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Geological Society, London, Special Publications; 1990; v. 52; p. 179-191;
DOI: 10.1144/GSL.SP.1990.052.01.13
© 1990 Geological Society of London

Geological significance of carbonate substitution in apatites: Israeli phosphorites as an example

Y. Nathan1, D. Soudry1 & A. Avigour2

1 Geological Survey of Israel, 30 Malkhei Israel Street, 95501, Jerusalem, Israel
2 Negev Phosphate Company, Tel Aviv 6100, Israel

The extent of carbonate for phosphate substitution in francolite (carbonate-fluorapatite) was measured in 177 selected phosphorite samples of Santonian to Eocene age from Israel. The results show that unaltered apatites from different stratigraphic positions have different CO2 concentrations while those from the same stratigraphic level have a relatively narrow range of concentrations. As a rule phosphorite samples from rocks with a low P2O5 content have a higher carbonate substitution than those from rocks with a high P2O5 content. Apatites from the carbonate-rich Santonian-Campanian Menuha Formation contain on average 5.5% CO2, while the average content of the apatites from the phosphatic Campanian-Maastrichtian Mishash Formation is 3.7%. These changes are probably a reflection of the early diagenetic environment and, more specifically, are a function of the varying composition of the interstitial waters.

Epigenesis and/or weathering bring about a significant decrease in the CO2 concentrations causing an overprint which masks the above picture. Therefore, it is possible to use a CO2 value for geological interpretation only after making a petrographic analysis, in order to distinguish between early diagenesis and late epigenetic processes.





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A. J. G. Notholt
Phosphate rock: factors in economic and technical evaluation
Geological Society, London, Special Publications, 1994; 79: 53 - 65.
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