ZEOLITES 53 



members of the alkali family in hornblende. The 

 augites are double silicates of iron, manganese, cal- 

 cium, and magnesium. Quite frequently, phosphoric 

 acid is present in chemical combination with the iron. 

 The members of this group are readily distinguished 

 by their color which is black, brown, or brownish 

 green. The hornblendes are insoluble in acids, hence 

 unavailable as plant food, and when disintegrated do 

 not as a rule form very fertile soils. 



61, Mica, Mica is quite complex in composition, 

 is an abundant mineral, and is composed of silica, 

 iron, alumina, manganese, calcium, magnesium, and 

 potassium. Mica is a polysilicate. The color may 

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

 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 disintegrated mica are usually 

 fertile, owing to the variety of essential elements 

 present. Frequently small pieces of undecomposed 

 mica are found in soils. 



62. Zeolites. The zeolites are a large group of 

 secondary or derivative minerals formed from disin- 

 tegrated rock. They are polysilicates containing 

 alumina and members of the alkali and lime families, 

 and all contain water held in chemical combination. 

 They are partially soluble in dilute hydrochloric acid 



