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 Mn2.67Fe0.33Ca0.05AI1.97Si2.70O10.7F1.0(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 Ia3d were observed.

The structure was refined to a final R of 0.054 (Rwt = 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 F44 substitution for SiO44. Garnets may thus preserve a record of variable F activity in hydrothermal processes.

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