COMPARISON OF TETRAGONAL AND MULTI-PHASE CUBIC GARNETS

Most garnets exhibit cubic symmetry, but exceptions with tetragonal symmetry have been reported. Henritermierite is a rare Mn³⁺-bearing hydrogarnet, ideally Ca₃Mn₂[(SiO₄)₂(O₄H₄)₁]_Σ3, with only two known occurrences. It has been reported with tetragonal symmetry and space group I4₁/acd. The tetragonal symmetry has been attributed to Jahn-Teller distortion of the octahedral Mn³⁺ cation. (OH,F)-bearing spessartine is a pyralspite-group garnet, ideally Mn₃Al₂[(SiO₄)₂(O₄H₄,F₄)₁]_Σ3, and has been reported with either cubic or tetragonal symmetry and space group Ia-3d or I4₁/acd. It has been reported that the cause of the tetragonal symmetry is ordering of (OH,F) groups or atoms. This study focuses on the crystal structure and chemical analyses of henritermierite and (OH,F)-bearing spessartine garnets using electron probe microanalysis (EPMA) and synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. EPMA gave near end-member compositions of Ca_2.97{Mn_1.95Al_0.04Fe_0.03}_Σ2.02[(SiO₄)_2.00(O₄H₄)_1.00]_Σ3 and (Mn_2.82Fe²⁺_0.14Ca_0.04)_Σ3{Al_1.95Fe³⁺_0.05}_Σ2[(SiO₄)_2.60(O₄H₄)_0.28(F₄)_0.12]_Σ3 for henritermierite and spessartine, respectively. These tetragonal garnets will be compared with a two-phase cubic kimzeyite garnet. Structural mismatch between the two cubic phases in a crystal gives rise to strain-induced optical anisotropy.