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1 Laboratorium für chemische und mineralogische Kristallographie, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
2 Institut für Mineralogie und Kristallographie, Universität Wien–Geozentrum, Althanstrasse 14, A-1090 Vienna, Austria
3 Laboratory of Crystallography, ETH Zentrum, CH-8092 Zürich, Switzerland
4 Institut für Mineralogie, Ruhr-Universität Bochum, Germany
5 Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park 2006, South Africa
Correspondence: * E-mail: armbruster{at}krist.unibe.ch
Henritermierite, space group I41/acd, at 293 K a = 12.489(1), c = 11.909(1) Å, Z = 8, with close to end-member composition (Ca2.98Na0.01Mg0.01)VIII(Mn1.95Fe0.01Al0.04)VI[SiO4]2.07[H4O4]0.93 from the N’Chwaning II mine at the Kalahari manganese fields, Republic of South Africa, has been studied by single-crystal X-ray diffraction at 100 and 293 K at ambient pressure and up to 8.7 GPa in a diamond-anvil cell at 293 K. Polarized FTIR spectroscopy at 80 and 293 K was also performed. The Mn3+O6 octahedra display a tetragonally elongated type of Jahn-Teller distortion where the oxygen atoms of the elongated O-Mn-O axis (Mn-O: 2.2 Å) are moderately hydrogen bonded (O-H...O: 2.76 Å) to the H4O4 tetrahedra, which replace 1/3 of SiO4 tetrahedra in an ordered fashion. Thus Jahn-Teller distortion and H4O4 arrangement are coupled and both are responsible for the tetragonal bulk symmetry. The H4O4 tetrahedra have a center-to-O distance of 1.98 Å and the H atoms are slightly above the tetrahedral faces as similarly observed in the synthetic katoite end-member, Ca3Al2[H4O4]3. However, in henritermierite the O-H...O hydrogen bond is considerably bent (ca. 131°) and gives rise to an OH stretching mode at 3432(5) cm–1. Additional, though weak, IR absorptions at 3508(2) and 3553(2) cm–1 may be due to more remote hydrogen-bond acceptors (O-H...O: 3.29 Å) within the H4O4 tetrahedra.
Compressibility data for a third-order Birch-Murnaghan equation of state yield a bulk modulus of K0 = 97.9(9) GPa with a pressure derivative of K' = 5.3(3). The axial compressibilities indicate a pronounced compressional anisotropy which is explained by the orientation of the elongated axes of the Jahn-Teller distorted MnO6 octahedra along the slightly more compressible [100]tetr directions compared to the c-axis. The crystal structure was refined at a pressure of 8.6 GPa. The MnO6 octahedra were observed to show anisotropic compression towards a more isometric shape. Calculated spontaneous strain reveals a trend towards a weaker tetragonal distortion.
If henritermierite is heated above 800 K in air it dehydrates and Mn3+ is partially oxidized to Mn4+. This topotactic transformation leads to a new garnet-like phase of Ia3d symmetry with a = 12.12 Å and of Ca3Mn2.26O2.32[SiO4]2.42 composition in which instead of H4O4 tetrahedra a new disordered octahedral site is occupied by Mn.
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