Structural CO2 in the apatite of the late cretaceous phosphorite resources in Egypt:: Effect on the crystal chemistry and geologic implications

Phosphorite deposits in Egypt, known as the Duwi Formation, are a part of the Middle East to North Africa phosphogenic province of late Cretaceous to Paleogene age. Based on the petrographical observation, the phosphatic grains in the phosphorites are classified into phosphatic mudclasts and phosphatic bioclasts. Both of them are composed of francolite. The structural CO2 contents in the francolite range from 3.3 to 7.2 % with an average of 5.3 %. Results indicated that the substitution with CO32- Of PO43- in the francolite decreases the unit cell volume and a-cell dimension, and increases the c/a ratio. Effect is more obvious in the a-cell dimension; therefore, it is more significant in distinction between the different apatite species. Lack of covariance between structural CO2 contents in the francolite and the carbonate minerals contents may render the supposition that the phosphorites formed as a result of replacement of preexisting calcareous sediments is doubtful. Similarity in CO2 content in both weathered and fresh samples indicates that the structural CO2 content in the phosphorites is not affected by weathering, and reflects the conditions and CO2 concentration of the depositional environment. Similarity in mineralogy and CO2, contents in the different phosphatic grains and higher CO2 content in the Egyptian phosphorites compared with the authigenic phosphates of Peru margin, which formed by the same mechanism as the Duwi phosphorites, suggest that the phosphatic grains in the Duwi Formation were francolitized during diagenesis by introducing CO2 from the surrounding pore water and diagenesis took place at an elevated temperature. Scattered values of structural CO2 contents suggest the reworking origin of the phosphatic grains in the late Cretaceous phosphorites in Egypt.