SOIL MINERALS 169 



Leucite is another potassium mineral which has the 

 formula KA1 (8^)3)2 — a metasilieate. It occurs in trans- 

 lucent to opaque grains of gray to white color. The potas- 

 sium becomes slowly available under the action of w r ater 

 and carbon dioxide. 



Another potassium mineral that is very common and very 

 familiar to nearly everybody is potash mica or muscovite. 

 The thin, transparent leaves that very easily cleave are too 

 common to need description. This is the white mica. It is an 

 orthosilicate the formula for which is H 2 (K or Na)Al 3 (Si0 4 )3. 

 It might be called an acid salt since the hydrogen atoms 

 replace bases — that is, they are true acid hydrogen atoms. 

 It is very resistant to the action of soil reagents but does 

 change very slowly under the action of water and carbon 

 dioxide, allowing potassium to go into solution. 



(c) Sulphur Minerals. — The principal sulphur mineral is 

 gypsum or land plaster, CaS0 4 .2H 2 0. It is soft, white, 

 granular, or compact, sometimes silky and fibrous. Occasion- 

 ally it is crystalline. It is soluble in water to the extent of 

 1 part in 400. On being heated to 130° C. it loses one mole- 

 cule of water and becomes "plaster of Paris," which has the 

 power of reabsorbing the lost molecule of water and " setting" 

 to gypsum again. This property is made use of in making 

 casts, etc. There is another sulphur mineral called anhy- 

 drite which is CaS04. This is more or less common. 



(d) Calcium Minerals. — Aside from apatite and gypsum 

 which contain calcium there are a number of important 

 calcium minerals. By far the most common is calcite, 

 CaC0 3 . This occurs as w T hite or yellowish, transparent 

 crystals of many shapes, but the amorphous variety known 

 as limestone is the commonest, and is too well known to need 

 description. It occurs pure, and with magnesium, when it 

 is called dolomite, Ca.Mg(C0 3 )-: 



o— Ca— o 



/ \ 



o=c c=o 



\ / 



O— Mg— O 



Calcium carbonate is soluble in pure water only to the extent 

 of 1 part in 20,000, but when the water contains carbon 



