The reactions of secondary lead orthophosphate with approximately 10-1 M sodium fluoride and sodium bromide solutions have been investigated at 25°C. Interpretation of the solubility data resulted in solubility product constants for fluoropyromorphite and bromopyromorphite of 10-71.6 and 10-78.1, respectively. According to these constants, the stability sequence for lead pyromorphites is Pb5(PO4)3Cl > Pb5(PO4)3Br > Pb5(PO4)3OH > Pb5(PO4)3F. The derived free energy data have been used to evaluate the respective stabilities of fluoro-pyromorphite and bromopyromorphite within the systems PbF2-PbO-P2O5-H2O and PbBr2-PbO-P2O5-H2O and to predict the equilibrium behavior of the Pb5(PO4)3F-Pb5(PO4)3OH solid solution under aqueous conditions. © 1973.
Nriagu, J. O. (1973). Lead orthophosphates-III. Stabilities of fluoropyromorphite and bromopyromorphite at 25°C. Geochimica et Cosmochimica Acta, 37(7), 1735–1743. https://doi.org/10.1016/0016-7037(73)90159-2