MOLYBDOMENITE-P21/c, POLYMORPHOUS WITH MOLYBDOMENITE AND THE NATURAL ANALOGUE OF SYNTHETIC α-PbSe4+O3 , FROM THE EL DRAGON MINE, POTOSI, BOLIVIA

A polymorph of PbSe4+O3, designated as molybdomenite-P2(1)/c, has been discovered from the El Dragon mine, Potosi Department, Bolivia. It occurs as thin-bladed crystals. Associated minerals are Co-bearing krut'aite-penroseite, chalcomenite, and dolomite. Molybdomenite- P2(1)/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/cm(3). An electron microprobe analysis yielded an empirical formula (based on 3 O apfu) of Pb1.00Se1.00O3. Molybdomenite-P2(1)/c is the natural counterpart of synthetic alpha-PbSe4+O3, which is dimorphous with grandfathered P2(1)/m molybdomenite (or beta-PbSe4+O3). It is monoclinic with space group P2(1)/c and unit-cell parameters a = 9.154(8), b = 8.071(6), c = 8.780(7) A, beta = 103.15(8) degrees, and V = 631.6(6) angstrom(3). The crystal structure of molybdomenite-P2(1)/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 threedimensional net system. Topologically, the crystal structure of molybdomenite-P2(1)/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-P2(1)/c.