process by which a newly deposited, unconsolidated sediment is changed into a coherent, solid rock through cementation, compaction, desiccation, and crystallization.  Lithification may occur concurrent with, soon after, or long after deposition. 

lithosphere  (lith'-o-sphere)  In plate tectonics, the solid outer layer of the Earth that deforms brittlely relative to the underlying ductilely-deforming asthenosphere.   The lithosphere includes the crust and part of the upper mantle and ranges in thickness from less than 1 km at the oceanic spreading ridges to about 100 km in mountainous continental areas. 

luster  (lus'-ter) The way in which a mineral reflects light from its surface.  Luster is a function of the smoothness of the mineral surface and its reflectivity.  Categories of luster include metallic, submetallic, and within nonmetallic; vitreous, adamantine, pearly, earthy, and dull.

magma  (mag'-ma)  Naturally occurring molten rock, generated within the Earth and capable of intrusion and extrusion, from which igneous rocks are derived through solidification and related crystallization processes.  Magma may or may not contain suspended solids (such as crystals and rock fragments) and/or gas phases.

metamorphic (met'-a-mor-phic)  A preexisting rock or mineral that undergoes mineralogical, chemical, and/or structural changes while in the solid state in response to marked changes in temperature, pressure, shearing stress, and chemical environment.  This generally occurs at depth in the Earth's crust during contact metamorphism, continental collision or subduction.  Metamorphic rocks are one of the three main classes of rocks, the others being igneous and sedimentary.

meteoric water  (me-te-or'-ic wa'-ter))  Ground water of atmospheric origin.  Meteoric water reaches the Earth's surface as precipitation (rain, snow, hail, or sleet).

mineral  (min'-er-al)  A naturally occurring, usually inorganic,  solid homogenous material with a definite chemical composition variable within fixed limits and a highly ordered atomic arrangement.  For a detailed explanation, see Just What Is a Mineral.

mineraloid  (min'-er-al-oid)  A naturally occurring, usually inorganic solid that is not considered to be a mineral because it is amorphous, that is, it lacks a long-range three dimensional ordering of its structure.

Mohs scale  A standard represented by ten mineral of varying hardnesses by which the hardness of other minerals may be rated.  From softest to hardest (numbered one to ten), the scale includes: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and diamond.

mollusc  (mol'-lusc)  A solitary invertebrate belonging to the phylum Mollusca, characterized by a nonsegmented body that is bilaterally symmetrical, and by a radially or biradially symmetrical mantle and shell.  Among the classes included in the molluscs are the gastropods (snails), bivalves (pelecypods), and cephalopods (squid, nautiluses, ammonites, and octopi).


neutron  (neu'-tron)  A fundamental particle of matter having almost the same mass as the proton, but without an electric charge.  It is a constituent of the nucleus of all elements except hydrogen.  See atom.

obsidian  (ob-sid'-i-an)  A black or dark-colored volcanic glass, usually of granitic or rhyolitic composition, characterized by conchoidal (shell-shaped) fracture, produced by quick cooling of lava on or near the Earth's surface.

opal  (o'-pal)  A transparent to nearly opaque mineral or mineraloid (SiO2.nH2O) consisting of packed spheres of silica and interstitial (i.e.: filling the spaces between the spheres) water (typically 3 to 9%).  Opal may have a marked play of colors in the case of precious opal, or opalescence (more of a diffuse glow) in the case of common opal.

ophiolite  (o'-phi-o-lite)  A stratigraphic section consisting of a suite of mafic and ultramafic igneous rocks and associated marine sediments and their metamorphic equivalents representing the oceanic crust and upper part of the mantle formed either at a oceanic spreading ridge or in a back arc basin behind a island arc.  A complete section consists of a basal peridotite representing the upper layer of oceanic mantle, layered ultramafic and mafic igneous rocks ranging in composition from peridotite through gabbro, a dike complex consisting of steeply inclined basic feeder dikes, basaltic lavas and pillow lavas (formed by submarine eruption of basaltic lavas), capped by a layer of chalcedony and other marine sediments.  This sequence of rocks is typically extensively altered to serpentine and other secondary minerals, and strongly tectonically deformed. 

optical properties  (op'-ti-cal prop'-er-ties)  A subset of the physical properties of a mineral that are a function of the interaction of light with mineral composition and atomic structure.  These properties include color, luster, index of refraction, dispersion, pleochroism, play of color, luminescence, and fluorescence.  For more information on this topic, see Science of Minerals.

organic  (or-gan'-ic)  Containing carbon compounds and produced by living organisms.

orthoclase  NEED A DEFINITION.

orthorhombic  NEED A DEFINITION.

