72 



Lithology 



eral that forms small disseminated grains, 

 layers in oolites, and replacements of origi- 

 nally calcareous foraminiferal tests and 

 mollusk shells. This mineral is anistropic 

 and probably is francolite or dahlite, close 

 relatives of collophanite. Chemical analyses 

 show that the phosphorite from the sea floor 

 has very nearly the same composition as re- 

 ported for large deposits of phosphorite on 

 land in the United States and elsewhere 

 (Table 6). It has the following approximate 

 empirical formula, neglecting sulfate, which 

 is probably also present: 



10Ca3(PO4)2 • SCaCO., • 4CaF., • CaO • nH.O 



On the assumption that phosphorite has this 

 composition, the purity of samples ranges 

 up to about 85 per cent. 



Origin 



The presence of phosphorite in a wide- 

 spread surface blanket, its unique internal 

 layering, its nodular shape, the large size, its 

 usual purity, and the fact that the impurities 

 that do exist are identical with the enclosing 

 sediment strongly indicate a primary or syn- 

 genetic origin in situ. The microcrystalline 

 character of the collophanite is regarded as 

 evidence that the material accumulated as 

 colloids rather than by the molecular attrac- 

 tion by which crystals grow in saturated solu- 

 tions. The originally calcareous tests and 

 shells within the nodules are the only mate- 

 rials that clearly have been secondarily or 

 epigenetically phosphatized. Such phospha- 



tization to francolite or dahlite is probably 

 to be expected, owing to the large volume of 

 collophanite that enclosed calcareous or- 

 ganic remains. 



Catastrophic killing of many animals was 

 suggested by Murray and Renard (1891, p. 

 133) and others as the origin of phosphorite. 

 Such seems not to be true for these deposits, 

 because the internal layering and the com- 

 mon inclusion of older phosphorite in nodu- 

 lar conglomerates indicate many periods of 

 accumulation. Blackwelder (1916), Mans- 

 field (1927, p. 210), and Rankama and 

 Sahama (1950, p. 591) suggested that phos- 

 phorites accumulate in stagnant water where 

 regeneration of phosphate from falling or- 

 ganic debris cannot occur. However, most 

 of these deposits occur only in the most 

 oxidizing environment of the entire conti- 

 nental borderland, the tops of banks; and in 

 any event none of the water off the coast is 

 stagnant. Others have tried to relate phos- 

 phorite deposition to times of cool climate 

 or to times of volcanism with indifferent 

 success. 



An examination by Dietz, Emery, and 

 Shepard (1942) of the solubihty of tricalcium 

 phosphate in sea water suggested that it may 

 well be oversaturated. If so, then it is rea- 

 sonable to expect that a colloidal phase may 

 be present (Emery and Dietz, 1950) and 

 available for deposition. Only the most 

 sketchy chemical data were available for the 

 computations and the situation is little bet- 

 ter now, except that Rittenberg, Emery, and 

 Orr (1955) have shown that the southern 



Table 6 



Chemical Analyses of Phosphorite' 



California Sea Floor Samples 



Dietz, Emery, and Shepard (1942); Emery and Dietz (1950). 



