356 A TREATISE ON METAMORPHISM. 



THE PHOSPHATES. 



The only important rock-making mineral among' the phosphates is 

 apatite. 



Apatite: 



CaF.Ca 4 P 3 12 , or CaCl.Ca 4 P 3 0i 2 , or a mixture of the two. 



Hexagonal. 



Sp. gr. 3.17-3.23. 



occurrence. — Apatite is one of the most widespread, if not the most wide- 

 spread, of all the subordinate constituents of rocks. It is a common, if not 

 an almost universal, constituent of the plutouic rocks, occurs almost as 

 broadly in the volcanic rocks, and is found in many varieties of unaltered 

 or little altered, sedimentary rocks, such as limestones, shales, sandstones, 

 etc; and, finally, it is almost everywhere found in the metamorphosed 

 igneous and sedimentary rocks. 



Alterations. — The only alteration which is recorded for apatite is to osteo- 

 lite, which is reported as having the same composition as apatite, except 

 that there has been a loss of part or all of the fluorine or chlorine. 



It is certain, however, that in the belt of weathering of the zone of 

 anamorphism apatite is slowly dissolved. This is shown by comparative 

 analyses of the weathered with the unweathered varieties of the same 

 rock. This fact has been frequently noted in reference to the iron ores, 

 because here the presence or absence of phosphorus is of such great impor- 

 tance. It may be stated that in the iron ores it is the general rule that those 

 parts of deposits which have been long subjected to weathering bear a 

 smaller proportion of apatite than the continuations of these same deposits 

 in the belt of cementation. 



The depletion of the surface rocks in apatite would seem to furnish an 

 adequate source for the apatite in veins, this mineral being taken into 

 solution near the surface and redeposited deeper down, thus being trans- 

 ported from the belt of weathering to the belt of cementationr 



