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Aerial Gamma-Ray Surveys in Alaska
Joseph S. Duval
U.S. Geological Survey Open-File Report 01-128

Radioactive Rock-Forming Minerals

The following definitions were taken from the second edition of the Glossary of Geology.

adularia  A moderate to low-temperature mineral of the alkali feldspar group: KAlSi3O8.  It is weakly triclinic (formerly regarded as apparently monoclinic) and typically occurs in well-developed, usually transparent, and colorless to milky white (and often opalescent) pseudo-orthorombic crystals in fissures in crystalline schists, esp. in the region of the Swiss Alps.  Adularia displays pearly internal reflections and a fascinating variety of optical behavior between crossed nicols.  It typically has a relatively high content of barium.

alkali feldspar (a) A group of feldspars composed of mixtures, or mixed crystals, of potassium feldspar (Or, or KAlSi3O8) and sodium feldspar (Ab, or NaAlSi3O8) in any ratio; a group of feldspars containing alkali metals but little calcium. (b) A mineral of the alkali feldspar group, such as microcline, orthoclase, sanidine, adularia, albite, anorthoclase, and plagioclase in which the proportion of the An molecule is less than 20%.  Synonym: alkalic feldspar.

allanite  A monoclinic, cerium-bearing mineral of the epidote group: (Ce, Ca, Y)(Al, Fe)3(SiO4)3(OH).  It is typically an accessory mineral in igneous rocks (granite, syenite, diorite, pegmatite) and their metamorphic equivalents.  Synonyms: orthite; cerine; bucklandite; treanorite.

apatite  (a) A group of variously colored hexagonal minerals consisting of calcium phosphate together with fluorine, chlorine, hydroxyl, or carbonate in varying amounts and having the general formula: Ca5(PO4, CO3)3(F, OH, Cl).  Also, any mineral of the apatite group, such as fluorapatite, chlorapatite, hydroxylapatite, carbonate-apatite, and francolite; when not specified, the term usually refers to fluorapatite.  The apatite minerals occur as accessory minerals in almost all igneous rocks, in metamorphic rocks, in veins and other ore deposits; and most commonly as fine-grained and often impure masses as the chief constituent of phosphate rock and of most or all bones and teeth.  Synonym: calcium phosphate. (b) A group of hexagonal minerals having the general formula: A5(RO4)3(F, OH, Cl), where A = Ca, SR, or Pb, and R = P, As, V, or less commonly Si.  Examples include svabite, hedyphane, mimetite, pyromorphite, vanadite.

biotite (a) A widely distributed and important rock-forming mineral of the mica group: K(Mg, Fe+2)3(Al, Fe+3)Si3O10(OH)2.  It is generally black, dark brown, or dark green, and forms a constituent of crystalline rocks (either as an original crystal in igneous rocks of all kinds or a product of metamorphic origin in gneisses and shists) or a detrital constituent of sandstones and other sedimentary rocks.  Biotite is useful in the potassium-argon method of age determination. (b) A general term to designate all ferromagnesian micas.  Synonyms: black mica; iron mica; magnesia mica.

carnallite  A milk-white to reddish orthorhombic mineral: KMgCl36H2O.  It occurs as a saline residue and is a raw material of fertilizer manufacture in some European districts.

feldspar (a) A group of abundant rock-forming minerals of general formula: MAl(SiAl)3O8, where M = K, Na, Ca, Ba, Rb, Sr, and Fe.  Feldspars are the most widespread of any mineral group and constitute 60% of the Earth's crust; they occur as components of all kinds of rocks (crystalline shists, migmatites, gneisses, granites, most magmatic rocks) and as fissure minerals in clefts and druse minerals in cavities.  Feldspars are usually white or nearly white and clear and translucent (they have no color of their own but are frequently colored by impurities), have a hardness of 6 on the Mohs scale, frequently display twinning, exhibit monoclinic or triclinic symmetry, and possess good cleavage in two directions intersecting at 90o as in orthoclase and at about 80o as in plagioclase).  On decomposition, feldspars yield a large part of the clay of soil and also the mineral kaolinite. (b) A mineral of the feldspar group, such as alkali feldspar (orthoclase, microcline), plagioclase (albite, anorthite), and celsian.  Synonyms: felspar, felspath.

glauconite  (a) A dull-green earthy or granular mineral of the mica group: (K, Na)(Al, Fe+3, Mg)2(Al, Si)4O10(OH)2.  It has often been regarded as the iron-rich analogue of illite.  Glauconite occurs abundantly in greensand, and seems to be forming in the marine environment at the present time; it is the most common sedimentary (diagenetic) iron silicate and is found in marine sedimentary rocks from the Cambrian to the present.  Glauconite is an indicator of very slow sedimentation.  (b) A name applied to a group of green minerals consisting of hydrous silicates of potassium and iron.

kainite  A usually whitish monoclinic mineral: MgSO4Kcl3H2O.  It is a natural salt occurring in irregular granular masses, and is used as a source of potassium and magnesium compounds.

leucite  A white or gray mineral of the feldspathoid group: KAlSi2O6.  It is an important rock-forming mineral in alkalic rocks (esp. lavas), and usually occurs in trapezohedral crystals with a glassy fracture.  Synonyms: amphigene; grenatite; white garnet; Vesuvian garnet; vesuvian.

microcline  A clear, white to gray, brick-red, or green mineral of the alkali feldspar group: KAlSi3O8.  It is the fully ordered, triclinic modification of potassium feldspar and is dimorphous with orthoclase, being stable at lower temperatures; it usually contains some sodium in minor amounts.  Microcline is a common rock-forming mineral of granitic rocks and pegmatites, and is often secondary after othoclase.  It is generally characterized by cross-hatch twinning.

