The crystal structure of bartelkeite, with a revised chemical formula, PbFeGeVI(Ge2IVO7)(OH)2{middle dot}H2O, isotypic with high-pressure P21/m lawsonite

Abstract

Bartelkeite from Tsumeb, Namibia, was originally described by Keller et al. (1981) with the chemical formula PbFeGe 3 O 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 VI Ge 2 IV O 7 (OH) 2 ·H 2 O (Z = 2). Most remarkably, bartelkeite is isostructural with the high-pressure P2 1 /m phase of lawsonite, CaAl 2 Si 2 O 7 (OH)·H 2 O, which is only stable above 8.6 GPa and a potential host for H 2 O in subducting slabs. Its structure consists of single chains of edge-sharing FeO 6 and Ge 1 O 6 octahedra parallel to the c-axis, cross-linked by Ge 22 O 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 2 O 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.