THE MICAS 21 



and also occurs in many syenites, andesites, and other eruptive 

 rocks. 



The aluminous yarieties undergo alteration into chloritic and 

 ferruginous products, while the non-aluminous give rise to ser- 

 pentine, either process being attended by the separation of 

 free calcite. 



The OrthorJiomlic Pyroxenes. — These are essentially silicates 

 of magnesia and iron, the latter replacing the former in varying 

 proportions up to 25%. Two principal varieties are recognized, 

 the distinction being founded mainly upon their optical prop- 

 erties which seem to be affected very largely by the percentages 

 of iron. Enstatlte is the theoretically pure magnesian silicate 

 of the formula MgSiOg, but which, as a matter of fact, usually 

 contains from 2 to 10% or more of iron. The highly ferruginous 

 varieties are known as tronzite, from their bronze-like lustre* 

 Eypersthene contains from 10 to 25% of ferric oxide. 



Both enstatite and hypersthene are common constituents of 

 basic igneous rocks, such as the gabbros, norites, and perido- 

 tites. Enstatite is also a common constituent of meteorites. 

 Both forms are liable to alteration, giving rise to serpentinous 

 pseudomorphs to which the name hastite has been given, and to 

 talcose and chloritic products. The general character of the 

 decomposition products of the pyroxenes, as well as the methods 

 by which the decomposition progresses, are in every way similar 

 to those of the amphiboles, and need not be further dwelt upon 

 here. 



The Micas. — There are several species of mica which are 

 prominent as rock constituents, the more important being the 

 white variety, muscovite, and the dark variety, biotite. Both 

 occur in platy forms, splitting readily into thin, elastic folia, 

 which in crystalline form are hexagonal in outline. The folia 

 are often bent and distorted, and the mineral frequently under- 

 goes alteration into a chloritic or sericitic product. The micas, 

 exercise an important influence upon the rocks containing them, 

 on both color and structural grounds. Other things being equal, 

 the muscovite-bearing rocks are lighter in color than those carry- 

 ing biotite. If the mica plates are arranged in definite planes, the 

 rock assumes a schistose structure and splits more or less readily 

 into sheets — an important feature from an economic stand- 

 point. Muscovite, or potash mica, a silicate of alumina and 

 potash, is a constituent of many granites, gneisses, and schists. 



