MINERAL CONSTITUENTS OF ROCKS. 55 



fine as flax, the mineral is called asbestus. The principal constituents 



of the mineral are silica, magnesia, oxyd of iron, and lime ; but, unlike 



the feldspars, it contains little or no alumina. 



Pyroxene (including Augite). Like hornblende in most of its 



characters, its variety of colors and its chemical 



Fig. 40 

 composition. But the crystals, as in the annexed 



figures, 40, 41, instead of being prisms of 124° 



30', are prisms of 87° 5' or nearly (angle I on 



I), and are often eight-sided from the truncation 



of the four edges, as in Fig. 41. Black and 



dark-green pyroxene in short crystals is called 



Augite ; it is an iron-bearing kind, and is common in igneous rocks. 



Talc, Serpentine, Chlorite. Talc and serpentine are silicates 

 of magnesia containing water. They are soft minerals, talc being 

 easily impressed with the nail, and serpentine easily cut with a knife ; 

 and both, but especially the talc, feeling greasy in the fingers. 



Talc occurs in broad, pale green or whitish plates, looking like mica ; 

 but the plates are much softer, and have no elasticity. Common steatite 

 or soapstone is nothing but a massive talc. Talc consists of silica 

 62-12, magnesia 32-94, water 4-94 = 100. 



Serpentine is usually compact massive, not granular at all, of a dark- 

 green color, but varying from pale green to greenish black. There is 

 a fibrous variety occurring in seams in massive serpentine, which is 

 called chrysotile. The species contains silica 43-6, magnesia 43-4, 

 water 13-0 = 100. 



Chlorite occurs of dark green color, sometimes thin foliated like 

 mica, but inelastic, oftener granular massive. It is a very soft mineral, 

 being in hardness between talc and serpentine. Besides silica, mag- 

 nesia and water, it contains alumina and oxyd of iron. 



Among the Carbonates, the most common is Calcite, or carbonate 

 of lime, one of the most universal of minerals. It is the ingredient of 

 a very large part of the limestones of the world, and these include the 

 various true marbles. When free from impurities, it consists of car- 

 bonic acid 44-0, lime 56*0 = 100. It is easily scratched with the 

 point of a knife-blade ; and, when dropped in powder into muriatic 

 ^chlorhydric) acid diluted with one half water, it effervesces strongly, 

 giving off carbonic acid. The following are some of the forms it pre- 

 sents when crystallized. It cleaves alike in three directions making the 

 angle 105° 5' with one another, and the resulting form, Fig. 42 A, is 

 called a rhombohedron. When crystallized, calcite is often transparent 

 and colorless. But the mineral occurs of various colors from white to 

 black, and the massive kinds from translucent to opaque. 



Dolomite, or carbonate of lime and magnesia, resembles calcite so 



