Structure refinement of a synthetic knorringite, Mg3(Cr0.8Mg0.1Si0.1)2(SiO4)3

The crystal structure of a polycrystalline knorringite, Mg3(Cr0.8Mg0.1Si0.1)2(SiO4)3, synthesized at 11 GPa and T = 1500 °C in a multi-anvil press, has been refined from high-resolution synchrotron X-ray powder diffraction data. The structure is cubic, space group Ia3d, a = 11.5935(1) Å, V = 1558.27(4) Å3, Dcalc = 3.79 g/cm3. The structural formula indicates that knorringite is susceptible to majorite substitution at these synthesis conditions. The Cr-O distance, 1.959(7) Å, 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 [4]Si-O-Cr angle decrease with decreasing synthesis pressure and with increasing X-cation size.