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Stabilization of trivalent Mn in natural tetragonal hydrogarnets on the join ‘hydrogrossular’–henritermierite, Ca₃Mn³⁺₂ [SiO₄]₂[H₄O₄]

Department of Mineralogy, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

View larger version (110K): [in a new window]   FIG. 1. BSE image of a zoned Mn3+-bearing ‘hydrogrossular’ from the Wessels mine. The grain displays a core (light grey) richer in Mn than the broad rim (dark grey). In addition, a thin complex intermediate zone (white) slightly enriched in Fe is observed.

View larger version (28K): [in a new window]   FIG. 2. Polarized optical absorption single-crystal spectra of the close to end-member henritermierite (sample #97) from the N’Chwaning II mine. Positions 1–4 indicate band peaks and shoulders. The absorber thickness was 19 µm.

View larger version (31K): [in a new window]   FIG. 3. E||O optical spectra of the four investigated tetragonal hydrogarnet samples. Numbers attached to each spectrum indicate the molar fraction of henritermierite in the corresponding sample. Absorber thickness was 43 µm for all samples except for crystal #97, for which it was 19 µm.

View larger version (27K): [in a new window]   FIG. 4. Fitted optical absorption spectra of henritermierite (sample #97). The thicker solid line represents the experimental data and the thinner lines show the four fitted Mn3+ bands as well as the fitted UV-background absorption.

View larger version (22K): [in a new window]   FIG. 5. Correlation plot of the net integral absorption of the absorption envelope at ~21500 cm–1 (A2+A3+A4; filled squares) and the absorption band at ~12500 cm–1 (A1; filled circles) in E||O spectra with the Mn3+ content. The lines represent linear least-square fits.