258 MATERIALS AND THEIR ARRANGEMENT 



quantities in other sedimentary rocks. It is a very soft mineral, and when free 

 from grit usually has a soapy feel, but differs from talc in that it becomes plastic 

 if ground up and moistened with water. Kaolin is earthy in appearance, and 

 breaks like an earthy substance. No cleavage is apparent, because the mineral 

 is not commonly crystallized, and when crystallized the individual crystals are too 

 small to show cleavage without the aid of the microscope. Its color is nearly 

 white when pure, but more commonly is brown, or bluish gray, according to the 

 impurities it contains. 



Pyrite. FeS?. H. 6-6.5; Sp. gr. 5.0. Pyrite occurs in minute crystals in 

 igneous rocks, in large masses in some veins and in metamorphic rocks, and is 

 not uncommon in limestones, sandstones and shales. It is abundant in some 

 coal beds and is the source of the sulphurous odor of coal smoke. Pyrite is very 

 hard, has a bright metallic luster resembling that of light colored brass, but its 

 streak is black. Crystals of pyrite may be cubes, octahedrons, or more complex 

 forms; slightly deformed cubes with striated faces are the most common. When 

 exposed to weathering, pyrite rusts that is changes to limonite. If the original 

 form of the pyrite crystal is retained by the limonite, it is called a pseudomorph. 

 Pyrite is used in the manufacture of sulphuric acid, and to a very small extent as 

 a source of low-grade iron. ,v a'*f\A'x%A-v^* 



Serpentine. Hydrated silicate of magnesium) 1 H. 4; Sp. gr. 2.5-2.6. Ser- 

 pentine occurs chiefly in metamorphic rocks, and as an alteration product in basic 

 igneous rocks. Serpentine has a greasy feel, but less marked than that of talc. 

 Most of it is yellowish green or yellow in color, but may be stained brown by 

 iron oxides. One variety of serpentine consists of fine, closely packed, flexible 

 fibers, called asbestos. Most asbestos occurs in vein-like masses in massive 

 serpentine, the fibers running across the vein. Serpentine is used as a building 

 stone and for interior decoration; asbestos is used in fire-proofing and in the thermal- 

 insulation of material of various sorts. 



Talc. A hydrous magnesium silicate. H. i; Sp. gr. 2.75. Talc is an alteration 

 product of magnesium silicates, especially those free from alumina, and abounds in 

 their metamorphic products. It may occur about the mineral grains in weathered 

 igneous rock, but occurs more abundantly in metamorphic rocks, such as soap- 

 stone and talc schist. It is very soft and has a peculiar greasy feel that 

 usually serves as a valuable aid in identification. Some specimens show very 

 good cleavage, resembling mica except for the fact that the thin plates are 

 not elastic. In translucent specimens most talc is light green in color; in 

 opaque specimens, it varies from nearly white to dark gray. Some varieties 

 resemble kaolin (pure clay), but may be distinguished readily from it by moisten- 

 ing some of the finely powdered mineral; kaolin becomes plastic while talc does 

 not. 



Classification of Igneous Rocks 



Several features are involved in the classification of igneous rocks. Some 

 of them have been noted already, but may be recapitulated here. All fragmental 

 igneous rocks are pyroclastic, and pyroclastic rocks may be iujfs, agglomerates, etc. 

 (p. 263). Rock formed from lava without the development of crystals, is obsidian, 

 if not porous. If porous (hardened rock-froth), the rock is pumice, scoriaceous 

 glass, etc. If the rock is largely glass, but partly of small crystals, it is sometimes 

 called pitchstone, because its freshly fractured surface looks like pitch or resin. 

 When the cavities of scoriaceous rock become filled by minerals deposited from 



