Francolites, carbonate fluorapatites, occur in temporally and spa- tially related sedimentary phosphorite deposits that range in age from the Proterozoic to the Neogene. Previous mineralogical studies of francolites have established that systematic relationships exist between chemical composition and crystdlographic data. The unit-cell dimensions and indices of refraction of francolites decrease with increasing substitution of CO;’ for POT3 in the francolite structure. Accompanying cation substitutions of Na+’ and Mg+’ for Ca+’ also are significant. Fluorine contents of more than 2 moles/unit-cell also are typical of francolites, and the ‘excess’ fluorine is related to the basic CO;’ for PO;’ substitution.
Francolites are thermodynamically metastable with respect to fluorapatite. Studies of francolites from deposits of various geologi- cal ages demonstrate systematic trends in chemical composition and crystallographic properties with time. Francolites in rocks of Palae- ozoic or older ages usually have compositions close to fluorapatite. Mesozoic and younger francolites have wide range of compositions that vary with the type and intensity of post-depositional alteration. These alteration processes can result in mobilization of cations and anions, decarbonation of the francolite, and changes in crystal- lographic properties. The extent and intensity of alteration proc- esses often are restricted by variations in francolite composition and associated accessory minerals.