Gemological And Mineralogical Properties And Color Mechanism Of Chrome Diopside From Siberia,Russia

Recently, a lot of chrome diopsides appeared on China’s jewelry market, and were called as "Siberian emerald". Gemological properties and coloring mechanism were analyzed in this subject through conventional gemological testing methods and infrared spectroscopy, Raman spectroscopy, electron microprobe, laser ablation inductively coupled plasma mass spectrometry, X-ray fluorescence and photoelectron spectroscopy and other modern means of testing.The color of Siberia chrome diopside is mostly green and yellowish green. It shows green-yellowish green pleochroism, and strong vitreous luster, with high transparency and multiphase fluid inclusions. The refractive index is 1.670~1.705,the density is 3.25~3.38g/cm3, Mohs hardness is 6~6.5, and Vickers hardness is792.5~952.04 kgf/mm2. Ultraviolet fluorescence is dark red or none, and it keeps green under Charles color filter. Compared with emerald, chrome diopside has stronger luster, higher refractive index and clarity degree. But the color of chrome diopside is mostly overall dim, and large particles are less.In the Q-J figure and Wo-En-Fs classification ternary diagram, research samples belong to Ca-Mg-Fe pyroxenes and diopside group, containing Fe, Cr, Na, Al, Ti and other trace elements, in which Fe content is 1.012% ~ 2.281%, and Cr content is mainly 0.338%~0.853%. In the X-ray fluorescence spectroscopy detection, sample with minimum chromium content(0.117%) can still detect the peak of Cr, while chromium-free sample have no chromium peak. Diopside which has chromium peak in XRF(containing Cr) and has absorption lines in band of red wavelength can be named as chrome diopside.Infrared absorption and raman scattering peaks are 405、420、445、490、530、600、652、681、935、993、1030cm-1 and 323, 389, 665, 1011cm-1 respectively, due to Si-O and M-O vibrations. 3600cm-1 is constitution water absorption peak, and 3360,3430 cm-1 are assigned to adsorbed water. Samples with different chromium contents did not differ between IR and Raman spectroscopy.Color of chrome diopside is mainly determined by Cr and Fe. 4A2�'2T1,4A2g�'4T2g(F), 4A2g�'2Eg spin-forbidden transitions of Cr3+ ions in octahedral coordination in the M1 sites cause green, while tetrahedrally coordinated Fe3+-O charge transfer and transitions of Fe 3+ ions in octahedral coordination in the M1 sites produce yellow. Dark samples with low value and chroma have higher content of Ti and V, but less Ge. With the yellow hue enhancing, Fe content of chrome diopside grows, Cr content and Cr/Fe ratio reduce; and refractive index, density, hardness increase.