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

Henritermierite, space group I4(1)/acd, at 293 K a = 12.489(1), c = 11.909( 1) Angstrom, Z = 8, with close to end-member composition (Ca2.98Na0.01Mg0.0 1)(VIII)(Mn1.95Fe0.01Al0.04)(VI)[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 an d up to 8.7 GPa in a diamond-anvil cell at 293 K. Polarized FTIR spectrosco py at 80 and 293 K was also performed. The Mn3+O6 octahedra display a tetra gonally elongated type of Jahn-Teller distortion where the oxygen atoms of the elongated O-Mn-O axis (Mn-O: 2.2 Angstrom) are moderately hydrogen bond ed (O-H . . .O: 2.76 Angstrom) to the H4O4 tetrahedra, which replace 1/3 of SiO4 tetrahedra in an ordered fashion. Thus Jahn-Teller distortion and H4O 4 arrangement are coupled and both are responsible for the tetragonal bulk symmetry. The H4O4 tetrahedra have a center-to-O distance of 1.98 Angstrom and the H atoms are slightly above the tetrahedral faces as similarly obser ved in the synthetic katoite end-member, Ca3Al2[H4O4](3) However, in henrit ermierite the O-H . . .O hydrogen bond is considerably bent (ca. 131 degree s) and gives rise to an OH stretching mode at 3432(5) cm(-1). Additional, t hough weak, IR absorptions at 3508(2) and 3553(2) cm(-1) may be due to more remote hydrogen-bond accepters (O-H . . .O: 3.29 Angstrom) within the H4O4 tetrahedra. Compressibility data for a third-order Birch-Murnaghan equation of state yi eld a bulk modulus of K-0 = 97.9(9) GPa with a pressure derivative of K' = 5.3(3). The axial compressibilities indicate a pronounced compressional ani sotropy 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 ref ined at a pressure of 8.6 GPa. The MnO6 octahedra were observed to show ani sotropic 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 p artially oxidized to Mn4+. This topotactic transformation leads to a new ga rnet-like phase of Ia3d symmetry with a = 12.12 Angstrom and of Ca3Mn2.26O2 .32[SiO4](2.42) composition in which instead of H4O4 tetrahedra a new disor dered octahedral site is occupied by Mn.