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A mineral of variable composition which is also known as lithium mica and lithionite, K<sub>2</sub>(Li,Al)<sub>5−6</sub>- (Si<sub>6−7</sub>,Al<sub>2−1</sub>)O<sub>20−21</sub>(F,OH)<sub>3−4</sub>. Rubidium (Rb) and cesium (Cs) may replace potassium (K); small amounts of Mn, Mg, Fe(II), and Fe(III) normally are present; and the OH/F ratio varies considerably. Polithionite is a silicon- and lithium-rich, and thus aluminum-poor, variety of lepidolite.
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A mineral of variable composition: in its purest state, NaAlSiO<sub>4</sub>; often nearly Na<sub>3</sub>K(AlSiO<sub>4</sub>)<sub>4</sub>; but generally (Na,K,□,Ca,Mg,Fe<sup>2+</sup>,Mn,Ti)<sub>8</sub>(Al,Si,Fe<sup>3+</sup>)<sub>16</sub>O<sub>32</sub>, where □ represents vacant crystallographic sites, and Ca, Mg, Fe<sup>2+</sup>, Mn, Ti, and Fe<sup>3+</sup> are usually present in only minor or trace amounts. The most important variations in nepheline composition are due to crystalline solution of KAlSiO<sub>4</sub> (the mineral kalsilite), and substitution of □ for K. Crystalline solution of KAlSiO<sub>4</sub> in (Na,K)AlSiO<sub>4</sub> nepheline is limited to about 25 mole % at 25°C (77°F), but increases with increasing temperature (<i>T</i>) to 65–70 mole % at 1000°C (1832°F). Substitution of □ for K can be viewed and treated as crystalline solution of alkali feldspar or (alternatively) “excess silica.” The amount of “excess silica” that can be present stably in any nepheline is very small at <i>T</i> < 500°C (932°F), and only a few mole % can be dissolved in K-rich nephelines at any temperature. However, crystalline solution of “excess silica” increases steadily in Na-rich nephelines at <i>T</i> > 500°C (932°F), and at 1000°C (1832°F) as much as 30 mole % can be dissolved in K-free nepheline.
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A mineral or other material that has sufficient beauty for use as personal adornment and has the durability to make this feasible. With the exception of a few materials of organic origin, such as pearl, amber, coral, and jet, and inorganic substances of variable composition, such as natural glass, gems are lovely varieties of minerals.
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A mineral series in the apatite group, or in the larger grouping of phosphate, arsenate, and vanadate-type minerals. In this series lead (Pb) substitutes for calcium (Ca) of the apatite formula Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>(F,OH,Cl), and little fluorine (F) or hydroxide (OH) is present. Substitution between phosphorus (P) and arsenic (As) gives rise to a complete mineral series with pyromorphite, Pb<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl, and mimetite, Pb<sub>5</sub>(AsO<sub>4</sub>)Cl, as pure end members. Substitution between As and vanadium (V) produces another series between mimetite and vanadinite, Pb<sub>5</sub>(VO<sub>4</sub>)<sub>3</sub>Cl.
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A mineral sorosilicate having the composition Zn<sub>4</sub>Si<sub>2</sub>O<sub>7</sub>(OH)<sub>2</sub> · H<sub>2</sub>O; an ore of zinc. It crystallizes in the orthorhombic system, pyramidal class, and thus the prismatic crystals have different forms at top and bottom (see <b>illus.</b>). Also, as a result of the symmetry, the vertical axis is polar and crystals display the properties of pyroelectricity and piezoelectricity. There is perfect prismatic cleavage. Botryoidal and staclactic aggregates, showing a crystalline surface and frequently impure, are common. Crystals are usually colorless and the aggregates white, but in some cases there are faint shades of green, yellow, and blue. The mineral has a vitreous luster, a hardness of 4½ to 5 on Mohs scale, and a specific gravity of 3.45. Hemimorphite frequently resembles the zinc carbonate, or smithsonite, but can be distinguished by the reaction with hydrochloric acid. Hemimorphite is slowly soluble but smithsonite effervesces in cold acid.
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A mineral with a framework structure in which all the aluminosilicate tetrahedral vertices are linked, thus allying it to the feldspars, feldspathoids, and zeolites. The structure is cubic, space group Ia3d, a = 1.371 mm. Its formula is Na(H<sub>2</sub>O)(AlSi<sub>2</sub>O<sub>6</sub>); in this sense it is a tectosilicate.
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A mineral with chemical composition (Fe,Mn)WO<sub>4</sub>, intermediate between ferberite, the iron tungstate, and huebnerite, the manganese tungstate, which form a complete solid solution series.
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A mineral with chemical composition BaSO<sub>4</sub>. It is isostructural with celestite, SrSO<sub>4</sub>, and anglesite, PbSO<sub>4</sub>. Barite is orthorhombic, space group Pnma, <i>a</i> = 0.887 nanometer, <i>b</i> = 0.545 nm, <i>c</i> = 0.714 nm. The <b>illustration</b> shows typical crystals. Barite possesses one perfect cleavage, and two good cleavages as do the isostructural minerals. Anhydrite, CaSO<sub>4</sub>, although also orthorhombic, possesses a different space group and a different structure. Solid solution in the series BaSO<sub>4</sub>–SrSO<sub>4</sub> occasionally occurs in the natural environments. Solid solution in BaSO<sub>4</sub>–PbSO<sub>4</sub> is uncommon, although up to 30 mole % PbSO<sub>4</sub> in barite has been reported. Infrequency of natural solid solution between these two end members may stem from the different origins of the two minerals, anglesite being an oxidation and weathering product of galena ores. Since Ba<sup>2+</sup> has the largest ionic radius of the isostructural series with the ionic radius of Ba<sup>2+</sup> ≈ Pb<sup>2+</sup> > Sr<sup>2+</sup> ≫ Ca<sup>2+</sup>, separation of these species may be a consequence of this geometrical distinction. Barite can be distinguished from celestite and anhydrite by its superior specific gravity of approximately 4.5. The mineral is relatively soft, approximately 3 on Mohs scale. The color ranges through white to yellowish, gray, pale blue, or brown, and a thin section is colorless. Barite is often an accessory mineral in hydrothermal vein systems, but frequently occurs as concretions or cavity fillings in sedimentary limestones. It also occurs as a residual product of limestone weathering and in hot spring deposits. It occasionally occurs as extensive beds in evaporite deposits.
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A mineral with chemical composition Na<sub>3</sub>AlF<sub>6</sub>. Although it crystallizes in the monoclinic system, cryolite has three directions of parting at nearly 90°, giving it a pseudocubic aspect. Hardness is 2½ on Mohs scale and the specific gravity is 2.95. Crystals are usually snow-white but may be colorless and more rarely brownish, reddish, or even black (see <b>illus.</b>). The mean refraction index is 1.338, approximately that of water, and thus fragments become invisible when immersed in water. Cryolite, associated with siderite, galena, and chalcopyrite, was discovered at Ivigtut, Greenland, in 1794. This locality remains the only important occurrence.
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A mineral with composition AgCl (see <b>illus.</b>). Its structure is that of the isometric NaCl type, but well-formed cubic crystals are rare. The hardness is 2.5 on Mohs scale and specific gravity 5.5. Cerargyrite is colorless to pearl-gray but darkens to violet-brown on exposure to light. It is perfectly sectile and can be cut with a knife like horn; hence the name horn silver. Bromyrite, AgBr, is physically indistinguishable from cerargyrite and the two minerals form a complete series. Both minerals are secondary ores of silver and occur in the oxidized zone of silver deposits.
Industry:Science
