山东、辽宁金伯利岩中石榴石的类型、标型特征及其与金刚石的关系

金伯利岩中的镁铝榴石是寻找金刚石的重要指示矿物。镁铝榴石的Cr2O3、CaO等氧化物含量及其颜色、折光率等物理光学性质可作为这种石榴石的标型特征。 本文将山东、辽宁等地金伯利岩中的石榴石的电子探针分析结果按分子数配分的方法计算了端员分子百分数及其他化学参数。用MPV—1显微光度仪测定了石榴石的透射率色散值,通过计算求得颜色指数。按Dawson等人的分类,本次研究样品以铬镁铝榴石为主,其次是低钙-铬镁铝榴石、钙铬-镁铝榴石和镁铬-钙铬-镁铝榴石;钛镁铝榴石和钙镁铝-铁铝榴石等少量。作者用铬、钙、镁三组份数绘制了石榴石与金刚石关系判别图解,分为五个区(A、B、C、D、E)、落入A区的低钙-铬镁铝榴石、铬镁铝榴石和钙铬-镁铝榴石等与金刚石的形成关系最密切。

Types, Typomorphic Characteristics of Garnet from Kimberlites in Shandong and Liaoning Province and Its Relationship with Diamond

Pyrope in kimberlites is an important indicator mineral in search for diamond,Electron microprobe analyses of garnet from kimberlites in Shandong and Liaoning were used to calculate its end member percentage and other chemical parameters by means of molecular number partitioning (Table 1). The transmittance dispersion values of garnet were determined by MPV-1 type microphotometer, and the color indices were calculated by equivalent ordinate method with equalenergy light source S_E as illumination means (Table 3). In this way, the colors of garnet have been described quantitatively. Aocording to classification by Dawson and Stephens (1975), our studied samples are mainly chrome--pyrope (DG 9), and subordinately knorringite-uvarovite-pyrope (DG 12), uvarovite-pyrope(DG 11) and low-calcium chrome-pyrope (DG 10), with small amounts of titanium pyrope (DG 1) and magnesian almandine(DG 5).A discriminant diagram showing relationship between garnet and diamond was drawn with Cr, Mg and Ca as components (Fig. 7), and this diagram can be divided into five areas (A, B, C, D and E). Pyrope falling into Area A has the closest relationship with diamond. The typomorphic characteristics of such pyrope are as follows.1. Chrome-pyrope(DG 9), low-calcium chrome-pyrope(DG 10) and uvarovite-pyrope(DG 11) are dominant.2. Chemically, it is relatively high in Cr and low in Ca(generally Cr_2O_3=5.73-16.41% and CaO=3.22-6.38%),belonging to pyrope which contains fairly high uvarovite and knorringite members(Table 2).3. Its color is mainly purple and red with various hues and, next, bluish green and yellowish green (Table 3).4. Its refraction indices increase with increasing Cr_2O_3 content, and pyrope with N 1.765-1.779 is most closely related to diamond.5. Genetically, it might have been mainly derived from xenoliths of dunite-harzburgite and websterite.