PHOSPHATES 111 



(5) PHOSPHATES 



The mineral apatite, a phosphate of lime^ as already noted, is 

 a common accessory, in the form of small crystals, in crystal- 

 line rocks of all ages, both metamorphie and eruptive. In 

 rare instances, as among certain Laurentian rocks of Canada, it 

 occurs in coarsely granular aggregates of a green or pinkish 

 color and of such dimensions as to constitute true rock masses. 

 Here we have to do, however, more with the amorphous, fibrous, 

 or concretionary forms to which the name phosphorite is com- 

 monly applied. These occur nearly if not quite altogether as 

 secondary products, due to the leaching out of phosphatic mate- 

 rial from older rocks, and its redeposition in clefts and cavities 

 at lower levels. It is thus that the phosphorites of Estre- 

 madura, Spain, are accounted for. From these very pure, 

 semi-crystalline masses, to the amorphous nodular and earthy 

 forms, such as are found in the eastern Carolinas, Florida and in 

 Tennessee, there are no well-defined lines of demarcation. All 

 have resulted apparently either from the leaching out of the phos- 

 phate as above, or from the dissolving and carrying away of the 

 lime carbonate in a phosphatic limestone, leaving the phosphatic 

 material to accumulate as a residual product. Some of the latter 

 products, like the phosphatic sandstones of the Carolinas, might 

 with equal propriety be classed with the fragmental rocks, as 

 are the residual clays. (See p. 112.) 



(6) CHLORIDES 



Sodium chloride, or common salt, is one of the most wide- 

 spread constituents of the earth's crust, and from the standpoint 

 of human comfort a most important constituent as well. The 

 theoretically pure mineral consists of 66.6 parts of sodium and 

 39.4 parts of chlorine, though in nature it is almost univer- 

 sally contaminated with chlorines, sulphates, and carbonates 

 of potassium, calcium, and magnesium, together with oxides of 

 iron and aluminum, A large number of analyses of rock-salts 

 from world-wide sources show them to range from 94 to 99% 

 sodium chloride. The pure mineral is white in color, but 

 shows often yellow, red, or purplish hues due to iron oxides or 

 organic matter. When crystallizing freely from solution, it 

 ordinarily assumes the form of a cube, the faces being frequently 

 cavernous or hopper-shaped; rarely it occurs in octahedrons, 

 and occasionally in fibrous forms. Sodium chloride in solution 



