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Crystal-structure refinement of a F-bearing spessartine garnet

Smyth, Joseph R. and Madel, Robin E. and McCormick, Tamsin C. and Munoz, James L. and Rossman, George R. (1990) Crystal-structure refinement of a F-bearing spessartine garnet. American Mineralogist, 75 (3-4). pp. 314-318. ISSN 0003-004X.

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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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Rossman, George R.0000-0002-4571-6884
Additional Information:© 1990 Mineralogical Society of America. Manuscript received September 5, 1989. Manuscript accepted December 2, 1989. The X-ray data collection for the structure refinement was conducted by Curt Haltiwanger, Department of Chemistry, University of Colorado. Jeffrey Swope (University of Colorado) helped with the data collection for the verification of the space group. John W. Drexler (University of Colorado) is thanked for numerous microprobe analyses. This study was supported by NSF grants EAR-8709224 and 8904164 to J .R.S. and EAR-8618200 to G.R.R. We thank Climax Molybdenum Company for permission to obtain garnet specimens from drill core at the Henderson mine, and R. B. Carten for help with the sampling. R. Madel thanks the Undergraduate Research Opportunities Program (UROP) at the University of Colorado for support. We thank George A. Lager of the University of Louisville for a constructive review of the manuscript.
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Official Citation: Joseph R. Smyth, Robin E. Madel, Tamsin C. McCormick, James L. Munoz, and George R. Rossman Crystal-structure refinement of a F-bearing spessartine garnet American Mineralogist, April 1990, v. 75, p. 314-318
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ID Code:38517
Deposited By: Ruth Sustaita
Deposited On:15 May 2013 16:54
Last Modified:26 Nov 2019 11:15

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