Molybdomenite-P21/c, Polymorphous with Molybdomenite and the Natural Analogue of Synthetic α-PbSe4+O3, from the El Dragón Mine, Potosí, Bolivia

Abstract

A polymorph of PbSe4+O3, designated as molybdomenite-P21/c, has been discovered from the El Dragón mine, Potosí Department, Bolivia. It occurs as thin-bladed crystals. Associated minerals are Co-bearing krut’aite–penroseite, chalcomenite, and dolomite. Molybdomenite- P21/c is pale yellow in transmitted light and has a greasy luster. It is brittle and has a Mohs hardness of 3. No cleavage was observed. The calculated density is 7.034 g/cm3. An electron microprobe analysis yielded an empirical formula (based on 3 O apfu) of Pb1.00Se1.00O3. Molybdomenite-P21/c is the natural counterpart of synthetic α-PbSe4+O3, which is dimorphous with grandfathered P21/m molybdomenite (or β-PbSe4+O3). It is monoclinic with space group P21/c and unit-cell parameters a = 9.154(8), b = 8.071(6), c = 8.780(7) Å, β = 103.15(8) °, and V = 631.6(6) Å3. The crystal structure of molybdomenite-P21/c consists of two symmetrically distinct Pb (Pb1 and Pb2), two Se (Se1 an Se2), and six O atoms. Each Se4+ cation is bonded to three O atoms, forming a typical (Se4+O3)2− trigonal pyramid. Both Pb1 and Pb2 atoms are coordinated by nine O atoms. Each [PbO9] polyhedron is linked to six (SeO3)2− pyramids, three by sharing edges and three by sharing corners. The [PbO9] polyhedra are connected to one another through sharing edges and by (SeO3)2− groups, giving rise to a complex three-dimensional net system. Topologically, the crystal structure of molybdomenite-P21/c is identical to that of molybdomenite. Raman spectroscopic measurements reveal that there is only one strong peak centered at 790 cm−1, attributable to the Se4+–O symmetric stretching vibrations within the (SeO3)2− groups, for molybdomenite, whereas there are two such strong peaks, one at 788 cm−1 and the other at 804 cm−1, for molybdomenite-P21/c.