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Geological Society, London, Special Publications; 1990; v. 52; p. 23-31;
DOI: 10.1144/GSL.SP.1990.052.01.03
© 1990 Geological Society of London

Mineralogy of sedimentary apatites

G. H. McClellan1 & S. J. Van Kauwenbergh2

1 Department of Geology, University of Florida, Gainesville, Florida 32611, USA
2 Fertilizer Technology Division, International Fertilizer Development Center (IFDC), Muscle Schoals, Alabama 35662, USA

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.





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