新疆库鲁克塔格兴地塔格群中石榴石的矿物学特征研究

摘要

The mineralogical characteristics of garnets from Xinditaga Group, Kuruk Tag, Xinjiang, have been rarely discussed by former researchers.In this paper, the mineralogical features of the red garnets produced in this area were investigated by using hydrostatic methods, refractometer, spectroscope, electron microprobe analysis and X-ray powder diffraction analysis.The crystal forms of garnets are mainly rhombic dodecahedron, icositetrahedron and their combination form.The garnets are bright red to dark red in color, and have abundant fractures and internal crystal inclusions, causing the tremendous decrease of the hosts' transparency.The relatively light elements Ca2+ , Mg2+ and Mn2+ substitute for Fe2+ so that the density of the garnets is lower than the average value of almandine.The strong bands in optical spectra of almandine indicate that Mg2+ rarely replaces Fe2+.With regard to garnets formed in different metamorphic stages, the results of EPMA show that from the garnet's core to rim, the amount of almandine end-member tends to increase, namely the mole fractions of the almandine end-members of samples Garnet 1, Garnet 2, Garnet 3 and Garnet 4 are 63.906％ ～72.408 ％, 65.663 ％ ～ 71.880％, 64.689％ ～ 71.711 ％ and 50.011 ％ ～ 71.780 ％ respectively.Moreover, the content of spessartine end-member shows a trend of decrease from the garnet' s core to rim, that is, the mole fractions of spessartine end-members of samples Garnet1 to Garnet4 are 23.417％ ～ 12.900％, 20.211％ ～ 12.258 ％, 21.124 ％ ～ 13.297％ and 41.943％ ～ 18.523 ％ respectively.In addition, the pyrope end-member increases in content from core to rim in which the mole fractions from sample Garnet1 to sample Garnet4 are 8.963％～11.880％, 10.200％～12.681％, 11.170％ ～12.500％ and 5.411％～7.302％.However, the calcium series of garnets are relatively low in amount and relatively stable.The calcium series consists of grossu-larite, andradite and uvarovite, in which the mole fractions from sample Garnetl to sample Garnet4 are 2.662％ ～4.320％, 3.066％～3.695％, 2.484％～3.406％ and 2.395％～4.222％.The chemical composition of garnets acts as an indicator of the metamorphic grade, namely the content of Fe2+ , Mg2+ increases and that of Mn2+ , Ca2+ decreases as metamorphism progressed.The content of spessartine end-member in garnet is higher in lower metamorphic grade, and with the increase of metamorphic grade, the stability of Mn2+ becomes lower, due to which the amount of Mn2+ decreases under the increasing pressure, Fe2+ substitutes for Mn2+ by entering the crystal structure of the garnet, due to which the amount of almandine increases subsequently.The content of pyrope end-member in garnets contained in metamorphic rocks indicates a higher metamorphic temperature.The content of calcium series garnet end-member reflects the forming pressure of a garnet.This suggests that the formation temperature generally increases from core to rim, implying that the metamorphic grade is getting higher; however, the pressure remains basically stable.X-ray powder diffraction analyses show that almandine and a small amount of quartz are contained in the garnet specimens.The refined cell parameter is 1.153 5 nm and the standard deviation is 0.00048, which is within the numbers 0.0001 and 0.0099.The cell parameter is higher than that of the theoretical value 1.1526 nm, because there are more larger ions like Ca2+ , Mn2+ than smaller ions like Mg2+ in replacing Fe2+.%通过电子探针测试和X射线粉晶衍射分析,对新疆库鲁克塔格兴地塔格群中红色石榴石样品的矿物学特征进行了研究.电子探针测试结果显示,不同变质期次形成的石榴石,铁铝榴石端员组分均由核部到边部逐渐增加,锰铝榴石均由核部向边缘逐渐减少,镁铝榴石由核部到边部逐渐增加,钙质系列石榴石总含量很低且变化稳定.研究表明,兴地塔格群石榴石由核部至边缘生长温度整体上升,形成压力基本稳定,石榴石形成环境的变质程度逐渐提高.根据X射线粉晶衍射数据计算出石榴石的晶胞参数较理论值大,是由Ca2+Mn2+两种大半径离子对Fe2+的置换数目多于小半径Mg2+对Fe2+的置换引起的.