高铬刚玉:湖南沅水金刚石包裹体中发现的一种高压矿物

A species of high pressure mineral:The high-chromium corundum(ruby) as an inclusion in diamond from the Yuanshui,Hunan,China

对湖南沅水金刚石中的包裹体矿物含铬刚玉进行了矿物学研究。确认含铬刚玉中铬的最高质量分数可达19.31%。根据EPMA分析,高铬刚玉的晶体化学分子式为(Al1.683,Cr0.298,Fe0.010,Mn0.008,Ti0.004,Si0.001)2.004O3,获得了该样品的X射线粉末衍射图谱,并在此基础上进行了Rietveld法的全谱拟合修正,得到的晶体学参数为:空间群R-3c,a=0.476 5nm,c=1.300 6nm,Z=6,三价的铬和铝离子在同一晶体化学位置。这些刚玉尽管从晶体学参数来看已回归常压下状态,但高的铬含量说明它曾经历过高压的环境,因而可以认为它是一种带有高压环境信息的标志性矿物。该区金刚石包体中刚玉的发现说明湖南沅水金刚石有来源于榴辉岩捕虏体的可能。

The mineralogy of a diamond inclusion that is the corundum containing high chromium from Yuanshui, Hunan, China has been investigated. It is confirmed that the highest content of chromium in the chromium-contained corundum may reach 19. 31%. According to the EPMA analysis, the formula of the high-chro- mium corundum is（All. 68a, Cr0. 298, Fe0. 010, Mno. 008, Tio. 004, Si0. 00, ）2. 004 03. Based on the X-ray powder diffraction pattern and the method of full spectrum Rietveld fitted refinement, the crystallographic parameters were obtained as the following： R-3c, a=O. 4765 nm, c=1. 3007 nm, Z=6, and the trivalent chromium and aluminum ions oceupy the same crystal-chemical position. Although the crystallographic parameters of the corundum suggest that it has regressed to the ordinary pressure state, the high chrome content implied that it had undergone a high-pressure environment. Thus, it can be regarded as a kind of index mineral indicating a high-pres- sure environment. The discovery of high Cr content of the Yuanshui corundum as a diamond inclusion suggested that the diamonds in this area were possibly from eclogite xenoliths.