Abstract

Bartelkeite from Tsumeb, Namibia, was originally described by Keller et al. (1981) with the chemical formula PbFeGe3O8. By means of electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy, we examined this mineral from the type locality. Our results show that bartelkeite is monoclinic with space group P21/m, unit-cell parameters a = 5.8279(2), b = 13.6150(4), c = 6.3097(2) Å, β = 127.314(2)°, and a revised ideal chemical formula PbFeGeVIGe2IVO7(OH)2·H2O (Z = 2). Most remarkably, bartelkeite is isostructural with the high-pressure P21/m phase of lawsonite, CaAl2Si2O7(OH)·H2O, which is only stable above 8.6 GPa and a potential host for H2O in subducting slabs. Its structure consists of single chains of edge-sharing FeO6 and Ge1O6 octahedra parallel to the c-axis, cross-linked by Ge22O7 tetrahedral dimers. The average <Ge-O> bond lengths for the GeO6 and GeO4 polyhedra are 1.889 and 1.744 Å, respectively. The Pb atoms and H2O groups occupy large cavities within the framework. The hydrogen bonding scheme in bartelkeite is similar to that in lawsonite. Bartelkeite represents the first known mineral containing both 4- and 6-coordinated Ge atoms and may serve as an excellent analog for further exploration of the temperature-pressure-composition space of lawsonite.

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