22 PHYSICAL GEOLOGY 



greatly in distinguishing the minerals when they occur in large 

 crystals. 



Mica is a complex silicate, and may be identified readily by the 

 fact that it is the only common mineral that splits or cleaves into 

 very thin (paperlike), elastic leaves. It is soft, and ranges in color 

 from white to green and black. A light-colored variety of mica, 

 called muscwite, a silicate of alumina and potash, is often used 

 (under the name of isinglass) in stove doors and lanterns. A dark- 

 colored variety, an iron-magnesia-silica compound, is called biotite. 

 Hornblende, augite, and biotite are all iron-magnesia-silica com- 

 pounds, and are known as ferromagnesian minerals. 



Calcite (calcium carbonate, CaCO 3 ) is the principal constituent 

 of limestone. It is scratched easily with a knife, and effervesces 

 when touched with acid ; by these tests it may be distinguished 

 readily from feldspar, which it frequently resembles in general ap- 

 pearance. Calcite cleaves readily along three planes, so arranged 

 as to make a rhombic pattern. 



Gypsum (hydrous calcium sulphate, CaSO 4 + 2 H 2 O) is a white 

 mineral, softer than calcite. It usually occurs in masses of small 

 grains or fibers in which the crystal form is not visible, and is fre- 

 quently stained brown or gray by the impurities it contains. 



Olivine (a silicate of magnesia and iron) is a hard, glassy mineral, 

 which may often be recognized by its grass-green or bottle-green 

 color. It rarely forms good crystals in rocks, but occurs as grains 

 and small masses without definite shape. The fracture is uneven. 

 Olivine is rarely found in the presence of quartz. 



Kaolin (a hydrous silicate of alumina) forms the basis of clay. 

 It is very soft, and the individual particles are not visible. Pure 

 kaolin is white and is known as " porcelain clay " because of its 

 use in the manufacture of chinaware. 



Hematite (iron oxide, FesOs) is steel-gray and hard when in the 

 pure crystalline form, but soft, red varieties are common, and con- 

 stitute the most important sources of iron ore in the Lake Superior 

 district, at Birmingham, Alabama, and in many other places. 

 This mineral occurs in many igneous rocks and is the red coloring 

 matter of many soils and of bricks. It gives a red streak when 

 rubbed with a harder surface, a fact by which it may be distin- 

 guished from other iron minerals. 



Magnetite (iron oxide, FeO4) is black, and acts as a magnet. Its 

 crystals are often cubes or double pyramids. Like hematite, it is 

 an important ore of iron. It gives a black streak. 



Limonite (hydrous iron oxide, 2 Fe-jOs + 3 H-jO) is often 

 called brown hematite. It is found frequently in marshes, 



