ROCKS AND MINERALS 57 



able as plant food, and when disintegrated do not as a 

 rule form very fertile soils. 



61. Mica. — Mica is quite complex in composi- 

 tion, is an abundant mineral, and is composed of sil- 

 ica, iron, alumina, manganese, calcinm, magnesium, 

 and potassium. Mica is a polysilicate. The color 

 may be white, brown, black, or bluish green owing 

 to the absence of iron, or to its presence in various 

 amounts. The chief physical characteristic of the 

 members of this group is the ease with which they 

 are split into thin layers. It is to be observed that the 

 mica group contains all of the elements of both feld- 

 spar and hornblende. 



Soils formed from the disintegration of mica are 

 usually fertile owing to the variety of essential ele- 

 ments present. Frequently small pieces of undecom- 

 posed mica are found in soils. 



62. Zeolites. — The zeolites form a large group of 

 secondary minerals. The}' are polysilicates containing 

 alumina and members of the alkali and lime fami- 

 lies, and all contain water held in chemical combina- 

 tion. They are soluble in dilute hydrochloric acid 

 and belong to the group of compounds which are ca- 

 pable, to a certain extent, of serving as plant food. In 

 color they are white, gray, or red. Zeolites are quite 

 abundant in clay and are an important factor in soil 

 fertility. It is this group which takes such an impor- 

 tant part in the process of fixation. The zeolites, when 



