III. THE SOIL 35 



Minerals. The word mineral is used in several senses ; as an adjec- 

 tive it is often employed as synonymous with inorganic, e.g., the 

 mineral constituents of soil or of food. As a noun, in popular 

 language it is used as the name for any deposit which is obtained from 

 the earth by mining thus gold, silver, coal, and even sandstone are 

 often spoken of as minerals. But in scientific language, as a noun, it 

 has a more restricted meaning. 



A mineral, in the sense in which the word is used by a geologist, is 

 characterised chemically by possessing a perfectly definite molecular 

 structure and yet showing great variation in composition. This is due 

 to the power which similar isomorphous elements exhibit of replacing 

 each other in a compound without altering its crystalline form or 

 general characteristics. Thus in felspar, K 2 O.A1 2 O 3 . 6Si0 2 , as it is 

 usually represented, the potash is almost always replaced to some ex- 

 tent by soda, and the mineral can be traced through all intermediate 

 stages to albite, with the ideal composition, Na 2 O.Al 2 O 3 .6Si0. 2 So, too, 

 in calcite, which is theoretically CaC0 3 , traces of magnesium are in- 

 variably present, and the replacement of Ca by Mg may go on until 

 the composition of the substance would be more correctly represented 

 by MgC0 3 . 



Minerals important in agriculture because of their abun- 

 dance are : 



Quartz. This is, practically, pure silica, SiO 2 , and is extremely 

 abundant. It occurs in granite and many other igneous rocks. It is 

 practically insoluble in water and so is little affected by weather. 

 Usually, however, the other constituents of the rocks in which it occurs 

 (e.g., the felspar in granite, the cementing material calcium carbon- 

 ate, clay, or ferric oxide in sandstone, etc.] disintegrate by weather- 

 ing, and the quartz fragments become detached and are thus removed. 

 Quartz, though the most abundant constituent in most soils, is of practi- 

 cally no value as a plant food. 



Felspar may be orthoclase, K 2 O.Al 2 O 3 .6SiO 2 , 

 albite, Na 2 O.Al 9 O 3 .6SiO 2 , 

 oligoclase, 2 (Na 9 : Ca)0."2AL0 3 .6Si0 2 , 

 or labradorite, (Na 2 : Ca)O.Al 2 6 3 .3SiO 2 . 



Orthoclase, the most important of these, is very abundant, forming an 

 essential ingredient in granite, gneiss, syenite and many other rocks. 

 Though a hard substance, felspar is very easily decomposed by the 

 influence of the weather. Water containing carbonic acid attacks it 

 readily, removing the larger portion of the potash and a portion of the 

 silica in a soluble form and leaving, eventually, a residue containing 

 pure clay or kaolin, Al 2 3 .2H 2 0.2Si0 2 . Clay, however, is usually 

 contaminated with partially decomposed felspar containing still a por- 

 tion of its potash. Felspar furnishes a considerable portion of the 

 potash of a soil. 



Mica, 3Al 2 O 3 .K 2 O.4Si0 2 , always contains considerable quantities of 

 ferric oxide, which partially replaces the alumina ; magnesia, soda and 

 lime, also, are usually present. This .mineral occurs in many rocks, 



