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University of Cambridge, Department of Earth Sciences, Cambridge, United Kingdom
Pyrope-grossular solid solutions, (Mg,Ca) 3 Al 2 Si 3 O 12 , of composition Py 85 Gr 25 and Py 75 Gr 25 were synthesized at 1000, 1200, and 1400 degrees C and 30 kbars in a piston-cylinder device. The synthetic garnets were characterized using optical, microprobe, and X-ray methods and their Ca-Mg distributions were investigated using 29 Si MAS NMR spectroscopy. The syntheses produced 100% garnet except for those undertaken at 1000 degrees C, where small amounts (up to 3%) of clinopyroxene were present. X-ray powder refinements showed differences up to 0.01 Aa in the unit-cell dimension of the garnets synthesized at the three different temperatures. A general decrease of the X-ray diffraction line widths with increasing synthesis temperature is observed. The 29 Si NMR spectra of Py 85 Gr 15 show little change as a function of synthesis temperature. In the spectra of Py 75 Gr 25 small but measurable changes in the relative peak intensities, depending upon synthesis temperature, were observed with a more random cation distribution corresponding to a higher synthesis temperature. None of the garnets investigated has a completely random Ca-Mg distribution. The reduction in configurational entropy compared to the disordered state is estimated to be less than 2 J/mol.K.
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