SOIL MINERALS 171 



oxidizes almost instantly, if in aerated soils, to the hvdrated 

 oxide. If, however, the decomposition takes place in sub- 

 soils and in water-logged soils, ferrous carbonate may remain 

 as siderite, FeCOs. Iron compounds give the yellow, red, 

 and brown color to soils when it is not masked bv humus. 

 A phosphate mineral is vivianite, Fe 3 (P04)2, a bluish-green, 

 earthy mass. 



(/) Magnesium Minerals. — Some of the magnesium in 

 soils is derived from dolomite, already mentioned under the 

 calcium minerals. There are also many silicates containing 

 magnesium, hornblende, already mentioned, and biotite or 

 black mica, an orthosilicate of aluminium, magnesium, hydro- 

 gen, and potassium, thus: (H or K) 2 (Mg or Fe) 2 Al 2 (Si0 4 )3. 

 This mineral is similar to muscovite or white mica, except 

 as to color, which is dark green or black. 



(g) Silicon Minerals. — All of the silicate minerals con- 

 tain silicon, of course, but quartz, or SiCK, is the only one to 

 be considered here. It is, next to feldspar, the most common 

 mineral in the earth's crust and occurs in many varieties 

 and all colors from transparent and white to red, blue, 

 green, and brown. Small quantities of impurities give the 

 color to it. Ordinarily, however, it is a hard, brittle mineral, 

 clear to white, and in hexagonal crystals of all sizes, although 

 frequently amorphous. Sandstone and quartzite are massive 

 varieties of quartz. It is ordinarily very insoluble, but some 

 varieties dissolve appreciably to the silicic acids. 



(h) Sodium Minerals. — Common salt, or halite, XaCl, is 

 the most familiar mineral of both sodium and chlorine, but 

 it does not occur to any extent in agricultural soil, being 

 confined to beds located in many parts of the world. Albitc, 

 or the soda feldspar, is a silicate mineral of soda and is the 

 counterpart of orthoclase, being Xa2Al2(Si 3 8 )2. It occurs 

 in white granular masses or plates. Its solubility is similar 

 to that of orthoclase. 



(/) Chlorine Minerals. — Halite mentioned above is the 

 only one of importance. There is very little chlorine in 

 soils ordinarilv. 



(j) Aluminium Minerals. — The feldspars and many 

 other silicates contain aluminium, and these break down as 



