Quantitative Study on Chemical Composition of Omphacite:Based on Laser-induced Breakdown Spectrometer

Five jadeite samples whose components are known and thirty two samples whose components are unknown were measured by Laser-induced Breakdown Spectrometer(LIBS) developed by Gemmological Institute of China University of Geosciences(Wuhan).The authors adopted optimized external standard method to analyze the testing stability and measurement accuracy of LIBS.The test average was less than 1% deviation from the true value,and the test standard deviation was better than 1.2%.The chemical formula of the sample is easily calculated with the help of test results.The ratio of the sum of different crystal cations coefficient is(0.913 to 1.075) ∶(0.902 to 1.182) ∶(1.909 to 2.087),which is basically in line with the theoretical value of jadeite cations crystallographic sites coefficient 1∶1∶2.The result verified the feasibility of using LIBS to distinguish jadeite from omphacite.The LIBS result of quantitative analysis of the test sample shows that the contents of SiO₂ in jadeite and omphacite are usually stable at around 59%,the contents of Na₂O and Al₂O₃ in jadeite are about 13% and 23% respectively,and the total contents of CaO,FeO_T,and MgO is less than 4%.However,the contents of Na₂O and Al₂O₃ in omphacite is less than that in jadeite.In jadeite,the contents of CaO,FeO_T and MgO increases as omphacite increases and it could reach to 27%.In addition,the contents of Na₂O and CaO in omphacite shows negative linear correlation,the correlation coefficient r₁ =-0.990 1,and the contents of Al₂O₃ and MgO shows negative linear correlation,the correlation coefficient r₂ =-0.966 6.