Grandidierite is an extremely rare aluminum borosilicate.Due to the limitation of produ-cing area and yield,there are few studies on its spectral characterization and colour formation mech-anism.In this paper,the gemmological characteristics,chemical compositions and spectral charac-teristics of gem-quality grandidierites from Madagascar have been studied in detail using convention-al gemmological testing methods,electron microprobe,UV-Vis spectrometer,Fourier transform in-frared spectrometer and laser Raman spectrometer.Chemical composition analysis showed that the samples contains trace elements such as Ca,Mn,Cr,Ti and Zn in addition to the main elements Al,Si,Mg and Fe,and the average content of FeOT of light and dark samples is 0.63％and 1.29％,respectively,X(X=wFe/w(Fe+Mg))is 0.024 and 0.049,respectively.With the higher value of the X-ratio,the higher the saturation of the colour,and the intensity of the polychromatici-ty and the polychromatic hue gradually deepened,the refractive index value is positively correlated with X.The characteristic absorption peaks in the UV-Vis spectra of grandidierite at 387 nm and 443 nm are due to the d-d leap 6A1→T2(4D)and the spin-barrier leap 6A1→(4A1g,4Eg)of the Fe3+ion in the octahedral crystal field,respectively,ⅤFe2+in the triangular bipyramidal structure resembles the distorted octahedral coordination field,so 458 nm is associated with the spin-barrier leap 5 T2g→3 T2g for the ⅤFe2+"five-state"(6D)and"three-state"(3H)splitting terms,and 477 nm is associated with the d—d leap of ⅤFe2+ions,515 nm is caused by the spin-barrier leap of ⅥFe2+in the octahedral crystal field,746,771,797 nm and 823 nm absorption peaks may be associated with the Fe2+/Fe3+IVCT between Fe3+and ⅤFe2+ions.The infrared spectra test results show that the infrared absorption peaks at 400-500 cm-1and 630-800 cm-1 are the v4 and v3 vibrations of[AlO5]+[AlO6],respectively;the absorption at 530-630 cm-1 belongs to the v4 vibration of[SiO4];the absorption at 800-1 100 cm-1 is due to the asymmetric stretching vibration of the silica-oxygen backbone Si-O-Si and the absorption of boron oxygen;the absorption bands of 1 300-1 500 cm-1belong to the v3 vibration of[BO3];3 400-4 200 cm-1is related to lat-tice vibration of OH.Raman spectra 1 300-1 500 cm-1 Raman shift is the v3 antisymmetric stretching vibration of[BO3].
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