Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal Mn3+ hydrogarnet, henritermierite
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Thomas Armbruster
,
Thomas Kohler
Abstract
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
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Self diffusion of Si and O in dacitic liquid at high pressures
- The effect of anhydrous composition on water solubility in granitic melts
- Stability and phase relations of Ca[ZnSi3]O8, a new phase with feldspar structure in the system CaO-ZnO-SiO2
- Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcán San Pedro, Chilean Andes: Evidence for a solvus relation between phlogopite and aspidolite
- The influence of T, aSiO₂, and fO₂ on exsolution textures in Fe-Mg olivine: An example from augite syenites of the Ilimaussaq Intrusion, South Greenland
- An experimental study of the external reduction of olivine single crystals
- Determination of site population in olivine: Warnings on X-ray data treatment and refinement
- Structural properties of ferromagnesian cordierites
- A calorimetric study of zoisite and clinozoisite solid solutions
- Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentière, France)
- Experimental mixtures of smectite and rectorite: Re-investigation of “fundamental particles” and “interparticle diffraction”
- Hydrothermal reactivity of Lu-saturated smectites: Part I. A long-range order study
- Hydrothermal reactivity of Lu-saturated smectites: Part II. A short-range order study
- Pulsed field gradient proton NMR study of the self-diffusion of H2O in montmorillonite gel: Effects of temperature and water fraction
- Structural environment and oxidation state of Mn in goethite-groutite solid-solutions
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- Electric field gradient tensors at the aluminum sites in the Al2SiO5 polymorphs from CCD high-resolution X-ray diffraction data: Comparison with 27Al NMR results
- Sodium cation dynamics in nitrate cancrinite: A low and high temperature 23Na and 1H MAS NMR study and high temperature Rietveld structure refinement
- O-D…O bond geometry in OD-chondrodite
- Refinement of hydrogen positions in synthetic hydroxyl-clinohumite by powder neutron diffraction
- In situ dehydration of yugawaralite
- Molecular dynamics simulation of phase transitions and melting in MgSiO3 with the perovskite structure—Comment
- Reply to Comment on “Molecular dynamics simulation of phase transitions and melting in MgSiO3 with the perovskite structure”
