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A mineral having the composition SnO<sub>2</sub>. Cassiterite is the principal ore of tin. Its crystals are of the tetragonal rutile structure type, usually in prisms terminated by dipyramids. Twins with a characteristic notch called visor tin are common. Cassiterite is usually massive granular, but may be in radiating fibrous aggregates with reniform shapes (wood tin). The hardness is 6–7 (Mohs scale), and the specific gravity is 6.8–7.1 (unusually high for a nonmetallic mineral). The luster is adamantine to submetallic. Pure tin oxide is white, but cassiterite is usually yellow, brown, or black because of the presence of iron.
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A mineral inosilicate with composition Ca<sub>2</sub>NaSi<sub>3</sub>O<sub>8</sub> (OH), crystallizing in the triclinic system. Crystals are usually acicular in radiating aggregates (see <b>illustration</b>). There is perfect cleavage parallel to the front and basal pinacoids yielding splintery fragments elongated on the <i>b</i> crystal axis. The hardness is 5 on Mohs scale, and the specific gravity is 2.75. The mineral is colorless, white, or gray with a vitreous to silky luster. Pectolite, a secondary mineral occurring in cavities in basalt and associated with zeolites, prehnite, apophyllite, and calcite, is found in the United States at Paterson, Bergen Hill, and Great Notch, New Jersey.
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A mineral inosilicate with composition CaSiO<sub>3</sub>. It crystallizes in the triclinic system in tabular crystals (see <b>illus.</b>). More commonly it is massive, or in cleavable to fibrous aggregates. There are two good cleavages parallel to the front and basal pinacoids yielding elongated cleavage fragments. Hardness is 5–5½ on Mohs scale; specific gravity is 2.85. On the cleavages the luster is pearly or silky; the color is white to gray. Wollastonite is the most common of three polymorphic forms of CaSiO<sub>3</sub>, the other two being pseudowollastonite and parawollastonite. Pseudowollastonite, a high-temperature triclinic form, is very uncommon in rocks but may be a constituent of synthetic CaO-SiO<sub>2</sub> systems and of slags and glasses. Parawollastonite, a monoclinic form, is only rarely found in Ca-rich rocks. Wollastonite, by far the most common polymorph, occurs abundantly in impure limestones that have undergone contact metamorphism. Resulting assemblages may consist of calcite-diopside-wollastonite with variable amounts of tremolite, clinozoisite, and grossularite. Wollastonite occurs sporadically in regionally metamorphosed calcareous sediments as well. It is found in large masses in the Black Forest of Germany; Brittany, France; Chiapas, Mexico; and Willsboro, New York, where it is mined as a ceramic material.
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A mineral inosilicate with composition MnSiO<sub>3</sub>. Rhodonite crystallizes in the triclinic system in crystals that are commonly tabular parallel to the base (see <b>illus.</b>). More often it is in cleavable to compact masses or in embedded grains. Crystallographically, rhodonite is closely related to the pyroxenes and thus has two cleavage directions at about 88 and 92°. Hardness is 5.5–6 on Mohs scale, and specific gravity is 3.4–3.7. The luster is vitreous and the color is rose red, pink, or brown. Rhodonite is similar in color to rhodochrosite, manganese carbonate, but it may be distinguished by its greater hardness and insolubility in hydrochloric acid. It has been found at Langban, Sweden; near Sverdlovsk in the Ural Mountains; and at Broken Hill, Australia. Fine crystals of a zinc-bearing variety, fowlerite, are found at Franklin, New Jersey.
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A mineral neosilicate, composition CaBSiO<sub>4</sub>(OH), crystallizing in the monoclinic system. It usually occurs in crystals showing many faces and having an equidimensional habit. It may also be fine granular or compact and massive. Hardness is 5–5½ on Mohs scale; specific gravity is 2.8–3.0. The luster is vitreous, the crystals colorless or white with a greenish tinge. Datolite is a secondary mineral found in cracks and cavities in basaltic lavas or similar rocks associated with zeolites, apophyllite, prehnite, and calcite. It is found in the Harz Mountains, Germany; Bologna, Italy; and Arendal, Norway. In the United States fine crystals have come from Westfield, Massachusetts; Bergen Hill, New Jersey; and various places in Connecticut. In Michigan, in the Lake Superior copper district, datolite occurs in fine-grained porcelainlike masses which may be coppery red because of inclusions of native copper.
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A mineral of composition CaF<sub>2</sub> and an important structure type with space group Fm3m, cell edge = 0.546 nm. The fluorite structure type is written <i>RX</i><sub>2</sub>, where the <i>R</i> cations are in cubic (eightfold) coordination by the <i>X</i> anions. Alternatively, the <i>X</i> atoms are tetrahedrally (fourfold) coordinated by the <i>R</i> atoms. The fluorite arrangement may occur when the ratio of <i>r</i>(<i>R</i>), the ionic radius of the <i>R</i> atoms, to <i>r</i>(<i>X</i>), the ionic radius of the <i>X</i> atoms, is equal to or greater than 0.73. The arrangement <i>XR</i><sub>2</sub> is termed the antifluorite arrangement. Here the anions are eight-coordinated and the cations are four-coordinated. Halides of larger divalent cations, oxides of larger quadrivalent (4+) cations, some intermetallic compounds, and <i>XR</i><sub>2</sub> alkali univalent oxides frequently belong to the fluorite structure type. Approximately 100 synthetic compounds are known to have this arrangement; distortions may result in similar arrangements but with lower symmetry, such as in ZrO<sub>2</sub>, which is monoclinic. A vast number of complex structures can be derived from the fluorite arrangement by ordering vacancies and different cations or anions over the fluorite atomic positions.
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A mineral of composition CuAl<sub>6</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>8</sub> · 5H<sub>2</sub>O in which considerable ferrous ion (Fe<sup>2+</sup>) may substitute for copper. Ferric ion (Fe<sup>3+</sup>) may also substitute for part or all of the aluminum (Al), forming a complete chemical series from turquoise to chalcosiderite (CuFe<sub>6</sub>(PO<sub>4</sub>)<sub>4</sub>(OH)<sub>8</sub> · 5H<sub>2</sub>O). Turquoise with a strong sky-blue or bluish-green to apple green color is easily recognized, and such material is commonly used as a gem. Some variscite, of composition AlPO<sub>4</sub> · 2H<sub>2</sub>O with minor chemical substitutions of Fe<sup>3+</sup> and or chromium ion (Cr<sup>3+</sup>) for aluminum and with a soft, clear green color, may be marketed as green turquoise.
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A mineral of composition KAl<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>. Alunite occurs in white to gray rhombohedral crystals or in fine-grained, compact masses. It has a hardness of 4 and a specific gravity of 2.6 - 2.8.
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A mineral of the feldspathoid group, crystallizing in the isometric system, with chemical composition Na<sub>4</sub>Al<sub>3</sub>Si<sub>3</sub>O<sub>12</sub>Cl. Its tektosilicate framework, in which half the tetrahedrally coordinated Si atoms are replaced by Al, is arranged in four- and six-membered rings. The rings define sets of channels which intersect to form cavities. Na atoms reside in the channels and Cl in the cavities. Crystals are rare, usually dodecahedrons. Sodalite is usually massive or granular with poor cleavage. The Mohs hardness is 5.5–6.0, and the density 2.3. The luster is vitreous, and the color is usually blue but may also be white, gray, or green.
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A mineral of the mica group with an ideal composition of KAl<sub>2</sub>(AlSi<sub>3</sub>)O<sub>10</sub>(OH)<sub>2</sub>. Sometimes it is referred to as a white mica or potash mica.
Industry:Science
