Local relaxation around [6]Cr3+ in synthetic pyrope-knorringite garnets, [8]Mg3[6](Al1-X CrX3+)2 [4]Si3O12, from electronic absorption spectra

Pyrope-knorringite garnets, Mg-3(Al1-x Cr-x(3+))(2)Si3O12 with X = 0.25, 0.50, and 1.00, were synthesized between 9 and 16 GPa and 1300 and 1600 degreesC, using multianvil high-pressure techniques. The garnets with X = 0.25 and 0.50 are fine-grained, pink and violet in color. The end-member knorringites with X = 1.00 are black when compact and gray when coarse-grained. The fine powder is greenish gray in natural light and pale pink under a tungsten lamp. Powder remission spectra in the wavenumber range 30 000-10 000 cm(-1) on finely powdered crystals were measured by two different methods: (I.) by the use of a small integrating sphere for small samples or (II.) microscope-spectrometric measurement using diffusely reflected radiation from a 45degrees illuminated microsample. Both methods yielded similar diffuse reflectance spectra. The following crystal-field parameters of Cr-[6](3+) were determined for garnets with X = 0.25, 0.50, 1.00: 10 Dq = 17 856, 17596, 17 286 cm(-1); and B = 654, 677, 706 cm(-1); nephelauxetic ratio beta = (B-field/B-free) = 0.71, 0.74, 0.77. The beta-values indicate decreasing covalency of the Cr-O bond with increasing Cr content. The 10 Dq value for XCr3+ = 1.00, together with the mean Cr-O distance in end-member knorringite, 1.96 Angstrom (Novak and Gibbs 1971), were used to calculate from the spectral data, local mean Cr-O distances (Langer 2001a) as a function of composition. The results indicate relatively strong local site relaxation with a value of epsilon = 0.77.