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

Abstract

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.

Keywords

• quartzite jade
• mineral composition
• mineralization
• polarizing microscope
• x-ray powder diffraction
• x-ray fluorescence spectrometry
• inductively coupled plasma-mass spectrometry