CONSTITUENT MINERALS OF ROCKS. 55 



2. MINERALS CONSTITUTING ROCKS.* 



1. Quartz, and Silicates containing alumina, without water. 



54. (1.) Quartz. — Quartz is the first in importance. It occurs in 

 crystals, like figs. 32 and 33 ; also massive, with a glassy lustre. 

 Hardness too great to be scratched with a knife ; varies in color 

 from white or colorless to black, and in trans- 



parency from transparent quartz to opaque. 

 It has no cleavage, — that is, it breaks as easily 

 in one direction as another, like glass. Be- 

 fore the blowpipe it is infusible, unless heated 

 with soda, when it fuses easily to a glass. 

 Clear kinds are called limpid quartz ; violet 

 crystals are the amethyst; compact translucent, 



with the colors in bands or clouds, agate ; or without bands or clouds, 

 chalcedony; massive, of dark and dull color, with the -edges trans- 

 lucent, flint ; the same with a splintery fracture, hornstone ; the same 

 more opaque, lydianstone or basanite ; the same of a dull red, yellow, 

 or brown color, and opaque, jasper; in aggregated grains, sandstone 

 or quartzite; in loose, incoherent grains, ordinary sand. 



Silica also occurs in another state, constituting opal, a well-known 

 mineral. In this state it is easily dissolved in a heated solution of 

 potash, while quartz is not so dissolved. Opal usually contains some 

 water, and is a little softer than quartz. 



55. (2.) Feldspar. — Feldspar is next in abundance to quartz. 

 Under this name several species are included, all of which contain 

 silica and alumina ; but one has, in addition, potash, and is a potash- 

 feldspar ; another, soda, — a soda-feldspar ; another, lime, — a lime-feld- 

 spar ; and others, both soda and potash, or soda and lime. They 

 are all similar in being nearly as hard as quartz ; in having a lustre 

 somewhat like quartz, though partly pearly on smooth faces ; in 

 general, only light colors, white and flesh-red being most common ; 

 also a broad, even, lustrous cleavage-surface, with a second cleavage 

 nearly or quite at right angles with the other, and but little less 

 perfect. Specific gravity, between 2.4 and 2.8. Before the blowpipe. 



® The ordinary characters by which minerals are distinguished are — relative 

 hardness, as ascertained by a file, a point of a knife, or by scratching one mine- 

 ral with another; specific gravity, or relative weight; lustre and color ; crystal- 

 line form ; cleavage (cleavage being a facility of cleaving or breaking in some 

 one or more directions, and affording even, lustrous surfaces, as in mica, gypsum, 

 feldspar); fusibility ; chemical comjiosition. 



