Abstract

Members of the apatite family are the most common phosphate minerals. Apatites in igneous and metamorphic rocks generally approach fluorapatite in composition. In sedimentary rocks, the apatite is usually the carbonate fluorapatite variety, francolite, which produces x-ray diffraction patterns similar to but distinctive from those of fluorapatite. Variations in francolite compositions and physical properties have also been identified. The substitution of CO₃⁻² for PO₄⁻³, from zero to about 25% of phosphate sites on a 1:1 basis, influences all other changes in composition and crystallography of francolites. Statistical models have reduced the francolite composition to a single parameter problem, the dominant factor being the variation in unit-cell a-value with increasing CO₃⁻² substitution. Within a deposit, variations in francolite composition with stratigraphical position and particle size may indicate the extent and type of post-depositional alteration. Replacements of SO₄⁻² for PO₄⁻³ and Ca⁺² by Na⁺² and Mg⁺² are also discussed but these are of more limited significance.