Gemmological and Mineralogical Characteristics of Nephrite from Ya’an,Sichuang Province

Recently，a distinctive deposit that produces green to light green nephrites has been discovered in Ya’an，Sichuan Province.This nephrite deposit differs from the others in terms of output state and appearance，as well as nephrite texture，structure and its metallogenic features.The deposit located at the southeast section of Mount Longmen Fault Zone，Sichuan Province，where fault structure was rich and magmatic actions were developed.Nephrite deposit from Ya’an is related to ultrabasic rock clusters，mainly distributing around the contact zone of augite peridotite and marble with stratoid and lentoid output.In general，augite peridotites went through tremolitization，serpentinization，chloritization and siliconization to various extents，and alteration zone has a length of 1to 3meters without obvi－ous zoning.In this paper，the mineral components，microstructure，spectroscopy，micro－area chemical compositions characteristics of nephrites from Ya’an were studied through testing methods like X－ray powder diffraction，scanning election microscope，Fourior transform infrared spectrometer and electron microprobe.Combined XRD results with field occurrence and specimen observation，it is suggested that nephrites from Ya’an are mainly composed of tremolites（95%～98%），and a few uvarovites，calcites and chromites，etc.The further observation results from SEM show that crystalline particle of tremolite mineral in nephrite from Ya’an is relatively big compared with similar nephrites at other deposits home and abroad.Most particles are in the shape of long columnar and plate columnar，and some parts are of fiber/column shaped interwoven structure distributing along a certain direction.Most particles have a width of about 30μm，a length about 80μm and a thickness about 1μm.The particles have no obvious deformation，indicating the nephrites were in a stable environment with rare reformation in the later period.FTIR results show that the numbers of absorption bands and the positions of the peaks are matched with tremolite mineral.Micro－aera chemical analysis reveals the average chemical compositions of nephrites from Ya’an are w（SiO₂）being 58.60%，w（MgO）being 22.39%and w（CaO）being 13.04%.The nephrites are poor in Mg，but rich in Fe，Cr，with w（FeO_T）being 1.810%～2.365%，w（Cr₂O₃）being 0.019%～0.152%.Comparing with nephrites of the other places，the presence of Fe and Cr could be the main colour causing factors of green nephrite from Ya’an.