oxidation state  (ox-i-da'-tion state) (equivalent to valence)  The number of electrons that must be added to or subtracted from an atom during chemical bonding to convert it to the elemental form.  The oxidation states of iron (Fe) and oxygen (O) in the mineral hematite (Fe2O3) are +3 and +2, respectively.  The formula of the mineral magnetite is usually written Fe3O4, but it is more informative to write Fe3+(Fe2+, Fe3+)2O4 to indicate that  iron in magnetite occurs in two oxidation states.  Many elements, especially the other transition metals, are similar to iron in that they have multiple oxidation states.  For instance, the element vanadium (V) occurs in four oxidation states: +2, +3, +4, and +5.  

pearl  A dense, variously colored, and usually lustrous concretion formed of concentric layers of nacre (nacre is  82-86% orthorhombic calcium carbonate (CaCO3) in a cryptocrystalline (microcrystalline) aggregate, 10-14% organic material, and 2% water).  Pearls occur as abnormal growths around sand grains or other irritants within the shell of some molluscs.  They are used as gems.

pegmatite (peg'-ma-tite)  A typically coarse-grained igneous rock formed through the crystallization of the very-last stages of magma typically granitic in composition that is enriched in water and other volatile elements (F, Cl) and, in some cases, highly incompatible elements (lithium, beryllium, boron, fluorine, niobium, tantalum, uranium, and the rare earth elements) that do not fit into the structures of minerals crystallized at higher temperatures and pressures.  Although most pegmatites are composed of the common minerals feldspar, quartz, muscovite, and biotite, pegmatites rich in incompatible elements may also contain rare minerals.  These incompatible elements make up a tiny fraction of the original magma, but because they are concentrated in the water-rich residual melt, they can occur in large enough quantities to produce large, well-formed crystals of beryl, chrysoberyl and euclase (beryllium), tourmaline (boron), topaz (fluorine), apatite (chlorine and fluorine), spodumene and lepidolite or lithium-rich pyroxene and mica, monazite (rare earth elements), tantalite (tantalum), and columbite (niobium).

periodic table of the elements  A listing of all known chemical elements by their atomic number and chemical behavior.  A periodic table is provided by the Los Alamos National Laboratory CST Division.

petroleum  (pe-tro'-le-um) A naturally occurring complex liquid hydrocarbon, that after distillation and removal of impurities, yields a range of combustible fuels, petrochemicals, and lubricants.

physical properties  (phys'-i-cal prop'-er-ties)  The characteristics of a mineral that are a direct result of their chemical composition and atomic structure, and that can be used to determine the identity of the mineral, including but not limited to crystal form and habit, cleavage, fracture, density, tenacity, color and streak (color of powdered mineral), luster, and magnetism. 

piezolectric  (pie-zo-lec-tric)  Capable of producing a surface electric charge when deformed elastically.  Only minerals that lack a center of symmetry may have this property.

play of colors  A flash of colors produced in a mineral such as precious opal  when it is viewed from different angles.  This phenomenon is due to optical diffraction.

pleochroism  (ple'-o-chro-sim)  The phenomenon in which the color of a mineral  varies or changes with orientation.

polycrystalline  (po-ly-crys'-tal-line)  An aggregate of crystals of the same mineral species.  

polymorph  (pol'-y-morph)  A chemical composition that can occur in more than one atomic or mineral structure.  Graphite and diamond (C), calcite and aragonite (CaCO3), andalusite and sillimanite and kyanite (Al2SiO5), low quartz,  high quartz, cristobalite, tridymite, coesite, and stishovite (SiO2) are all examples of polymorphs.

porphyry  (por'-phyry)  An igneous rock that contains larger crystals in a fine-grained groundmass or matrix typically indicating at least two rates of cooling and crystallization.

porphyry copper deposit   (por'-phyry cop'-per de-pos'-it)  A large body of rock, typically a porphyry (an igneous rocks that contains larger crystals in a fine-grained groundmass or matrix), that contains disseminated chalcopyrite and other sulfide minerals. Such deposits are mined in bulk on a large scale, generally in open pits, for copper and byproduct molybdenum. Most deposits are 3 to 8 km across, and of low grade (less than 1% Cu). They are always associated with intermediate to felsic hypabyssal porphyritic intrusive rocks. Distribution of sulfide minerals changes outward from dissemination to veinlets and veins. Supergene enrichment has been very important at most deposits, as without it the grade would be too low to permit mining. 

porphyry deposit  (por'-phyry de-pos'-it)  A deposit in which minerals of copper, molybdenum, gold, or less commonly tungsten and tin, are disseminated or occur in a number of small veinlets within a large mass of hydrothermally altered igneous rock. The host rock is commonly an intrusive porphyry, but other rocks intruded  by a porphyry can also be hosts for ore minerals.  

