Gemmological Characteristics of“Yacui”from Sichuan Province and Discussion of Naming

Shimian county, Ya'an City, Sichuan Province produces in recent years a precious green jade material with soft texture, delicate and sparkling quality, and moderate hardness, known commercially as "Yacui".For an in-depth analysis of its category and naming, "Yacui" samples from Baishuihe mine in Shimian county are tested by using conventional gemmological methods, polarizing microscope, FTIR, XRD and WDXRF.First, physical properties of "Yacui" samples are observed.Samples have massive structure and rare granular metal minerals, and some samples have some dendritic inclusions and yellow dyeing fabric.The main colour is green, bluish green, yellowish green, and light bluish green.Samples are being of waxy luster to oily luster and from opaque to transparent.Second, conventional gemmological methods are used to get the basic gemmological characteristics of the stone samples.The refractire index is between 1.56 and 1.57 (point measurement).The density is between 2.81 and 2.91g/cm³ (due to minor differences in mineral component).The Mosh hardness is between 2 and 3 (soft than a knife).The green parts show no fluorescence under both SW and LW, while the white part shows pale yellow in LW.The green parts show weakly narrow absorption band in the red area under the spectroscope, and turn red under the Chelsea colour filter, and do not react with HCl, while the parts bubble.Third, the mineral slices observed under polarizing microscope show that the main mineral component is closely packed mica with structure of microscopic flakes or fibrous aggregates.The interference colour is gorgeous and average size of mineral particles is about 0.05⁰.20mm.Secondary minerals are calcite, quartz, pyrite, the proportions of which are as microscopic as trace elements, and they are mainly enriched in surrounding rocks.Fourth, samples are tested under infrared spectrometer.The spectra of the seven stone samples are the same as sericite in the standard map, and it can be inferred, from the interlayer water content concerned absorption peak at 3 475cm^-1 and 1 645cm^-1, that interlayer water content has a certain effect on the quality of the samples.Fifth, X-ray powder diffraction analyses of the green parts, which are obtained from grinded samples, show that the principal mineral is sericite, about 74%, and minor minerals are kaolinite, talc, quartz, with content of 15%, 3%, 8% respectively.Sixth, analysis by wavelength dispersive X-ray fluorescence spectrometer shows that high concentration of Al, Si, K in the chemical composition match those of sericite.Appearance of trace element Cr₂O₃ in samples, together with characterisations under the colour filter and the spectroscope, indicates that Cr is colouring element of "Yacui".The results show that the main components of the "Yacui" are densely aligned sericite, with minor elements of quartz, kaolinite, talc, calcite and other minerals.Based on analyses above, this paper categorizes "Yacui", conforming to "Six virtues standards" of "fine, clean, warm, moist, greasy and condensate", into sericite QingTian stone.