Crystal Chemistry and Structure of kimzeyite, Ca3Zr2[Al2Si]Σ3O12, henritermierite, Ca3Mn2[(SiO4)2(O4H4)1]Σ3, (OH,F)-spessartine, Mn2+3Al2[(SiO4)2(O4H4,F4)1]Σ3, and hausmannite, Mn3O4
Creators: name: Cruickshank, Laura Ann
Publisher: name: Graduate Studies
Year: 2018
Subjects: subject: crystal chemistry
subject: synchrotron high-resolution powder x-ray diffraction
subject: single crystal x-ray diffraction
subject: rietveld refinement
subject: electron probe microanalysis
subject: optical anisotropy
subject: crystal symmetry
subject: garnet
subject: henritermierite
subject: spessartine
subject: kimzeyite
subject: hausmannite
subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: Geology
subject: Mineralogy

description: This study considers the crystal chemistry of some rare garnet-group minerals of the general formula [8]X3[6]Y2[4]Z3O12 including kimzeyite, Ca3Zr2[Al2Si]Σ3O12, henritermierite, Ca3Mn2[(SiO4)2(O4H4)1]Σ3, and (OH,F)-spessartine, Mn2+3Al2[(SiO4)2(O4H4,F4)1]Σ3. Most garnets have cubic symmetry, with space group Ia3 ̅d, but a few uncommon exceptions have been reported with tetragonal symmetry, and space group I41/acd. A sample of kimzeyite, from type-locality, Magnet Cove, Arkansas, USA, a sample of henritermierite, from Wessels Mine X, Kalahari manganese field, Northern Cape Province, South Africa, and a sample of (OH,F)-bearing spessartine, from Tongbei, near Yunxiao, Fujian Province, China were studied using electron-probe microanalysis (EPMA), back-scattered electron imaging (BSE), single crystal X-ray diffraction (SCXRD) and synchrotron high-resolution powder X-ray diffraction (HRPXRD). For kimzeyite, structural Rietveld refinements confirmed cubic space group Ia3 ̅d and achieved reduced χ2 and overall R(F2) values of 1.840 and 0.0647, respectively. The kimzeyite sample contains an intergrowth of two cubic phases that began as oscillatory growth zoning, with later fluid-enhanced dissolution and re-precipitation giving rise to patchy intergrowths. For henritermierite and (OH,F)-bearing spessartine, the SCXRD structure refinements confirmed tetragonal space group I41/acd, and produced a goodness of fit on F2 of 1.209 and 1.232 for henritermierite and (OH,F)-spessartine, respectively. In henritermierite, the deviation of unit-cell parameters from cubic symmetry is significant (a = 12.4908(2) Å, c = 11.6446(2) Å, c/a = 0.9534). Tetragonal henritermierite has a vacant Z2 site that contains the substituent O4H4 tetrahedron. The H atom is bonded to an O3 atom (O3 – H3) = 0.73(2) Å. Because of O2 – Mn3+ – O2 Jahn-Teller elongation of the Mn3+O6 octahedron, a weak hydrogen bond is formed to the under-bonded O2 atom. This causes the large deviation from cubic symmetry. In (OH,F)-spessartine, the Z2 site is fully occupied, but the Z1 site contains vacancies. The Z1 and Z2 sites occupied by Si atoms are surrounded by four O atoms, as seen in cubic garnets. When the Z site is vacant, a larger [(O2H2)(F2)] tetrahedron is formed, and is similar to the O4H4 tetrahedron in hydrogarnets. These results indicate a new possible end-member: Mn2+3Al2[(SiO4)2(O2H2)0.5(F2)0.5]Σ3, which remains unknown. Finally, this study also examines a tetragonal spinel mineral, hausmannite, ideally Mn3O4. The sample comes from the henritermierite specimen previously described, and has tetragonal space group I41/amd. The SCXRD structure refinement confirmed the tetragonal symmetry, and produced a goodness of fit on F2 of 1.144 and R indices of R1 = 0.0277 and wR2 = 0.0559. Henritermierite and hausmannite spinel occur as an intergrowth, observed in a BSE image. Hausmannite is thought to be an original mineral from which henritermierite was formed by a reaction that includes quartz, SiO2, calcite, CaCO3, and H2O.