IMPURITIES OV PHOSPHATE ROCK. 69 



deposits, usually as an incrustation, or as an alteration product 

 of other minerals. The iron minerals frequently form in bogs, and 

 it is an observed fact that the phosphate deposits in such localities 

 not infrequently contain more iron than do the same deposits when 

 found on the uplands. In such cases the iron is doubtless a com- 

 paratively recent infiltration, and may include phosphates of iron 

 as well as oxides and other iron minerals. Of the aluminum phos- 

 phates a large number are known, one of which, wavellite, as al- 

 ready stated, is mined as a source of phosphate. This mineral and 

 others of the aluminum phosphates are likely to occur in associa- 

 tion with calcium phosphate. 



Some of the large phosphate deposits have been formed by the 

 replacement of an original rock by calcium phosphate. In this 

 process parts of the original rock not infrequently remain un- 

 changed or incompletely phosphatized. Since the phosphatizing 

 processes proceed from the surface, the imperfectly phosphatized 

 remnant is likely to lie within the rock, thus giving rise to included 

 impurities that are difficult to eliminate. Moreover, small amounts 

 of clay and silica are usually found in the limestone and as these 

 substances do not readily phosphatize, if not worked out^ they re- 

 main as impurities in the rock. 



Aside from these related minerals, the materials associated 

 with the phosphate rock are varied in character. They include 

 clay, fragments of limestone, flints, gravel, silica in the form of 

 sand, and other resistant materials, the character of which is deter- 

 mined by the manner of formation of the deposits. The asso- 

 ciated materials of this nature make up the matrix in which the 

 phosphate rock is imbedded. It is scarcely possible in mining to 

 remove entirely all associated minerals, and the purity of the rock 

 as delivered to the market, is affected, without doubt, by the pres- 

 ence of more or less of these minerals, as well as by the constituent 

 impurities of the rock. 



OBJECTIONABLE IMPURITIES. 



Of the impurities contained in or associated with phosphate 

 rock, the most objectionable in the processes of manufacture of 

 acid phosphate for fertilizers, for which purpose the phosphate rock 

 is almost wholly used, are iron and aluminum. For this reason 

 practically all phosphate mined is sold under a guarantee that the 



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