湖南临武黑色石英岩质玉矿物组成特征及成因初探

石英岩质玉是一种市场上常见且产地较多的玉石品种,本文基于前人石英岩质玉的研究基础,通过常规宝石学测试、红外光谱测试、偏反光显微镜下观察,以及采用X射线粉晶衍射、X射线荧光光谱、电感耦合等离子体质谱仪对样品的宝石学特征、矿物组成进行分析,并对其成因进行讨论。结果表明:该地区黑色石英岩质玉多为中-细粒粒状结构,偏光显微镜观察可见多种变晶/变余结构;成分中主要矿物石英平均含量为44.7%,次要矿物云母、长石平均含量合计31.0%,黏土矿物平均含量为12.7%,另含有有机碳以及红柱石、铁铝榴石、黄铁矿等铁质矿物;结合样品结构、构造特征及矿物化学成分分析可知,样品为典型副变质岩系的中、低温热液交代型区域变质岩,属绿片岩相,原岩为富铝且富含石英、长石的沉积岩,其形成的构造环境属大陆边缘构造。本研究为该地区石英岩质玉的矿物组成鉴定提供了多手段技术支撑。

Mineral Composition and Genesis of Black Quartzite Jade from Linwu County, Hunan Province

BACKGROUND: Black quartzite jade is a type of commonly and commercially available jade. Many areas produce this gem.OBJECTIVES: To analyze the gemological characteristics and mineral composition of the jade samples and to discuss their genesis.METHODS: Conventional gemological tests, infrared spectrum tests, polarizing microscopy, X-ray powder diffraction analysis, X-ray fluorescence spectrometry, and inductively coupled plasma mass spectrometry were used.RESULTS: Black quartzite jade from the Linwu County, Hunan Province, showed a medium-fine grain texture. The polarizing microscopy results revealed many crytalloblastic or palimpsest textures. The content of the main mineral quartz was 44.7%, the content of secondary minerals (mica and feldspar) was 31.0%, and the content of clay minerals was 12.7%. Minor organic carbon, iron minerals, andalusite, almandine, and pyrite were also observed.CONCLUSIONS: Based on the main textures, structures, configuration features, and chemical compositions, the studied samples are regarded as typical low-to-moderate temperature hydrothermal regional metamorphic rocks, which are classified as greenschist facies. The source rock was composed of sedimentary rocks abundant with aluminum, quartz, and feldspar. The tectonic environment for its formation belonged to the continental margin. This study provides technical support for the identification of the mineral composition of quartzite jades from the Linwu County, Hunan Province.