Gemmological and Spectroscopic Characteristics of Grandidierite from Madagascar

Grandidierite is an extremely rare aluminum borosilicate. Due to the limitation of producing area and yield, there are few studies on its spectral characterization and colour formation mechanism. In this paper, the gemmological characteristics, chemical compositions and spectral characteristics of gem-quality grandidierites from Madagascar have been studied in detail using conventional gemmological testing methods, electron microprobe, UV-Vis spectrometer, Fourier transform infrared 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 FeO_T of light and dark samples is 0.63% and 1.29%, respectively, X(X=w_Fe/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 polychromaticity 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→(⁴A_1g, ⁴E_g) 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 ⁵T_2g→³T_2g 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^-1 and 630-800 cm^-1 are the ν₄ and ν₃ vibrations of AlO₅+AlO₆, respectively; the absorption at 530-630 cm^-1 belongs to the ν₄ 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^-1 belong to the ν₃ vibration of BO₃; 3 400-4 200 cm^-1 is related to lattice vibration of OH. Raman spectra 1 300-1 500 cm^-1 Raman shift is the ν₃ antisymmetric stretching vibration of BO₃.