Crystal-structure refinement of a F-bearing spessartine garnet

Abstract

The crystal structure of a F-bearing spessartine from the Henderson molybdenite deposit in Clear Creek County, Colorado, has been investigated by single-crystal X-ray methods. Electron-microprobe and infrared spectroscopic data indicate a composition of Mn_(2.67)Fe_(0.33) Ca_(0.05)Al_(1.97)Si_(2.70)O_(10.7)F_(1.0)(OH)_(0.35) with up to 3.8 wt% F. The garnet has a cubic unit-cell edge of 11.628(1)Å. No reflections violating space group Iɑ3d were observed. The structure was refined to a final R of 0.054 (R_(wt) = 0.038) using 2682 intensity measurements that reduced to 550 symmetry-independent reflections, of which 493 were greater than three times the standard deviation of the background count rate. The final oxygen positional parameters are x = 0.0336(1), y = 0.0481(1); and z = 0.6520(1). The total Z-site occupancy was found to be 0.90(1) Si and is consistent with the chemical analysis that indicates a Si cation deficiency of approximately 10%. The Si-O distance of 1.656(1)Å is longer than observed for fully occupied Si sites in spessartines, indicating that F substitutes for O in Si-absent tetrahedra, similar to the OH substitution in hydrogrossular. The study indicates that it is not necessary to have the strong polarity of OH^- to achieve a F^(4-)_4 substitution for SiO^(4-)_4. Garnets may thus preserve a record of variable F activity in hydrothermal processes.

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