gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1966.036.276.06 Experimental Studies of Apatite Crystallization in Parts of the System CaO-P2O5-H2O at 1000 Bars Biggar G. M. B.Sc., Ph.D. Grant Institute of Geology, University of Edinburgh 12 1966 36 276 1110 1122 2 5 1966 Copyright © 1966, The Mineralogical Society 1966 Summary

Solid-liquid-vapour phase equilibria for the join Ca(OH)2-Ca3(PO4)2-H2O at 1000 bars are determined in the temperature range 700 to 950°C. The isobaric invariant equilibrium portlandite + hydroxyapatite → liquid on the join Ca(OH)2-Ca3(PO4)2 involves a liquid with the composition 96 wt. % Ca(OH)2, 4 wt. % Ca2(PO4)2 and occurs at 765°C. The isobaric invariant equilibrium portlandite + hydroxyapatite + vapour → liquid on the join Ca(OH)2-Ca3(PO4)2-H2O is encountered at 735°C, and involves a liquid containing 92 wt. % Ca(OH)2, 4 wt. % Ca3(PO4)2, and 4 wt. % H2O. The apatites are shown by X-ray diffraction techniques to vary in composition with changes in the bulk composition from which crystallization occurred.

The apatite liquidus is steep; apatite is the first mineral to crystallize from calcium-hydroxide-rich liquids containing as little as 4 wt. % Ca2(PO4)2 (1·8 % P2O5) at temperatures as low as 735°C in the presence of vapour at 1000 bars pressure; and rapid crystal settling of apatite was experimentally noted. These observations suggest that concentrations of apatite in carbonatites are the result of crystal accumulation from liquids relatively poor in P2O5.