690 APPENDIX 



(H = 6). The cleavage planes are often striated and are not at right 

 angles to each other as in orthoclase. It is an important constituent 

 of some igneous rocks. Labradorite is used to a limited extent for 

 ornamental purposes. 



Muscovite Mica (isinglass, white mica), H 2 KAl 3 (Si04)3. — It is 

 usually transparent or gray. It can be scratched with the finger nail 

 (H = 2). The most distinctive characters of muscovite are its abil- 

 ity to be cleaved into thin leaves, its hardness, the elasticity of its leaves, 

 and its color. It is used in stove doors, for insulation in electrical 

 apparatus, and, when ground, as a lubricant. 



Biotite Mica (black mica), a complex silicate. — With the exception 

 of the color and chemical composition, biotite has the same characters 

 as muscovite. 



Chlorite, a complex silicate. The color is usually dark green. It is 

 so soft that it can be easily scratched with the finger nail (H = 1—2). 

 It occurs in dark green masses in which the flakes are usually so small 

 as to be distinguished with difficulty. Chlorite occurs commonly in 

 metamorphic rocks. 



Talc, a hydro-magnesian silicate. — The color is white, greenish, 

 or gray. It is readily distinguished by its soapy feel (H = 1), in which 

 it differs from gypsum. Talc commonly occurs in plates or leaves like 

 mica. It occasionally occurs in beds 15 or more feet in thickness. It 

 is ground to make " talcum powder " and has many other uses, such 

 as a filler for paper, a lubricant, and an adulterant. Large deposits 

 of talc occur in New York, Massachusetts, North Carolina, and other 

 states. 



Serpentine, a hydro-magnesian silicate. — The color is usually 

 green or yellow, and the hardness is less than that of steel (H = usually 

 about 3). There are two principal varieties, massive serpentine, a 

 compact mineral with a greasy or waxy luster, and asbestos or chrysotile, 

 a fibrous variety. The massive serpentine is polished for table tops 

 and other ornamental purposes ; the asbestos is used in the manufac- 

 ture of fire-proof articles, such as theater curtains, coverings of steam 

 pipes and boilers, and for firemen's suits. The province of Quebec 

 is the great center for asbestos. 



Hornblende, a silicate of several elements. — The color is commonly 

 black, and the hardness about that of steel (H = 5.6). The most dis- 

 tinctive character of hornblende is its occurrence, usually, in slender, 

 flat crystals, the larger angles of the crystals being about 124 degrees. 

 A fibrous variety known as hornblende asbestos has much the same ap- 



