合成的Mg3Cr2Si3O12的晶体结构
Crystal structure of synthetic Mg3Cr2Si3O12, the high-pressure Cr end-member of the knorringite-pyrope garnet series
摘要：
镁铝榴石石榴石系列（Nixon等人，1968）的Cr端元-钾镁铝榴石，通常以高比例出现在金伯利岩石榴石中，因此被用于追踪有利于金刚石形成的高压深地条件，在这种条件下，富含钾镁铝榴石的石榴石可以作为包裹体出现。然而，尽管文献中报道了钾镁矾的合成（Ringwood 1977; Irifune等1982; Taran等2004），但纯端元的结构尚未从实验数据中确定。在这项研究中，晶体结构的镁铬榴石，Mg3 Cr2（SiO 4）3，已从高分辨率同步辐射X射线粉末衍射数据记录在环境条件下的多晶样品在12 GPa合成的多砧装置进行了细化。其结构为立方晶系，空间群Ia-3d，a = 11.5935（1），V = 1558.27（4）π ~ 3，dcalc = 3.97 g·cm ~（-3）. Cr-O距离为1.957（2）nm，与EXAFS结果一致。这一短距离表明，与环境压力下的铬3 +-矿物（如铬钒矿）相比，CrO 6八面体受到了很大的压缩（<Cr-O>= 1.99 μ m，Andrut和Wildner 2002）。因此，我们的实验结果证实了早期基于一系列高压Cr-石榴石端元的经验预测（Fursenko 1981），表明Cr-O距离和Cr-O-Si角的值随着压力的增加和二价阳离子尺寸的减小而减小。
英文摘要：
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) Å3, dcalc = 3.97 g.cm-3. The Cr-O distance of 1.957(2) Å 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 Å, 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.
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