PHOSPHATE DEPOSITS 



517 



Table 104. — Marketed production of phosphate rock in the United States in each 10th year of 



the period of 1870 to 1970 and total production through 1971 



[In thousands of long tons. Data from U.S. Geological Survey "Mineral Resources of the United States" and U.S. 

 Bureau of Mines "Minerals Yearbook'*] 



' Alabama, Arkansas, and Pennsylvania in 

 ' Includes north Florida and North Carolina ; 

 ' Includes Alabama in 1970. 

 ' Includes California in 1970. 



1910; Kentucky in 1920. 



more than 4 million tons in 1970. Production has 

 leveled off somewhat in the last several years because 

 of the general soft market for phosphate rock. 



The total production of phosphate in the United 

 States has almost doubled in each succeeding decade 

 since 1870 (table 104) , except for the depression and 

 war years, and by 1970 production had reached 38 

 million tons. 



Total production of phosphate rock in the United 

 States is about 654 million tons through 1971 — 76 

 percent from Florida, 12 percent from Tennessee, 10 

 percent from the Western States, and 2 percent from 

 South Carolina. 



GEOCHEMISTRY 



Phosphorus is widely distributed in the earth's 

 crust, which contains about 0.23 percent P2O5. Most 

 of the phosphorus occurs in minerals of the apatite 

 group Caa„(P04,C03)6(F,OH,Cl)2. The structure of 

 apatite is such that small amounts of VO4, AS2O4, and 

 SO4 substitute for PO4, and Na, Sr, U, Th, and the 

 rare earths may substitute for Ca. 



Apatite is relatively insoluble in alkaline or neu- 

 tral waters, and its solubility increases as acidity in- 

 creases and temperature decreases. Thus, although 

 apatite may be residually concentrated in the first 

 stages of weathering, it eventually breaks down. 

 Some of the phosphate may be redeposited locally. 

 In limestone terranes, apatite is again formed ; in 

 igneous terranes, iron and aluminum phosphates 

 (variscite and strengite) are formed; and in clay 

 soils, aluminum phosphates (wavellite and crandal- 

 lite) are formed. 



Phosphorus is carried to the sea as phosphate min- 



erals adsorbed on iron or aluminum hydroxides or 

 clays, in particulate or dissolved organic compounds, 

 or in solution. 



The ocean is nearly saturated with phosphate, but 

 the phosphate content of the ocean is not uniform. 

 Deep cold water contains about 0.3 ppm PO4 and 

 warm surface water contains about 0.01 ppm. In the 

 ocean, cold, nutrient-rich waters may be brought to 

 the surface in several ways — by divergence upwell- 

 ing, dynamic upwelling, turbulence, and convection. 

 The solubility of phosphate in upwelling waters de- 

 creases as temperature and pH increase near the sur- 

 face, and apatite may then be precipitated by organic 

 or inorganic processes. Phosphorites and the accom- 

 panying rocks in the environment of upwelling — 

 black shale, chert, and limestone — are depositional 

 products from sea waters that change in composition 

 as the result of physical, chemical, and biologic 

 processes. 



TYPES OF DEPOSITS 



Minable concentrations of phosphate are found in 

 igneous rocks, as sedimentary phosphorite, and as 

 guano, or in deposits derived from guano. The term 

 "phosphate rock" is applied to any rock that contains 

 more than about 20 percent P2O5. "Phosphorite" is 

 equivalent, but is usually applied only to marine 

 phosphate rocks. 



IGNEOUS APATITES 



Apatite occurs as intrusive masses or sheets, as 

 hydrothermal veins or disseminated replacements, as 

 marginal differentiations, or as pegmatites. Largest 

 are the intrusive masses, which are commonly as- 