monazite  A yellow, brown, or reddish-brown monoclinic mineral: (Ce, La, Nd, Th)(PO4, SiO4).  It is a rare-earth phosphate with appreciable substitution of thorium for rare earths and silicon for phosphorous; thorium-free monazite is rare.  It is widely disseminated as an accessory mineral in granites, gneisses, and pegmatites, and it is often naturally concentrated in detrital sand, gravel, and alluvial tin deposits.  Monazite is a principal ore of the rare earths and the main source of thorium.  Synonym: cryptolite.

muscovite  (a) A mineral of the mica group: KAl2(AlSi3)O10(OH)2.  It is colorless to yellowish or pale brown, and is a common mineral in gneisses and schists, in most acid igneous rocks (such as granites and pegmatites), and in many sedimentary rocks (esp. sandstone).  Synonyms: white mica; potash mica; common mica; Muscovy glass; mirror stone.

orthoclase  (a) A colorless, white, cream-yellow, flesh pink, or gray mineral of the alkali feldspar group: KAlSi3O8.  It is the partly ordered, monoclinic modification of potassium feldspar and is dimorphous with microcline, being stable at higher temperatures; it usually contains some sodium in minor amounts.  Ordinary or common orthoclase is a common rock-forming mineral; it occurs esp. in granites, acid igneous rocks, and crystalline schists, and is usually perthitic.  Synonyms: common feldspar, orthose, pegmatolite.  (b)  A general term applied to any potassium feldspar that is or appears to be monoclinic; e.g. sanidine, submicroscopically twinned microcline, adularia, submicroscopically twinned analbite.

pyrochlore  (a) A pale-yellow, red, brown, or black isometric mineral: (Na, Ca)2(Nb, Ta)2O6(OH, F).  It is isomorphous with microlite, with Nb greater than Ta, and it usually cerium and titanium.  Pyrochlore occurs in pegmatites derived from alkalic igneous rocks and constitutes an ore of niobium.  Synonym: pyrrhite.  (b) An group of minerals of the general formula: A2B2O6(O, OH, F), where A = Na, Ca, K, Fe+2, U+4, Sb+3, Pb, Th, Ce, or Y, and B = Nb, Ta, Ti, Sn, Fe+3, or W.  It includes minerals such as pyrochlore, microlite, betafite, obruchevite, and pandaite.

sanidine  A high-temperature mineral of the alkali feldspar group: KAlSi3O8.  It is a highly disordered monoclinic form, occurring in clear, glassy, often tabular crystals embedded in unaltered acid volcanic rocks such as trachyte; it appears to be stable under equilibrium conditions above approximately 500o C.  Sanidine forms a complete solid-solution series with high albite, and some sodium is always present.  Synonyms: glassy feldspar, ice spar, rhyacolite.

sericite  A white, fine-grained potassium mica occurring in small scales and flakes as an alteration product of various aluminosilicate minerals, having a silky luster, and found in various metamorphic rocks (esp. in schists and phyllites) or in the wall rocks, fault gouge, and vein fillings of many ore deposits.  It is usually muscovite or very close to muscovite in composition, and may also include much illite.

sphene  A usually yellow or brown mineral: CaTiSiO5.  It often contains other elements such as niobium, chromium, fluorine, sodium, iron, manganese, and yttrium.  Sphene occurs in wedge-shaped or lozenge-shaped monoclinic crystals as an accessory mineral in granitic rocks and in calcium-rich metamorphic rocks.  Synonyms: titanite, grothite.

sylvite  A white or colorless isometric mineral: KCl.  It is the principal ore mineral of potassium compounds.  Sylvite occurs in beds as a saline residue with halite and other evaporites.  It has a sharper taste than that of halite.  Synonyms: sylvine; leopoldite.

thorite  A brown, black, or sometimes orange-yellow tetragonal mineral: ThSiO4.  It is isostructural with thorogummite, strongly radioactive, and usually metamict, and may contain as much as 10% uranium.  Thorite resembles zircon and occurs as a minor accessory mineral of granites, syenites, and pegmatites.  It is dimorphous with huttonite.

uraninite  A black, brown, or steel-gray octahedral or cubic mineral, essentially UO2, but usually partly oxidized.  It is strongly radioactive, and is the chief ore of uranium.  It is isomorphous with thorianite.  Uraninite often contains impurities such as thorium, radium, the cerium and yttrium metals, and lead; when heated, it yields a gas consisting chiefly of helium.  It occurs in veins of lead, tin, and copper minerals and in sandstone deposits, and is a primary constituent of granites and pegmatites.  Synonyms: ulrichite; coracite.

xenotime  A brown, yellow or reddish tetragonal mineral: YPO4.  Is is isostructural with zircon, and often contains erbium, cerium, and other rare earths, as well as thorium, uranium, aluminum, calcium, beryllium, zirconium, or other elements.  Xenotime occurs as an accessory mineral in granites and pegmatites.

zircon  A mineral ZrSiO4.  It occurs in tetragonal prisms, has various colors, and is a common accessory mineral in siliceous igneous rocks, crystalline limestones, schists, and gneisses, in sedimentary rocks derived therefrom, and in beach and river placer deposits.  It is the chief ore of zirconium, and is used as a refractory; when cut and polished, the colorless varieties provide exceptionally brilliant gemstones.  Synonyms: zirconite, hyacinth, jacinth.

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