IMPURITIES OF PHOSPHATE ROCK. /I 



requirements as to the grade of rock. If it is true, as elsewhere 

 stated in this paper, that the principal mineral of the massive phos- 

 phate deposits is a calcium carbono-phosphate, this fact will afford 

 an explanation of the presence of the desired amount of carbonate 

 in all phosphate deposits of this class. 



The fluorine found in phosphate rock, upon being attacked by 

 the acid, forms hydrofluoric acid gas which passes into the atmos- 

 phere, it being estimated that as much as from 50 to 66 2-3 per 

 cent of the fluorine present is eliminated in this way. Although 

 a small amount of acid is used up in this reaction and a propor- 

 tionate amount of calcium sulphate formed, yet it is seldom, if 

 ever, necessary to specify against the fluorine content of the rock, 

 the amount present being negligible. 



Among the numerous other impurities that may be present in 

 phosphate rock, silica and clay are perhaps the most common. 

 Here also should be mentioned moisture, which when present not 

 only adds bulk to the shipment but also interferes with the proc- 

 esses of manufacture. The excess of moisture must, therefore, be 

 removed by drying, not more than 3 or 4 per cent being allowable 

 in the rock as shipped. 



THE ORIGIN OF PHOSPHATE DEPOSITS. 



The origin of phosphate deposits is such that the presence of 

 associated minerals as well as constituent impurities is almost in- 

 variable. The original source of phosphorus, the constituent for 

 which phosphate rock is mined, is in the igneous and crystalline 

 rocks, v^rhere it exists in combination with other elements fomiing 

 phosphate minerals. These minerals, as indeed is true of all min- 

 erals, are soluble in water, the degree of solubility, however, vary- 

 ing with the different minerals, and with the diverse conditions 

 to which they are subjected. Indeed, some very interesting and 

 suggestive observations have been made on the relative solubility 

 of phosphates under varying conditions. Thus it has been shown 

 that the solubility of the phosphate minerals is increased by the 

 presence of decaying organic matter in water. They have also 

 been found to be appreciably soluble in carbonated waters. In 

 this connection Reese* has made the very important observation 



*Chas. L. Reese, Amer. Jour. Sci., 3rd Ser., Vol. 43, p. 402, 1892. 



