Phosphorous accumulation and sedimentation

Sedimentary phosphorites are predominantly marine sediments formed by an upwelling of phosphate-rich waters into relatively shallow marine settings, where 

Replacement. CFA may also crystallize through replacement of other minerals, particularly calcite or gypsum. This was first shown in a series of experiments in which powdered calcite was exposed to a phosphatic solution and converted to CFA (Ames 1959). This process has been widely reported in nature with phosphatization of foraminiferal and other shell material. Initially, this process was thought to involve P04 ions replacing COs ions within the structure, a realignment of the Ca, and addition of F. This method is unlikely because of the size differences of the phosphate and carbonate, the coordination difference of Ca between the two minerals, and completely different crystal structures. It has since been shown instead that the calcite or gypsum dissolves first before apatite crystallization (Nathan and Lucas 1972). Dissolution of either of these minerals would increase the Ca/Mg ratio and encourage crystallization of apatite. 

The methods and theories presented above are the most common for apatite crystallization. However, other workers have examined the role of fluoridation (Baturin and Shishkina 1973, Froelich et al. 1983, Ong and Davidson 1992) and still others have correlated phosphate formation at the oxygen minimum zone and humic acids (Slansky 1986). In addition, there are less popular theories not mentioned here, and certainly others still to be proposed. 

Mg-inhibition. Even if sediments or interstitial waters are at or near saturation with phosphate, there are still impediments to CFA crystallization. One of the most significant is the role of Mg-inhibition. Martens and Harris (1970) demonstrated in the laboratory that when phosphate and fluorine are added to a solution with a composition similar to that of ocean water in Mg VCa ratio, apatite does not crystallize. Instead, a Ca-phosphate gel precipitates with a Ca/P ratio of 1.35 compared to the ratio of 1.67 found in apatite. In their experiments, the gel was kept in the proxy seawater solution for 8 months with no appearance of apatite. However, when the gel was placed in a Mg-free solution, apatite quickly formed. Mg-inhibition results from the smaller Mg ion replacing Ca in the apatite structure, the resultant lattice distortion prevents effective crystal growth (Martens and Harris 1970). Later work by Gulbrandsen et al. (1984) showed that given enough time, in their experimentation seven years, apatite would indeed crystallize from the gel despite the Mg in solution. 

MINERALOGY AND CRYSTAL CHEMISTRY OF PHOSPHORITES Carbonate-fluorapatite nomenclature 

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