Uneven fracture

The natural niobites and tantalites axe usually black, and form iso-morphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8 2 with increase in tantalum content.6 When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen.7 Small quantities of helium have also been found occluded in them. 

PROUSTITE. This ruby-silver mineral crystallizes in the hexagonal system its name is a product of its scarlet-to-vermilion color when first mined It is a silver arsenic sulfide. AgjAsS, of adamantine luster Hardness of 2-2,5 specific gravity of 5.55-5.64. Usual crystal habit is prismatic to rhombohedral more commonly occurs massive. Conchoidal to uneven fracture transparent to translucent color, scarlet to vermilion red. Light sensitive must be kept in dark environment to maintain its primary character. A product of low-tcmpcraturc formation in most silver deposits. Notable world occurrences include the Czech Republic and Slovakia, Saxony, Chile and Mexico. Found in minor quantities in the United States the most exceptional occurrence at the Poorman Mine, Silver City District. Idaho where a crystalline mass of some 500 pounds (227 kilograms) was recovered m 1865, It was named for the famous French chemist, Louis Joseph Proust. 

STANNITE (Mineral). This mineral is a sulfo-stannate of copper and non, sometimes with some zinc, corresponding to the formula. Uii PeSnS j It is tetragonal brittle with uneven fracture hardness, 4 specific gravity, 4.3-4.5 metallic luster color, gray to black, sometimes tarnished by chalcopyiite stieak, black opaque. The mineral occuts associated witli cassilerile, chalcopyiite. tetrahedrite, and pyrite, probably the result of deposition by hot alkaline solution. Stannite occurs in Bohemia Cornwall, England Tasmania Bolivia, and in the United States in South Dakota. It derives its name from the Latin word for tin, stannum. 

SYLVITEA. A mineral, potassium chloride, KC1, occurring in cubes, or as cubes modified by octahedra. Sylvite is therefore isometric. It has a perfect cubic cleavage uneven fracture is brittle hardness. 2 specific gravity, 1.9 luster, vitreous colorless when pure but may be white, bluish, yellowish or reddish due to impurities. It is soluble in water. It is much rarer than halite and has been found as sublimates at Mt. Vesuvius and as bedded deposits al Slassfurl, Genii any. Extensive deposits occur in sedimentary deposits in the Permian basin of southwestern New Mexico, near Carlsbad, in die United States. 

One directional perfect cleavage Conchoidal Uneven fracture... 

Feldspars exhibit uneven fracture, and have a vitreous to pearly luster. Colors are generally white, cream and pink, hut also milky, clear, huff, hrown, red, gray, green and hluish. Property ranges for the major types of feldspar are sp. gr. 2.56—2.63 m.p. 1110-1532°C refiactive index 1.524—1.584. 

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