Mineralogical And Gemological Study Of Aquamarine From Keketuohai In Aletai Of Xinjiang

Base on pioneers’achievement, study and discussion of the gemologicalproperties, chemical compositions, spectrometry characteristics, thermodynamicparameters of fluid inclusions etc by using gemology measurement，electron probemicroscopic analyzer (EPMA), UV spectrum, IR spectrum, fluid inclusion testing.Aquamarine crystals from Keketuohai area mainly is light to medium blue，bluish green and greenish blue. Most of them are Hexagonal prismatic crystals.There is incompletely cleavage {0001} in the direction vertical to the c-axis,and longitudinal lines developed parallel to the c-axis. Through EMPA，the resultsshowed that in comparison with beryl from Pingwu Sichuan province, those crystalsfrom Koktokay area has a high content of Fe and a low content of alkali metal,as well as lower specific gravity, due to the content of alkali metal isproportional to the specific gravity.UV spectrum of aquamarine showed characteristic absorption of Fe. In theextraordinary ray, a broad band at830nm, due to Fe2+in a channel site; an absorptionband at700nm, due to arise from an Fe2+/Fe3+interaction，and contribute to theblue aquamarine; a narrow band at370nm, due to Fe2+in the Si4+site in the Si—O tetrahedron site. In the ordinary ray, a broad band at810nm or830nm, dueto Fe2+in the Al3+site in the Al—O octahedral site. A narrow band at465nm, inboth extraordinary ray and ordinary ray, due to Fe2+in the Si4+site in the Si—O tetrahedron site.The result of micro-IR showed that structural water occurs in aquamarine，the spectra of two types of water can be recognized with molecular vibrationalfundamentals at3694cm-1,3555cm-1and1542cm-1for Type I, and at3655cm-1,3592cm-1and1628cm-1for Type II, the channel occupied water is mainly of type II. The colourseems to be related to the content ratios of Type I to Type II, the crystal looksblue when the ratio near1:1. Molecules of CO2also are sometimes present in thechannels, producing a vibrational absorption frequency of2359cm-1and2360cm-1in the ordinary ray, providing important basis for identifications of natural andsynthetic aquamarine.There are many liquid inclusions in the samples of aquamarine, theseinclusions can be two-phase tubes that run parallel to the c-axis. The resultsof micro-temperature measurement showed that the fluids are of the NaCl-H2O type.The homogenization temperatures are concentrated in the range of150～320℃, thesalinities w (NaCleq) are in the range of1.74%to12.85%. So the fluids belongto low-medium temperature and low-medium salinity fluids. The ore fluid is featuredby density of0.72g/cm3to0.97g/cm3, concentrated in0.9g/cm3. The pressures arein the range of0.48MPa to9.47MPa, concentrated in2MPa to3MPa. The ore-formingdepth is concentrated in200m to300m, the deepest is up to1349m.