  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 used
for tracing high-pressure deep-earth conditions favorable to the formation of
diamonds, in which knorringite-rich garnet can occur as inclusions. However,
although the synthesis of knorringite is reported in the literature (Ringwood
1977; Irifune et al. 1982; Taran et al. 2004), the structure of the pure
end-member has not been yet determined from experimental data. In this study,
the crystal structure of knorringite, Mg3Cr2(SiO4)3, has been refined from high
resolution synchrotron X-ray powder diffraction data recorded under ambient
conditions on a polycrystalline sample synthesized at 12 GPa in a multi-anvil
apparatus. 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} is
consistent with EXAFS results on the same sample. This short distance indicates
a substantial compression of the CrO6 octahedron, compared to ambient pressure
Cr3+-minerals such as uvarovite (<Cr-O> = 1.99 {\AA}, Andrut and Wildner 2002).
Our experimental results thus confirm early empirical predictions based on
series of high-pressure Cr-garnet end-members (Fursenko 1981), showing that the
values of the Cr-O distance and the Cr-O-Si angle decrease with the
augmentation of pressure and with the diminution of the size of the divalent
cation.
