francolite

Francolite is a carbonate-bearing member of the apatite group, essentially a fluorapatite with substantial carbonate substitution. It is a phosphate mineral with calcium as a major constituent, and its crystal structure belongs to the hexagonal system. The carbonate substitutes for phosphate in the lattice (and can also substitute for hydroxide), producing a range of compositions described as carbonate-rich francolite. Fluoride is generally the dominant halogen substituent, with chlorine and hydroxide present in varying amounts.

Geology and occurrence: Francolite forms in marine sedimentary phosphorite deposits, typically in diagenetic environments where phosphate derived from organic matter concentrates alongside carbonate-rich fluids. It is commonly intergrown with minerals such as calcite, dolomite, quartz, and other phosphate phases.

Physical properties and associations: Francolite is usually colorless to white or brownish; crystals are rare and it more often occurs as grains or compact masses. It has a Mohs hardness of about 5 and a specific gravity of roughly 3.1–3.2, with a vitreous luster. Its carbonate content and halogen substitutions influence its optical properties and ore processing characteristics.

Uses and significance: The principal economic importance of francolite is as a source of phosphate for fertilizers. The presence of carbonate and halogens affects ore quality and processing behavior. It is also studied in research on paleoceanography and isotopic compositions in phosphate rocks.

Distribution: Francolite is found in sedimentary phosphate deposits worldwide, with occurrence in major phosphate provinces and nodules.