466 GEOLOGY. 



varieties; usually a magnesium-iron-calcium silicate; crystals usually thick and 

 stout, but varying greatly; sometimes lamellar and fibrous; H. 5-6; luster 

 vitreous inclining to resinous; green of various shades verging towards light 

 colors, occasionally more often to browns and blacks ; among the minerals belong- 

 ing to the pyroxene group are augite, bronzite, diallage, diopside, enstatite, 

 hypersthene, and others. 



Quartz — crystallized silica; rhombohedral ; crystals commonly six-sided 

 prisms capped by six-sided pyramids; without cleavage; H. 7; scratches 

 glass; usually transparent, glassy, colorless when pure, shaded by impurities 

 to yellow, red, brown, green, blue, and black; varieties, amethyst, purple, or violet; 

 false topaz, yellow, rose-quartz,'"smoky, milky, cat's eye, opalescent; aventurine, 

 spangled with scales of mica; chalcedony is a cryptocrystalline variety; carnelian, 

 a red chalcedony; chrysoprase, an apple-green chalcedony; prase, a leek-green 

 variety; agate, a variegated or banded chalcedony; moss-agate, a chalcedony 

 containing moss-like or dendritic crystallizations of iron or manganese oxide; 

 onyx, a chalcedony in layers ; sardonyx, like onyx in structure, but includes layers 

 of sard (carnelian); jasper, an opaque-colored quartz, usually red or brown; flint, 

 an opaque impure chalcedony; chert, an ill -defined term applied to an impure 

 flinty rock; hornstone, a translucent, brittle, flinty rock. 



Rutile — titanium oxide; tetragonal, crystals commonly in prisms; H. 6-6.5; 

 luster metallic, adamantine; reddish bro\\Ti, passing to red; sometimes yellowish, 

 bluish, violet, and black; occurs in crystalline rocks and is a common secondary 

 product in the form of microlites. 



Sanidine — a glassy variety of orthoclase feldspar, 



Satinspar — a variety of selenite or gypsum. 



Selenite — a distinctly crystallized transparent form of gypsum. 



Serpentine — a hydrous magnesium silicate; usually in pseudomorph forms; 

 also fibrous, granular, cryptocrystalline, and amorphous; H. 2.5-4; luster 

 subresinous to greasy, pearly or earthy, resinous or waxlike; feel, smooth and 

 somewhat greasy; leek-green to blackish green and siskin green verging into 

 brownish and other colors; apparently derived most commonly from chrysolite 

 or olivine and also from other magnesian minerals; sometimes constitutes the 

 bulk of rock masses. 



Siderite — iron carbonate; rhombohedral; H. 3.5-4.5; luster vitreous, more 

 or less pearly, ash-gray, yellowish or greenish, also brownish; occurs as extensive 

 iron deposits and in crystalline rocks. 



Smaragdite — a form of amphibole or hornblende (q.v.). 



Spherosiderite — a globular form of siderite. 



Spinel — a magnesium-aluminum oxide; crystals, octahedrons; red of various 

 shades, passing into other colors; spinel -ruby is a variety. 



Staurolite — a complex hydrous iron-magnesium-aluminum silicate; ortho- 

 rhombic; disposed to cruciform shapes; occurs in schists and gneisses. 



Steatite — soapstone, a variety of talc (q.v.) ; a hydrous magnesium silicate. 



Sulphur — a well-known element occurring native in volcanic regions; also 

 formed by the decomposition of sulphides, particularly pyrites. 



Talc — a hydrous magnesium silicate; usually in foliae; granular or fibrous 

 forms; also compact; easy cleavage into thin flexible laminse, but not elastic; 



