MATERIALS OF THE EARTH 75 



Alabaster. A fine-grained variety of gypsum (q. v.), either white or del- 

 icately colored. 



Albite. A soda feldspar (q. v.), an aluminum-sodium silicate ; H. 1 5-6; 

 cleavage .perfect in two planes ; luster vitreous or pearly white ; occasionally 

 bluish gray, reddish, greenish; sometimes opalescent. 



Amethyst. A variety of quartz or corundum of purple or bluish-violet 

 color, due probably to manganese. 



Amphibole. The type of an important group of rock- 

 forming minerals known as the amphibole or hornblende 

 group; a ferromagnesian silicate; monoclinic; H. 5-6; luster 

 vitreous to pearly; fibrous varieties often silky; black, ranging 

 through various shades of green to light colors; embraces the 

 magnesium-calcium varieties, tremolite and nephrite; the 

 magnesium-calcium-iron variety actinolite; the aluminous- 

 magnesium-iron-calcium variety hornblende, the most important 

 member of the group, and others. 



Andesine. A plagioclase feldspar (q. v.) ; a sodium-calcium- 

 aluminum silicate, intermediate in composition between albite Amphibole 

 and anorthite; H. 5-6; white, gray, grayish, yellowish, flesh crystal, 

 red; luster sub vitreous, inclining to pearly. 



Andalusite. An aluminum silicate; luster vitreous; whitish, rose red, 

 flesh red, variety pearly gray, reddish brown, olive-green; H. 7.5, infusible; 

 impurities sometimes so arranged in the interior as to exhibit a colored, 

 crossed, or tesselated appearance in cross-section (chiastolite). 



Anhydrite. A calcium sulphate; H. 3-3.5; luster pearly to vitreous; 

 white, sometimes bluish or reddish; differs from gypsum in absence of water 

 and in its greater hardness. 



Anorthite. A plagioclase feldspar (q. v.) ; a calcium-aluminum silicate; 

 varies much by impurities and admixtures; H. 6-6.5; pearly or vitreous 

 luster; white, grayish, reddish. 



Anthracite. Hard coal; hydrocarbon with impurities; supposed to be 

 derived from bituminous coal by metamorphism. 



Apatite. Essentially calcium phosphate with chlorine or fluorine; hex- 

 agonal; H. 5; luster vitreous or sub-resineous; colors usually greenish to 

 bluish, characterized by a hexagonal form. 



Aragonite. A calcium carbonate; differs from calcite in cleavage, and 

 in being orthorhombic ; H. 3.5-4; luster vitreous or resinous; white, also 

 gray, yellow, green, and violet. 



Asphaltum. Asphalt; mineral pitch, bitumen; a natural mixture of 

 different hydrocarbons; odor bituminous; melts at 90 to 100 degrees C.; 

 burns with a bright flame; graduates into mineral tars and through these 

 into petroleum; probably the residue of the latter. 



1 H. = Hardness. 



