Gemmological and Mineralogical Characteristics of Nephrite from Longxi, Sichuang Province

Based on the current situation and existing problems of the comprehensive development and utilization of nephrite resources in Longxi, Sichuan Province, X-Ray diffraction, electron probe, Fourier transform infrared spectrometer, laser Raman spectrometer, polarizing microscope and other analytical testing equipments were adopted. The chemical composition, mineral components, structure, vibration spectrum and other characteristics of nephrite in the area were studied. The results showed that the main mineral of nephrite from Longxi is tremolite, and the minor minerals are calcite, dolomite, talc and apatite. Nephrite has a fibrous metamorphic structure as a whole, and a partial metasomatic residual structure. Similar to nephrite produced from similar deposits in other mining areas, the chemical compositions of nephrite from Longxi are characterized by relatively poor Fe and rich Mg. In the mid-infrared wavenumber range, a set of infrared absorption bands caused by ν_as (Si-O-Si) antisymmetric stretching vibration is in the range of 1 000-1 200 cm^-1. The infrared absorption bands caused by ν_as (Si-O) antisymmetric stretching vibration and ν_s (Si-O) symmetric stretching vibration are located at 990, 955 cm^-1 and 926 cm^-1, respectively. The absorption band below 600 cm^-1 is caused by δ (Si-O) bending vibration and M-O stretching and vibration. In the near-infrared wavenumber range, the infrared absorption peaks caused by OH (MgMgMg), OH (MgMgFe²⁺) and OH (MgMgFe³⁺) frequency-doubled vibrations are located near 7 181, 7 151 cm^-1, and 7 133 cm^-1, respectively. Tectonic stress can easily lead to the development of microcracks in some samples of nephrite in this area, and affect the quality of nephrite from Longxi to varying degrees.