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Knorringite, the Cr-end-member of the pyrope garnet series (Nixon et al.1968), often occur in high proportions in kimberlite garnets and is thus usedfor tracing high-pressure deep-earth conditions favorable to the formation ofdiamonds, in which knorringite-rich garnet can occur as inclusions. However,although the synthesis of knorringite is reported in the literature (Ringwood1977; Irifune et al. 1982; Taran et al. 2004), the structure of the pureend-member has not been yet determined from experimental data. In this study,the crystal structure of knorringite, Mg3Cr2(SiO4)3, has been refined from highresolution synchrotron X-ray powder diffraction data recorded under ambientconditions on a polycrystalline sample synthesized at 12 GPa in a multi-anvilapparatus. The structure is cubic, space group Ia-3d, a = 11.5935(1), V =1558.27(4) {\AA}3, dcalc = 3.97 g.cm-3. The Cr-O distance of 1.957(2) {\AA} isconsistent with EXAFS results on the same sample. This short distance indicatesa substantial compression of the CrO6 octahedron, compared to ambient pressureCr3+-minerals such as uvarovite (<Cr-O> = 1.99 {\AA}, Andrut and Wildner 2002).Our experimental results thus confirm early empirical predictions based onseries of high-pressure Cr-garnet end-members (Fursenko 1981), showing that thevalues of the Cr-O distance and the Cr-O-Si angle decrease with theaugmentation of pressure and with the diminution of the size of the divalentcation.

Crystal structure of synthetic Mg3Cr2Si3O12, the high-pressure Cr end-member of the knorringite-pyrope garnet series