267 D33. Mineralogy/Crystallography (Posters)

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

COMPARISON OF TETRAGONAL AND MULTI-PHASE CUBIC GARNETS

Paper number: 267-5
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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 Ca2.97{Mn1.95Al0.04Fe0.03}Σ2.02[(SiO4)2.00(O4H4)1.00]Σ3 and (Mn2.82Fe2+0.14Ca0.04)Σ3{Al1.95Fe3+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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-301382

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