CHEMICAL CONSTITUENTS OF THE SOU, 151 



alkaline, turn the phenolphthalein red. A number of other 

 minerals behave in the same way. The quantity of alkali which 

 goes into solution is, however, very small. Only a small fraction 

 of felspar is dissolved by the concentrated hydrochloric acid used 

 in soil analysis and about i to 4 per cent, of the total potash. The 

 potash held in felspar has only :i slight value to plants, as it is dis- 

 solved very slowly. 



Micas are primary silicates, which are easily split into thin, 

 flexible, and elastic leaves. They have a complex and varying 

 composition, being silicates of alumina with potash, lithia, 

 magnesia, iron or manganese. Muscovite is light colored potash 

 mica, K 2 Al 6 Si 6 O l22 2H 2 O. Biotite, a dark colored mica, is a com- 

 plex hydrated silicate of alumina, iron, potash, and magnesia. 



Micas are decomposed very slowly. They persist for a long 

 time after the other rock minerals have been entirely changed by 

 weathering. Almost any soil derived from granite contains flakes 

 of mica, which are more easily seen if oxides of iron are removed 

 with a little hydrochloric acid. Micas aid in the decomposition 

 of rocks in which they are present, by allowing water to percolate 

 into their fissures. Mica is, to a large extent, dissolved by strong 

 acids, and it is probable that the plant food it contains is more 

 easily used than that of felspar. 



Hornblendes and Pyroxenes include a number of silicates of 

 varying composition, though with related properties. They are 

 complex silicates of magnesia, alumina, lime, and iron. They are 

 usually green, brown, or black in color. They are easily affected 

 by natural agencies. Olivine is a silicate of iron and magnesia 

 (MgFe) 2 Si 2 O 4 . 



Apatite Ca 3 PO 4 Ca(ClF) 2 is a crystallized phosphate of 

 lime. It is considered to be the chief form in which phosphoric 

 acid occurs in igneous rocks. 



Secondary Minerals. The minerals formed from the primary 

 minerals by processes of combination with water, by solution, 

 oxidation, or by partial or complete decomposition, are termed 

 secondary minerals. They occur in rocks formed by decomposi- 

 tion of the igneous rocks, and in soils. A few of them are 

 described in the following paragraphs. 



