SOIL MATERIAL AND ITS ORIGIN 



granite, quartz occurs as glassy masses which do not decompose as 

 most other minerals do, but remain as distinct grains of quartz 

 when the rock is broken down. In limestones it frequently occurs 

 as chert, an impure rather soft form, or as flint. Sand, sandstones, 

 and quartzite are formed principally of quartz. The fact that its 

 hardness is 7, that it is almost insoluble, decomposes very slowly and 

 possesses no cleavage, makes it very abundant among the coarser 

 constituents of soils. It may be distinguished by its glass-like ap- 

 pearance, hardness, shell-like fracture, lack of cleavage and its 

 resistance to the action of all acids with the exception of hydro- 

 fluoric. 



2. Feldspars. The feldspars include double silicates of potas- 

 sium, sodium, calcium and aluminum. They possess a hardness of 

 6, distinct cleavage and decompose rather readily in the presence 

 of carbonated water. The action of carbonic acid is to dissolve 

 out the base or bases, forming the soluble carbonates, leaving a 

 hydrated aluminum silicate, kaolin, and finely divided free silica 

 which constitute the clay of soils. The process is known as kaolin- 

 ization. The following table gives the composition of the principal 

 feldspars. 



Composition of Principal FcUlxpurs 2 

 Per cent 



As an illustration of the decomposition of feldspar, ortboclase 

 may be taken. Carbonated water coining in contact with this min- 

 eral dissolves out the potassium, forming potassium carbonate, which 

 being soluble is carried away. In the process the excess of silica, 

 which amounts to approximately -0 per cent in orthoclase^is sepa- 

 rated as extremely fine particles most of which are classed as clay. 

 The alumina is left in combination with silica as a hydrous alumi- 

 num silicate forming the mineral kaolin which also constitutes clay. 

 The result then of the decomposition of feldspar is a light-colored 

 clay composed of free silica and kaolin. 



3. Amphibole and Pyroxene. These groups of minerals are 



