COMPONENTS OF THE EARTH.' 8 CRUST, MINERALS AND ROCKS 121 



The feldspars are silicates of alumina combined with potash, 

 soda or lime. The more common species are: orthoclase and mi- 

 crocline, silicates of alumina and potash. 



AlUte, silicate of alumina and soda. 



Anorthite, silicate of alumina and lime. 



Oligoclase, andesite and labradorite, which contain both lime and 

 soda, and are intermediate between albite and anorthite. 



In crystallization orthoclase is monoclinic, the others named 

 are triclinic. 



The triclinic feldspars are usually called plagioclase in techni- 

 cal rock nomenclature, and are referred to collectively by this 

 term. 



The magnesia-iron silicates are classified in three principal 

 groups, the amphiboles, pyroxenes and micas. 



The amphiboles are monoclinic and comprise hornblende, actino- 

 lite and tremolite. 



Hornblende is a silicate of alumina, iron, lime and magnesia; 

 it is very tough and somewhat fibrous in fracture, its color 

 varies from dark green to blackish green. This is a very import- 

 ant constituent of granites and other crystalline rocks. 



Actinolite is a fibrous variety, generally light green in color and 

 containing less alumina. 



Tremolite is usually white and contains but little iron and no 

 alumina. It occurs generally in crystals scattered through crys- 

 talline limestone. 



Asbestus is a finely fibrous tremolite. 



The pyroxenes have very nearly the same chemical composition 

 as the amphiboles and are also monoclinic but crystallize with a 

 different prismatic angle. 



Augite, which corresponds closely to hornblende in composition 

 and resembles it in many ways, is an important constituent of 

 many eruptive rocks such as diabase, basalt, etc. 



Pyroxene is lighter in color than augite and similar to actinolite 

 in composition. 



Diopside corresponds closely to tremolite in composition and 

 like it, occurs in limestones. 



