NATURAL OCCURRENCE OF PHOSPHATES 25 



If the temperature be raised to. 200 the decomposition of the 

 molecule is hastened according to the following formula : 

 4CaH 4 (PO 4 ) 2 = Ca.P-A -f Ca(PO 3 )o-f CaHoPXL + 2H 3 PO 4 



The chemical changes during the drying of monocalcium 

 phosphates can be represented as follows, temperature 200 for 

 one hour: 



8[CaH 4 (PO 4 ) 2 H 2 O]=4CaH 4 (PO 4 ) 2 +Ca(PO 3 ) 2 +Ca,P 2 O 7 



+ CaH 2 P 2 O 7 -f 2H 3 PO 4 -h 1 2H 2 O. 



The further drying at 200 produces the following decomposi- 

 tion: 



4CaH 4 ( PO 4 ) 2 +Ca ( PO 3 ) 2 +Ca 2 P 2 O 7 +CaH 2 P 2 O 7 +2H 3 PO 4 



=2Ca(PO 3 ) 2 +4CaH 2 P 2 O 7 +Ca 2 P 2 O 7 +2H 3 PO 4 +5H 2 O. 



2Ca(PO 3 ) 2 +4CaH 2 P 2 O 7 +Ca 2 P 2 O 7 +2H 3 PO 4 := 



6Ca ( PO, ) 2 +2CaH 2 P 2 6 7 +5H 2 O. 



Finally, pyrophosphate at 210 is completely decomposed into 

 metaphosphate and water according to the following formula : 



6Ca(PO 3 ) 2 +2CaH 2 P 2 O 7 =8Ca(PO 3 ),+2H 2 O. 

 Provided the drying is made at once at 210 the sum of the 

 changes produced as indicated above, can be represented by the 

 following formula : 



8[CaH 4 (PO 4 ) 2 .H 2 OJ=8Ca(PO 3 ) 2 +2 4 H 2 O. 

 The equations are to be considered as applying only to a pure 

 monocalcium salt. 



MINERAL PHOSPHATES 



33. Natural Occurrence of Phosphates. Gautier calls attention 

 to the fact that the oldest phosphates are met with in the igneous 

 rocks, such as basalt, trachyte, etc., and even in granite and 

 gneiss. 13 It is from these inorganic sources, therefore, that all 

 phosphatic plant food must have been drawn. In the second order 

 in age Gautier places the phosphates of hydro-mineral origin. This 

 class not only embraces the crystalline apatites but also those phos- 

 phates of later formation formed from hot mineral waters in the 

 Jurassic, cretaceous, and tertiary deposits. These deposits are 

 not directly suited to nourish plants. 

 13 Comptes rendus, 1893, 116 : 1271. 



