The crystal structure of a polycrystalline knorringite, Mg-3(Cr0.8Mg0.1Si0.1)(2)(SiO4)(3), synthesized at 11 Gila and T = 1500 degrees C in a multi-anvil press, has been refined from high-resolution synchrotron X-ray powder diffraction data. The structure is cubic, space group la (3) over bard, a = 11.5935(1) angstrom, V= 1558.27(4) angstrom(3), D-calc = 3.79 g/cm(3). The structural formula indicates that knorringite is susceptible to majorite substitution at these synthesis conditions. The Cr-O distance, 1.959(7) angstrom, is similar to that in Cr-bearing pyrope Mg3Al2(SiO4)(3). This confirms that the magnitude of the Cr-O distance is not responsible for the difference in crystal Field splitting values between green knorringite and red Cr-pyrope. A comparison with the structure of other Cr-garnet end-members shows that the Cr-O distance and the Si--O-Cr angle decrease with decreasing synthesis pressure and with increasing X-cation size.
Juhin, Amélie Morin, Guillaume Elkaim, Erik Frost, Daniel J. Fialin, Michel Juillot, Farid Calas, Georges