The Study On Mineral Composition And Color Causes Of Sugilite

Sugilite jade is a collection of various minerals, and its main mineral is sugilite.In this paper, sugilite jade from South Africa is tested through a variety ofconventional gemological instruments to study its gemological characteristics;through various large-scale instruments to study its mineralogical characteristics andcolor causes.Observation tests showed that sugilite jade has aphanitic-granular structure, itscolor is from light purple to purple. It has vitreous luster, translucent-opaque, a Mohshardness of5.755to7.627, a refractive index of1.54to1.62, the relative density of2.50~3.84. A few samples show weak blue-white or white fluorescence in thelong-wave ultraviolet radiation, and weaker or no fluorescence in the short-waveultraviolet radiation.Tested by polarized light microscopy, infrared spectroscopy, electron microprobe,X-ray powder diffraction, the main mineral in sugilite jade is sugilite, and thesecondary minerals include quartz, pectolite, aegirine, braunite, limonite, andradite,aedelforsite, idocrase, hematite, eudidymite, cheralite. Among these minerals,hematite, eudidymite and cheralite are tested out for the first time. Combined with thegeology, sugilite jade is deduced as a kind of skarn, formed under the gas-liquidmetamorphism, when the alkaline hydrothermal solution flowed through the typecarbonate manganese ore.Observed under a polarizing microscope, sugilite jade has the seriate blasticstructure and columnar blastic structure, deep purple samples have more sugilite, lightpurple samples contain more impurity minerals. And dark minerals such as brauniteand limonite, can lead to a black and brown on the sugilite jade.By the Fourier transform infrared spectroscopy, sugilite has four peaks at1154cm-1,1065cm-1,770cm-1,674cm-1, which can be used as a basis for identifyingsugilite jade. Furthermore, the tests on the powder transmission method indicate thatsugilite jade contains crystal water. Tested by electron microprobe, the main chemical ingredient of sugilite are SiO2,Li2O, Na2O, K2O, Fe2O3, Mn2O3, Al2O3. In addition, sugilite also contain impuritieselements such as Cr2O3, P2O5, NiO, etc. As the purple tones of sugilite jade from lightto dark, the content of Mn2O3is generally showed a gradual increasing trend. And bythe UV-visible absorption spectra, sugilite jade has a strong absorption band at560nm,which is caused by the absorption of Mn element. However, the parti-colours ofsugilite jade is caused by impurity minerals.