Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal Mn³⁺hydrogarnet, henritermierite

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.