Letter: The crystal structure of bartelkeite, with a revised chemical formula, PbFeGe ^VI(Ge ₂ ^IVO ₇) (OH) ₂·H ₂O, isotypic with high-pressure P2 ₁/m lawsonite

Bartelkeite from Tsumeb, Namibia, was originally described by Keller et al. (1981) with the chemical formula PbFeGe ₃O ₈. 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 P2 ₁/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 PbFeGe ^VIGe ₂ ^IVO ₇(OH) ₂·H ₂O (Z = 2). Most remarkably, bartelkeite is isostructural with the high-pressure P2 ₁/m phase of lawsonite, CaAl ₂Si ₂O ₇(OH)·H ₂O, which is only stable above 8.6 GPa and a potential host for H ₂O in subducting slabs. Its structure consists of single chains of edge-sharing FeO ₆ and Ge1O ₆ octahedra parallel to the c-axis, cross-linked by Ge2 ₂O ₇ tetrahedral dimers. The average <Ge-O> bond lengths for the GeO ₆ and GeO ₄ polyhedra are 1.889 and 1.744 Å, respectively. The Pb atoms and H ₂O 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.