170 THE SOIL: INORGANIC MATTER 



dioxide to saturation the calcium carbonate is soluble to the 

 extent of 1 part in 1000 of water, being changed to the acid 

 carbonate, thus: 



CaC0 3 + H2O+ C0 2 = CaH 2 (C03) 2 . 



This form of calcium is the most important in the soil, 

 although some of the more soluble silicates supply small 

 amounts of this element. 



Anorthite or the lime-feldspar is an orthosilicate of calcium 

 and aluminium, CaAl 2 (Si0 4 )2, which slowly decomposes 

 under the action of water and carbon dioxide to kaolinite 

 and calcium bicarbonate. 



(e) Iron Minerals. — Iron occurs largely as the hydra ted 

 ferric oxide, Fe 2 3 .xH 2 0, in surface soils. There are a number 

 of minerals of this kind, of which limonite is the most common. 

 It is an amorphous, loose to compact, yellow or brown mineral, 

 occurring fairly well disseminated in soils. Its formula is 

 Fe 4 3 (OH)6, graphically: 



/OH 

 HO— Fe<^ 



O 

 HO— Fe/ 



O 

 HO— Fe/ 



O 



HO— Fe/ 



X OH 



Limonite is derived from silicate minerals containing iron, 

 hornblende for example. This occurs in columnar and gran- 

 ular crystals of green, brown, or black color. It is a meta- 

 silicate of any two or sometimes more of the following bases : 

 Calcium, magnesium, iron, and aluminium. The iron may 

 be ferrous or ferric. Under the action of water and carbon 

 dioxide, carbonates or bicarbonates of the bases are formed, 

 except ferric iron which is set free as such, usually in the 

 hydrated form and becomes limonite or similar minerals. 

 If the iron is ferrous it is changed to carbonate first but 



