MATERIALS OF THE EARTH 



77 



surface pearly, sometimes metalloid or brassy; an essential mineral in the 

 gabbros, as sometimes defined. 



Enstatite. One of the pyroxenes; essentially a magnesium silicate; 

 orthorhombic ; H. 5.5; luster a little pearly on cleavage surface; metalloidal 

 in the bronze variety (bronzite) ; grayish white, yellowish white, greenish 

 white to olive-green and brown; very infusible; a common mineral in certain 

 basic crystalline rocks. 



Epidote. A complex aluminum-calcium-iron silicate of varying com- 

 position; monoclinic; H. 6-7; luster vitreous, pearly, or resinous; color 

 usually pistachio-green, or yellowish green to brownish green; common in 

 many crystalline rocks, usually as a sec- 

 ondary product. 



The feldspars are aluminum silicates, 

 with either potassium, sodium, or calcium, 

 or two or more of these ; crystal systems, 

 monoclinic and triclinic ; possesses distinct 

 cleavage in two directions; H. 6-6.5; 

 range in color from white through pale 

 yellow, red, or green; and occasionally 

 dark; triclinic feldspars frequently called 

 plagioclase; the group embraces ortho- 

 clase, microcline, albite, oligoclase, ande- 

 sine, labradorite, anorthite, and numer- 

 ous varieties. 



Fluorite. Fluorspar; calcium fluoride; 

 luster vitreous, sometimes splendent; white, yellow, green, rose, crimson 

 red, violet, sky-blue, and brown; yellow, greenish, and violet most common; 

 occurs usually in veins or cavities in beautiful crystalline form. 



Galenite. Galena; lead sulphide; isometric, usually cubic ; perfect cubic 

 cleavage f luster metallic ; lead-gray; a common ore of lead; occurs in veins 

 and layers, also as linings of cavities. 



Garnet. A complex silicate of varying composition, embracing aluminum, 

 calcium, magnesium, chromium, iron, and manganese, but usually only two 

 or three of these are present in abundance, and the varieties are characterized 

 by the leading constituent; isometric, usually in dodecahedrons or trapezo- 

 hedrons; H. 6.5-7.5; luster vitreous to resinous; commonly red or brown, 

 sometimes yellow, white to blue, green or black; common in mica schist, 

 gneiss, hornblende schist; also in granite, syenite, and metamorphosed 

 limestone. 



Glauconite. Green-sand, a hydrous potassium-iron silicate usually im- 

 pure, amorphous, or earthy ; dull olive-green or blackish, yellowish, or grayish 

 green; opaque, commonly occurs as grains or small aggregations. 



Graphite. Plumbago, black lead; a form of carbon, usually impure; 

 rhombohedral, but rarely appearing as a crystal; more often as thin laminse 



Fig. 49. Feldspar crystals, 

 isometric, usually cubic; H. 4; 



