4 SOIL PHYSICS AND MANAGEMENT 



very abundant in some rocks and vary a great deal in composition 

 and physical properties. They have about the same hardness as 

 feldspar and possess more or less definite cleavage planes. They 

 may be either aluminous or non-aluminous. Magnesium, calcium 

 and iron are nearly always present. The iron is frequently in the 

 ferrous condition. As a general rule these groups of minerals 

 decompose somewhat readily, giving rise to hydrous magnesium 

 silicates and soluble carbonates, the latter of which are carried away 

 in solution. The hydrous magnesium silicate may be in the form 

 of serpentine or talc, the latter of which, because of its softness, is 

 readily broken down into clay. The ferrous iron present becomes 

 oxidized and generally gives a yellow or brownish color to the soil 

 formed. As these groups of minerals are frequently magnesian, the 

 soil resulting is not generally highly productive. 



4. Muscovite White Mica. This mineral is made up of 

 transparent laminae or folia possessing a hardness of 2 to 2.5. These 

 folia are thin, elastic and tough. 



The chemical composition and physical properties of this min- 

 eral seem to indicate that it would decompose rather readily, but 

 on the other hand it is very stable and resists decomposition so 

 well that in most cases the mica remains in the residue as distinct 

 shining flakes, giving the soil a peculiar glittering appearance 

 where the flakes are of considerable size. The first step in its de- 

 composition is hydration, resulting in a Hydra ted mica having a 

 pearly luster. When decomposition is complete the product is the 

 hydrous aluminum silicate or clay. Muscovite is found in granites 

 to a considerable extent, but is not very often associated with the 

 more basic rocks or those containing a large per cent of magnesium, 

 calcium or iron. 



5. Biotite Black Mica. Biotite differs from the preceding 

 inica in color, and in the fact that it decomposes more readily. It 

 contains aluminum and iron in both ferrous and ferric states with 

 both magnesium and potassium. It decomposes into a mixture of 

 hydrated aluminous and magnesian silicates, both of which con- 

 stitute clay. Biotite occurs associated with the more basic rocks. 



6. Zeolites. The zeolites comprise a group of secondary min- * . 

 crals of somewhat doubtful importance, whose _f unction, it is be- , 

 lieved, is to retain the potassium and calcium in the soil against 

 leaching. In the decomposition of minerals to form soil material 

 the potassium, sodium and calcium unite with the aluminum ami 

 silica in loose combinations instead of being carried away in solu- 



