Structure refinements of two natural pyromorphites, Pb₅(PO₄)₃Cl, and crystal chemistry of chlorapatite group, M₅(PO₄)₃Cl

Abstract

X-ray single-crystal structure analyses of green and brown pyromorphites, Pb₅(PO₄)₃Cl, from Kamioka Mine and Mikura Mine both in Japan, respectively, were performed at room temperature. They are hexagonal, P6₃/m, and the cell dimensions are 10.022(3) Å and 9.993(2) Å for a axis, and 7.348(9) Å and 7.334(6) Å for c axis for green and brown ones, respectively. The structures were refined to R's of 0.031 for green and 0.029 for brown ones. No significant difference was observed between their structures. In both of the pyromorphites, the site occupancies of Pb atoms which have two crystallographically independent sites, M(1) and M(2), are slightly larger at M(2) than at M(1), and those of impurities (minor cations) are slightly larger at M(1) than at M(2).

The thermal ellipsoids of the Cl atoms in the two pyromorphites were nearly isotropic. In the chlorapatite group, M₅(PO₄)₃Cl, the amplitude along [001] of the thermal ellipsoid of Cl atom becomes quantitatively large with decreasing the value of the ionic radius of M atom, r_M²⁺.