proton  (pro'-ton)  A fundamental particle of matter, having a mass of 1.67 x 10-24 gram and a positive electric charge.  Its mass is almost the same as that of a neutron, and 1837 times that of an electron.  Protons are constituents of all atomic nuclei, their number in each nucleus being the atomic number of the element (from 1 in hydrogen to 106 unihexium, through elements 116 and 118 discovered by particle physicists in 1999).

pyroelectric  (py'-ro-e-lec-tric)  Capable of producing a surface electric charge when temperature changes.  Only minerals that lack a center of symmetry have this property.

quartz  (quartz)  A hexagonal  mineral composed of SiO2 and polymorphous with tridymite,  cristobalite, coesite, stishovite, and keatite.  There are many varieties of quartz.  Amethyst is purple quartz resulting from the oxidation of Fe+3 to Fe+4 by irradiation by gamma rays. Aventurine is a quartz that includes tiny plates of  mica, hemitite, or other minerals.  False topaz or citrine is a yellow quartz.  Rock crystal is a watery clear variety.  Rose quartz is a pink variety. Rutilated quartz contains needles of rutile.  Smoky quartz is a brownish variety, sometimes called cairngorm.  Tigereye is crocidolite (an asbestisform mineral) replaced by quartz and iron oxide and having a chatoyant effect.

rare earth elements  A family of elements, consisting of scandium (atomic number 21), yttrium (atomic number 39), and the elements in the Lanthanides series, that chemically behave in similar ways because of the similarity in atomic structure, atomic radii and valances (3+ or 4+).  Lanthanum, (atomic number 57), cerium, praseodymium, niodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, through lutetium (atomic number 71) are the elements in the lanthanide series.

radical  (rad'-i-cal)  Negatively charged polyatomic (multiple) ions or molecular groups of ions that occur in many compounds, and which remain unchanged through chemical reactions.   Examples are the borate (BO3)-3, carbonate (CO3)-2, sulfate (SO4)-2, phosphate (PO4)-3, arsenate (AsO4)-3, vanadate (VO4)-3, and hydroxyl radicals (OH)-.

refraction  (re'-frac-tion)  The bending of light as it moves between materials with different indices of refraction.

refractive index  A mathematical constant equal to the ratio of the velocity of light in a vacuum (which is approximately equal to the velocityof light in air) to that in a substance or mineral.  The refractive index determines the angle at which light bends when it enters a substance at and oblique angle.

rhombohedral  NEED A DEFINITION.

rock  A well-consolidated or lithified assemblage of grains of one or more minerals.  Rocks are formed through igneous including hydrothermal, sedimentary, or metamorphic processes.

rhyolite  (rhy'-o-lite)  A fine-grained extrusive volcanic rock that has a composition roughly equivalent to the intrusive coarsely crystalline granite.  A group of extrusive igneous rocks, typically porphyritic (containing two or more sizes of crystals) and commonly exhibiting flow texture, with phenocrysts (large crystals) of quartz and alkali feldspar in a glassy to cryptocrystalline groundmass.  Most rhyolites form through the accumulation and compaction of volcanic material explosively ejected in a volcanic explosion.  Volcanic rocks formed in this manner whatever the composition are called tuffs or pyroclastic rocks.  Rhyolites very rarely make up extensive lava flows.  The extremely high silica content of a rhyolite magma results in very high viscosity that is resistant to flow.

schist  A strongly foliated (thinly layered) crystalline rock, formed through dynamic metamorphism, that has a well developed parallelism of the constituent minerals especially the micaceous or platy minerals.

sedimentary  (sedimentary)  A rock resulting from the consolidation of loose sediment that has accumulated in layers, a clastic rock (such as conglomerate, breccia, or  tillite) consisting of mechanically formed fragments of older rock transported from its source and deposited in water or from air or ice, or a chemical rock (such as rock salt or gypsum) formed by precipitation from evaporating sea water, or an organic rock (such as certain limestones) consisting of the remains or secretions of plants and animals.   Sedimentary rocks are one of the three main classes of rocks, the others being igneous and metamorphic.

shale  A fine-grained, detrital, sedimentary rock, formed by the compaction and lithification of clay, silt, or mud.  It has a finely laminated (composed of layers) structure, that gives it a fissility, or tendency to split along bedding planes.  Shale is well indurated, but not as hard as argillite or slate.


skarn  (skarn)  A metamorphic rock composed of silicate minerals produced through the metasomatic replacement of carbonate rocks (typically) in a the contact metamorphic aureole of an igneous intrusion.  The silicate rocks are produced through the metasomatic reaction of carbonate minerals and elements derived from or mobilized by the magma that typically is of intermediate composition.

specific gravity  (spe-cif'-ic grav'-i-ty)  A measure of density, equivalent to the ratio of the weight of a given volume of a substance to the weight of an equal volume of water.

subduction  (sub-duc'-tion)  The process of one oceanic lithospheric plate descending beneath another (usually continental) lithospheric plate.  

