The principal phosphate mineral in sedimentary phosphorites is carbonate fluorapatite of the variety francolite. Francolites exhibit several systematic isomorphous substitutions including sodium and magnesium for calcium, carbonate for phosphate, fluorine for oxygen, and hydroxyl for fluorine. These substitutions result in measurable variations of the crystallographic properties including unit-cell parameters and optical indices. Statistical models have been developed to relate compositional and crystallographic variations in the various francolites. Previous work has shown that francolite compositions are metastable with respect to fluorapatite. The effects of weathering, metamorphism and geological time causing systematic changes in francolite compositions are used as examples of these relationships.