Preliminary Study on Components of Crust of Jadeite Jade and Its Relationship with Interior Jade Quality

After a great deal of observation on jadeite jade rough in Pingzhou and Huadu, Guangdong Province, the jadeite jade samples with yellow, white and grayish black crusts are chosen as the major objects for study．Based on naked-eye and microscopic observation, X-ray diffraction and EPMA tests, the mineral components and chemical compositions of the crusts with different colours are studied．The results show that jadeite is the main mineral component, but the various secondary mineral components are detected in all the studied crusts．Kaolinite, gibbsite, pyrolusite and hematite are found in the yellow crusts, kaolinite and hydrogrossular in the white crusts, and kaolinite and chlorite minerals in the black crusts．Compared with the interior jade, the contents of the main chemical compositions such as Na₂O and SiO₂ are relatively lower but CaO, MgO and Fe are enriched in all the studied crusts．In the jadeite jade with yellow crusts, the higher Fe content in its interior jade, the darker the colour of the crust．The chemical compositions of the crust, which are subjects to the forming environment, are difficult to be used in predicting the interior jade quality．The relationship between the mineral component of the crust and its interior jade quality is summarized in the paper．