682 



AURHENIUS 



[chap. 25 



f)hos23hate minerals (mainly francolite, Ca5[F|P04C03]3) in shallow low- 

 latitude areas where saturation is reached. Upwelling of phosphate -rich deep 

 water produces exceptionally high concentrations of phosphate minerals in such 

 areas (Kazakov, 1950). The ensuing increase in organic productivity in the 

 euphotic zone leads to a high rate of accumulation of organic remains on the 

 bottom, and a high rate of crystallization of phosphates is maintained by 

 decomjDosition of the organic phosphorus compounds (McKelvey et al., 1953). 

 Bruejewicz and Zaytseva (1958) measured concentrations of dissolved phos- 

 phorus as high as 27 [xmole/l. in Pacific sediments of this type. 



Fig. 14. Dendritic intergrowth of manganese oxide in marine phosphorite from oxidizing 

 environment (Cape Johnson Guyot; 17° lO'N, 177° lO'W). Transmitted Ught. 



The solubility relations of francolite are complicated by variable substitution 

 of calcium with zirconium and rare earth ions, which drastically reduce the 

 solubility; zirconium phosphate concentrations up to 2800 ppm Zr have been 

 observed in marine inorganic apatite (Arrhenius and Korkisch, unpublished). 



The phosphorite rock accumulating in areas of high organic productivity 

 consists of a microcrystalline matrix of francolite with phosphatized tests of 

 Foraminifera (originally consisting of calcite), skeletal apatite of marine verte- 

 brates, thin flakes of opaline silica derived from diatom frustules, films and 

 grains of glauconite, interspersed dark-colored organic matter and terrigenous 

 minerals. Other widespread deposits of marine phosphorite occur, without 

 association with exceptionally high organic productivity, in shallow areas of the 

 tropical ocean, where calcareous deposits are exposed to relatively warm sea- 

 water, such as on seamounts and on drowned coral reefs; some of these are 



