Chemical Composition,Mineral Component and Microstructure of Light-Coloured Jadeitite with Different Values of Relative Density

Nowadays a large number of jadeitite containing associated minerals are sold as“jadeite jade”and parts of those jadeitite’s relative density is not in agreement with the prescribed scope of GB/T16553－2011，which brings new problems to gem and jade industry.In order to find the reason that causes the relative density of the light－coloured jadeitite varying from 2.88to 3.30and to provide the theory basis to identify this kind of jadeitite，186 pieces of light－coloured jadeitite samples are collected and divided into four groups.The relative density of Group A is greater than or equal to 3.25.The relative density of Group B is 3.19to 3.24.The relative density of Group C is 3.10to 3.18.The relative density of Group D is 2.88to 3.09.Chemical composition，mineral component and microstructure of the light－coloured jadeitite samples with different values of relative density are synthetically and comparatively studied by microscope，X－ray fluorescence spectrum （XRF）analysis，electron probe microanalysis，and X－ray powder diffraction（XRD）phase analysis method.Results indicate that main chemical composition of samples is Na₂O （average content 14.13%），Al₂O₃（average content 23.49%），SiO₂（average content 57.70%），and minor component is merely about 1%such as CaO （average content 1.17%），MgO （average content 0.86%），FeO （average content 1.26%）.As the relative density of sample decreases，the loss on ignition increases，which is considered to be relevant to mineral assemblages and content difference in samples.Samples of Group A are composed of jadeite（more than 90%）and a small amount of albite and analcime.Jadeite content of Group B is 83%to 93%.Jadeite content of Group C is 68%to 88%.Jadeite content of Group D is less than 70%.Secondary minerals of Group B，C and D are albite，analcime，soda mica and a small amount of omphacite，phlogopite，white mica，hematite，calcite，etc.With the reduction of jadeite content and the increase of content of albite and analcime etc，the relative density of the samples decreases.Thin section microscopic observation shows that four groups of jadeitite samples have significant primary texture（granitoid structure，columnar structure，columnar－fiber radial structure，chemically zonal structure，etc）and metasomatic filling structure，but no obvious deformation metamorphic structure.If both jadeite particle size and particle gaps are small and the metasomatic filling structure is not obvious，the value of relative density of sample is high.Otherwise，if the sizes of jadeite particle vary greatly and the metasomatic filling structure is more obvious，the value of relative density of sample is low.Studies indicate the reduction of jadeite content and the content increase of impurity minerals，such as albite and analcite，is due to the relative density decline.The differences in the microstructure have indirect effects on relative density change.Therefore，the authors ssuggest when naming the light－coloured jadeitite containing associated minerals，the jadeite content of jadeitite should be considered.And relative density and surface characteristics should be checked.