Compressibility of mimetite and pyromorphite at high pressure

High pressure X-ray diffraction experiments on mimetite [Pb ₅(AsO₄)₃Cl] and pyromorphite [Pb ₅(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: K₀ = 46(7) GPa, V₀ = 680(2) ų, and K₀^′ = 15(4) for mimetite; K₀ = 44(5) GPa, V₀ = 636(1) ų, and K₀^′ = 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.