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Henritermierite, space group I4 1 /acd, at 293 K a = 12.489(1), c = 11.909(1) Å, Z = 8, with close to end-member composition (Ca 2.98 Na 0.01 Mg 0.01 ) VIII (Mn 1.95 Fe 0.01 Al 0.04 ) VI [SiO 4 ] 2.07 [H 4 O 4 ] 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 Mn 3+ O 6  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 H 4 O 4  tetrahedra, which replace 1/3 of SiO 4  tetrahedra in an ordered fashion. Thus Jahn-Teller distortion and H 4 O 4  arrangement are coupled and both are responsible for the tetragonal bulk symmetry. The H 4 O 4  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, Ca 3 Al 2 [H 4 O 4 ] 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 H 4 O 4  tetrahedra.

Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal Mn 3+ hydrogarnet, henritermierite