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Guangyuanite, Pb 3 Cl 3 (Se 4+ O 3 )(OH), a new lead chloride selenite mineral from the El Dragón mine, Potosí, Bolivia

2023; Cambridge University Press; Volume: 88; Issue: 1 Linguagem: Inglês

10.1180/mgm.2023.93

1471-8022

Hexiong Yang, Xiangping Gu, James A. McGlasson, Ronald B. Gibbs, Robert T. Downs,

Solid-state spectroscopy and crystallography

Abstract A new mineral species, guangyuanite, ideally Pb 3 Cl 3 (Se 4+ O 3 )(OH), was discovered from the El Dragón mine, Antonio Quijarro Province, Potosí Department, Bolivia. It occurs as equant crystals. Associated minerals are Co-bearing krut'aite–penroseite, chalcomenite, schmiederite, olsacherite, phosgenite, anglesite, cerussite and franksousaite. Guangyuanite is pale yellow–brown in transmitted light, transparent with white streak and vitreous lustre. It is brittle and has a Mohs hardness of ~3. No parting or cleavage was observed. The calculated density is 7.63 g/cm 3 . An electron microprobe analysis yielded an empirical formula [based on 7 (O + Cl) atoms per formula unit] of Pb 3.02 Cl 3.01 (Se 4+ 0.99 O 3 )(OH), which can be simplified to Pb 3 Cl 3 (Se 4+ O 3 )(OH). Guangyuanite is isostructural with synthetic Pb 3 Br 3 (Se 4+ O 3 )(OH). It is orthorhombic, with space group Pnma and unit-cell parameters a = 11.0003(5), b = 10.6460(5), c = 7.7902 Å, V = 912.31(6) Å 3 and Z = 4. The crystal structure of guangyuanite contains two symmetrically-distinct Pb (Pb1 and Pb2) cations, with Pb1 coordinated by eight anions (4O + 4Cl) and Pb2 only by six anions (3O + 3Cl), forming a marked lopsided coordination typical of Pb 2+ with a stereochemically active 6s 2 lone electron pair. The Se 4+ cation forms a typical [Se 4+ O 3 ] trigonal pyramid. The crystal structure of guangyuanite can be described as consisting of layers of edge-sharing [Pb1O 4 Cl 4 ] polyhedra parallel to (100). These layers are linked together by sharing polyhedral corners (Cl atoms), as well as [Pb2O 3 Cl 3 ] and [Se 4+ O 3 ] groups. Chemically, guangyuanite is one of six lead chloride selenite minerals reported thus far and closely related to orlandiite Pb 3 Cl 4 (Se 4+ O 3 )⋅H 2 O.