Local Ca-Mg distribution of Mg-rich pyrope-grossular garnets synthesized at different temperatures revealed by Si-29 MAS NMR spectroscopy

Pyrope-grossular solid solutions, (Mg,Ca)(3)Al2Si3O12, of composition Py(85 )Gr(15) 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 chara cterized using optical, microprobe, and X-ray methods and their Ca-Mg distr ibutions were investigated using Si-29 MAS NMR spectroscopy. The syntheses produced 100% garnet except for those undertaken at 1000 degrees C, where s mall amounts (up to 3%) of clinopyroxene were present. X-ray powder refinem ents showed differences up to 0.01 Angstrom in the unit-cell dimension of t he garnets synthesized at the three different temperatures. A general decre ase of the X-ray diffraction line widths with increasing synthesis temperat ure is observed. The Si-29 NMR spectra of Py(85)Gr(15) show little change a s a function of synthesis temperature. In the spectra of Py(75)Gr(25) small but measurable changes in the relative peak intensities, depending upon sy nthesis temperature, were observed with a more random cation distribution c orresponding to a higher synthesis temperature. None of the garnets investi gated has a completely random Ca-Mg distribution. The reduction in configur ational entropy compared to the disordered state is estimated to be less th an 2 J/mol.K.