sulfide  (sul'-fide)  A mineral compound formed through the binding of sulfur and a metal, such as iron in pyrite (FeS), copper and iron in chalcopyrite (CuFeS2), and lead in galena (PbS).

sulfur  (sul'-fur)  An orthorhombic, nonmetallic mineral composed of the element sulfur (S).  Much of the sulfur formed at or near the Earth's surface is produced by bacteria.  Sulfur also crystallizes from magmas and around volcanic vents, from fumaroles (heated, sulfur-rich waters derived from magmatic activity), and hot springs.

supergene  (su-per-gene)  An ore enrichment process occurring near the Earth's surface typically produced by descending meteoric waters containing metal ions in solution.

supergene enrichment  (su-per-gene en-rich-ment)  A mineral deposition process in which near-surface oxidation of ore minerals produces acidic solutions that leach metals, carry them downward, and reprecipitate them, resulting in the enrichment of sulfide minerals already present. 

symmetry  (sym'-me-try)  The correspondence in shape or length of elements in a crystal, such as similar crystal faces, that indicates the ordered internal arrangement of a crystalline substance, as though repeated by a mirror, rotation about an axis, or inversion through a point (center of symmetry). 


tephra  (teph-ra)  The general term for airborne volcanic ejecta of any size.  Fragmental volcanic products between 0.25 cm to 6.35 cm (0.1 to 2.5 inches) in diameter are called lapilli.  Material finer than 0.1 inch is called ash.  Fragments larger than about 6.35 cm (2.5 inches) are called blocks if they were ejected as solids, and volcanic bombs if ejected as liquid or semi-solid lava.  In a major explosive eruption, most of the pyroclastic debris would consist of lapilli and ash.  Volcanic bombs are aerodynamically shaped by their passage through the atmosphere.  Cinders or scoria are lapilli- or bomb-size irregular fragments of lava containing abundarnt vesicles or gas cavities.  Pumice is a low density cinder riddled with gas vesicles that is light enough to float on water.

theory (the'-o-ry)  A scientifically accepted general principle or body of principles offered to explain natural phenomena that is consistent with evidence, data, and experimental results.  Theories can be disproved, but it is scientifically impossible to prove a theory correct.


transition element or metal  (trans-si'-tion el'-e-ment)  Any of a number of elements than can form bonds with the electrons from the outer two shells.  The transitions elements are all metals and that most of them are hard, strong, and lustrous, have high melting and boiling points, and are good conductors of heat and electricity.  Most of these elements have multiple oxidation states.  The transition elements include elements with atomic numbers 21 through 30 (scandium through zinc), 39 through 47 (yttrium through silver), 57 through 79 (lanthanum through gold) and occupy the middle portions of the long periods of the periodic table of elements in groups 3 through 12.  Transition elements that are common in minerals, or occur in significant amounts, are titanium, chromium, manganese, iron, cobalt, nickel, copper, molybdenum, silver, tungsten, gold, platinum, and mercury.  Yttrium, zirconium, niobium, lanthanum, hafnium, rhenium, and osmium are important industrially in semiconductors or for geologic dating. 

tuff (tuff)  A volcanic rock formed through the consolidation of pyroclastic fragments.  Tuffs are extrusive volcanic rocks that form through the accumulation and compaction of volcanic material ejected during a explosive volcanic eruption.  Most of this material consists of volcanic ash and glass shards.

twin  A rational intergrowth of two or more single crystals of the same mineral in a mathematically describable manner.  The symmetry of the two parts may be reflected about a common plane, axis or center.

valence  (va-lence) (equivalent to oxidation state)  An integer (whole number) that represents the power of one element to combine with another.  For example, if hydrogen (H) and chlorine (Cl) both have a valence of 1, oxygen (O) -2 and nitrogen (N) +3, the valence-balancing principle gives the formulas HCl (hydrogen chloride or hydrochloric acid), H2O (water), NH3 (ammonia), and HNO3 (nitric acid).  The subscript numbers indicate the relative numbers of atoms of each element.  Many elements have multiple valences; for example, the element vanadium (V) occurs in four valences, +2, +3, +4, and +5, and iron (Fe) occurs in two valences, +2 and +3.

volcano (vol-ca'-no)  A conical hill, low rounded mound, or rugged steep-sided mountain built around a vent that connects with reservoirs of molten rock below the surface of the Earth.  The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.  Volcanoes are composed of any of the following volcanic materials: lava (molten rock erupted on the Earth's surface), tephra (airborne volcanic material of any size including ash, lapilli, and volcanic bombs), cinders or scoria (lapilli- or bomb-size irregular fragments of lava containing abundarnt vesicles or gas cavities). Pumice is a low density cinder riddled with gas vesicles that is light enough to float on water.





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