Mineralogical and geochemical behavior of heated natural carbonate-apatite of the Ypresian series, Maknassy-Mezzouna basin, central Tunisia

Ypresian phosphate deposits of the Meknassy-Mezzouna basin (central part of Tunisia) correspond to an alternation of thick phosphatic layers and dolomitic marls. The present work aimed to follow the mineralogical and geochemical behavior of natural phosphate during thermal treatment. The representative sample of phosphate was investigated using XRD, chemical and infrared analyses, in addition to crystallite size computed from XRD patterns. The data obtained show that the sample is mostly constituted of carbonate fluorapatite associated with dolomite, calcite, opal CT, quartz, feldspars and few amounts of all clay minerals. XRD data of the heated samples showed a gradual removal of calcite, dolomite, opal CT, quartz and feldspars and a progressive concentration of carbonate-fluorapatite. This later is featured by slight shifting of their narrow XRD peaks towards low angles due to the increase of the unit cell volume of carbonate fluorapatite. This behavior is due to high PO43− content, which partially replaced by CO32−, rather than, the persistence of CO32− ions, which related to calcite mineral. This idea is confirmed by intense characteristic bands of PO43− ions recognized in infrared spectra of samples after heat treatment against large bands of CO32− of intact sample.