Compressibility of mimetite and pyromorphite at high pressure

Abstract

High pressure X-ray diffraction experiments on mimetite [Pb5(AsO4)3Cl] and pyromorphite [Pb5(PO4)3Cl] were performed up to 14.1 and 14.9 GPa, respectively, at 300 K, using in situ angle-dispersive X-ray diffraction and a diamond anvil cell. No phase transition of mimetite and pyromorphite was observed within the experimental pressure range. Fitting the P–V data under hydrostatic stress conditions with a third-order Birch–Murnaghan Equations of State (BM-EoS) we obtained: K 0=46(7) GPa, V 0=680(2) Å3, and K 0′=15(4) for mimetite; K 0=44(5) GPa, V 0=636(1) Å3, and K 0′=15(3) for pyromorphite. The axial compressibility was also calculated with a third-order ‘linearized’ BM-EoS. We obtained K 0a :K 0c =1:1.00 for mimetite, and K 0a :K 0c =1:1.28 for pyromorphite, indicating that mimetite and pyromorphite are elastically isotropic and slightly anisotropic, respectively. Comparing the previous equation of state data of vanadinite [Pb5(VO4)3Cl] with the current results of mimetite [Pb5(AsO4)3Cl] and pyromorphite [Pb5(PO4)3Cl], we found that the substitution of V5+ by As5+ and P5+ has an insignificant effect on the bulk modulus, but has a greater effect on the axial parameters, compression ratio, and elastic anisotropy.