CONSTITUENT MINERALS OF ROCKS. 53 



colorless to black, and in transparency from transparent quartz to 

 opaque. It has no cleavage, — that is, it breaks 

 as easily in one direction as another, like glass. 

 Specific gravity, 2-65. Before 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 ; the same, without bands or clouds, chalcedony ; mas- 

 sive, of dark and dull color, with the edges translucent, 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 quart- 

 zyte ; in loose, incoherent grains, ordinary sand. 



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

 mineral. In this state it is never crystallized, and is easily dissolved in 

 a heated solution of potash, while quartz is so with difficulty. Opal 

 usually contains some water, and is a little softer than quartz. Silica 

 exists also in a third state called tridymite, having the specific gravity 

 2-3. Unlike quartz, it crystallizes in hexagonal tables. 



(2.) Feldspar. — The feldspars are next in abundance to quartz. 

 They have a lustre nearly like quartz, but often somewhat pearly on 

 smooth faces ; are very nearly as hard as quartz, with about the same 

 specific gravity (2-4-2 # 6) ; and in general have light colors, mostly 

 white or flesh-colored, though occasionally dark gray, brownish, or 

 gree^. They differ from quartz in having a perfect cleavage in one 

 direction, yielding under the hammer a smooth lustrous surface, and 

 another nearly as perfect in a second direction, inclined 84° to 90° to 

 the first ; also in being fusible before the blowpipe, though not easily 

 so ; also in composition, the feldspars consisting of silica combined 

 with alumina and an alkali — this alkali being either potash, soda, or 

 lime, or two or all of these combined. 



(3.) Mica. — The transparent mineral often used in the doors of 

 stoves and lanterns is mica, often wrongly called isinglass. It is re- 

 markable for splitting easily into very thin elastic leaves or scales, — 

 even thinner than paper, — and for its brilliant lustre. It occurs color- 

 less to brown, green, reddish and black : and either in small scales 

 disseminated through rocks, — as in granyte — or in plates a yard in 

 diameter. Consists of silica and alumina with either potash, magnesia, 

 or iron, and some other ingredients. Fluorine is sometimes present. 

 It is of several kinds, which differ in composition and optical charac- 

 ters more than in appearance. Some of the varieties resemble crys- 

 tallized talc and chlorite, from which they differ in being elastic (un- 

 less weathered). 



