76 GEOLOGY 



Augite. One of the pyroxenes; an aluininum-calciuin-nmgiHvsium-iron 

 silicate; H. 5-6; monoclinic, crystals usually thick and stout; sometimes 

 lamellar; also granular; black, greenish black, deep green; an important 

 rock-forming mineral. 



Beauxite. Essentially hydrated alumina; occurs in concretionary 

 grains of clay-like form, whitish to brown; valuable as a source of aluminum. 



Barite. Barites, heavy-spar, barium sulphate; orthorhombic, H. 5-3.5; 

 luster vitreous to resinous, sometimes pearly; white, inclining to yellow, 

 gray, blue, red, or brown; very heavy, sp. gr. 4.3-4.7. 



Biotite. Black mica, a potash-aluminum-magnesium-iron silicate; 

 monoclinic; easy basal cleavage into thin laminse; sometimes occurs as a 

 massive aggregation of cleavable scales ; H. 2.5-3 ; luster splendent on cleavage 

 surface; black to dark green; cleavage surfaces smooth and shining; a very 

 common constituent of crystalline rocks. 



Bitumen. The same as asphaltum (q. v.). 



Calcite. Calcspar; calcium carbonate; rhombohedral, perfect rhombo- 

 hedral cleavage; often taking the forms known as dogtooth spar, nail-head 

 spar; frequently stalactitic and stalagmitic; H. 2.5-3.5; luster vitreous; 

 white, occasionally pale shades of gray, red, green, blue, violet, yellow, brown ; 

 strong double refraction; embraces variety called Iceland spar; a very 

 common mineral; the essential basis of limestone. 



Chalcedony. A crypto-crystalline variety of quartz having a wax-like 

 luster, either transparent or translucent; white, grayish, pale brown to dark 

 brown, black, sometimes delicate blue, occasionally other shades; frequently 

 occurs as the lining or filling of cavities, taking on a botryoidal or mamillary 

 form. 



Chlorite. The type of an important group of secondary minerals usually 

 characterized by a green color, softness, and smoothness or unctuousness of 

 feeling; they are usually aluminum-magnesium-iron silicates, with chem- 

 ically combined water; derived from several other species, as pyroxene, 

 amphibole, biotite, garnet, etc. ; embraces a number of species, among which 

 are clinochlore, penninite, prochlorite, and delessite. 



Chrysolite. Olivine; essentially a magnesium-iron silicate; ortho- 

 rhombic; H. 6-7; luster vitreous; green, commonly olive-green, sometimes 

 yellow, brownish, grayish green; highly infusible; a common constituent 

 of certain basic igneous rocks; the name olivine is more commonly used by 

 geologists. 



Corundum. Alumina; an oxide of aluminum; H. 9; rhomboliedral; 

 large crystals usually rough; luster vitreous; color blue, ml, yellow, irniy, 

 and nearly white; purer forms of fine colors are sapphires; the red variety 

 is ruby, the yellow, oriental topaz, the green, emerald, and the purple, 

 amethyst; dark colors, with iron oxide, emery. 



Diallage. A variety of pyroxene (q.v.); H. 4; characterized 1>\ thin 

 foliac; usually grayish green to grass-green, or deep green; luster on cleavage 



