Mineralogical characteristics and color genesis of black quartzite jade from Linwu, Hunan, China

The phanerocrystalline aggregate (single mineral particle size greater than 20 μm), which is mainly composed of α-quartz and has technological value, is called quartzite jade. Black quartzite jade from Linwu, Hunan Province, has gained significant market popularity due to its fine texture and aesthetic appeal. This study aims to provide a comprehensive analysis of this less-explored variety, focusing on its mineral composition, microstructure, spectral characteristics, and chemical properties. A combination of gemological assessments, polarizing microscopy, infrared spectroscopy, X-ray diffraction (XRD), total organic carbon (TOC) analysis, and X-ray fluorescence spectrometry (XRF) was utilized to investigate these features systematically. Additionally, the origin of color is discussed. Results indicate that the quartzite jade from Linwu primarily consists of α-quartz along with varying amounts of muscovite, andalusite, graphite, rutile, and other trace minerals. Infrared analysis reveals characteristic peaks at 479 cm^-1, 540 cm^-1, 778 cm^-1, 796 cm^-1, 1086 cm^-1 and 1173 cm^-1. The presence of absorption double peaks between 700-800 cm^-1 suggests enhanced particle arrangement within the internal structure of the sample; this indicates a well-organized Si-O bond configuration and a high degree of crystallization within the specimen. Based on metamorphic rock discriminant factor (DF) determinants showing negative values for all samples alongside an Al₂O₃ to TiO₂ ratio ranging from 19 to 29 implies these samples are medium to low-temperature metasomatic parametamorphic rocks formed via regional metamorphism, combined with the average total organic carbon (TOC) content of 1.27%, further suggesting that Linwu's quartzy autolith is silicon-rich clay shale. The predominant factor contributing to the sample's black coloration is attributed to its substantial graphite content.