Heterogeneity and Indicative Significance of Constitution Water in Nephrite from Ya'an, Sichuan Province

Nephrite from Ya'an is a basic-ultrabasic type nephrite newly discovered in Sichuan Province. At present, there are still some scientific problems to be solved for its genetic mechanism.For example: How the multi-stage ore-forming fluids in the area affected the formation process of nephrite and the various metamorphic structure of nephrite from Ya'an; The occurrence state, bonding form and evolution trend of water in a single tremolite grain; The manifestation and cause of grain-scale in homogeneity of constitution water and chemical composition of tremolite. In the paper, Fourier transform micro-infrared spectroscopy and electron probe were used to reveal the existing form and bonding mode of constitution water in the tremolite grains of nephrite from Ya'an in a microscopic point of view. In particular, the coupling relationship between constitution water and M1 and M3 cations was analyzed.The results show that the infrared absorption bands of tremolite in nephrite from Ya'an caused by (Mg²⁺, Mg²⁺, Mg²⁺)-OH, (Mg²⁺, Mg²⁺, Fe²⁺)-OH, (Mg²⁺, Fe²⁺, Fe²⁺)-OH, (Fe²⁺, Fe²⁺, Fe²⁺)-OH stretching vibration were located at 3 672, 3 659, 3 644, 3 623 cm^-1 respectively. The tremolite in nephrite from Ya'an generally shows grain scale heterogeneity of constitution water and chemical composition. The content of constitution water in a single tremolite grain shows a significant increasing trend from core to rim, while the MgO content shows an increasing trend and the FeO_T content shows a decreasing trend. The results are helpful to reveal the evolution of the ore-forming fluid in the mineralization process of basic-ultrabasic type nephrite deposit, and provide a scientific basis for analyzing its genetic mechanism.