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.

Keywords:

• mimetite
• pyromorphite
• synchrotron radiation X-ray diffraction
• equation of state

Acknowledgements

We thank Prof. Guohong Gong for his help with the conventional powder X-ray analysis. This investigation was financially supported by the National Natural Science Foundation of China (Grant Nos 41004035, 41274105 and 41274091), Doctor western special fund of the West Light Foundation of The Chinese Academy of Sciences (2011, to Dawei Fan), the DOE/NNSA (DE-FG5209NA29458 to Baosheng Li) and NSF (EAR1045630 to Baosheng Li). This research was partially supported by COMPRES, the Consortium for Materials Properties Research in Earth Sciences under NSF Cooperative Agreement EAR 10-43050.