Near-infrared Spectroscopy Study Of Silicate Gem Minerals

At present,the research of gemstones is mainly concentrated in the mid-infrared band,and there are relatively few studies on the near-infrared band.In this paper,near-infrared spectroscopy is used to test the silicate-based gemstone minerals with different silica-oxygen backbone structures.The near-infrared spectroscopy measured by the experiment and the spectral curve of the database are compared and combined with the structure of the silicate to determine the band assignment.Analysis found:There are two main types of near-infrared spectroscopy absorption of silicate gem minerals:1)absorption of electrons in crystal field changes induced by charge transfer and charge transfer,which is mainly reflected in islands with dense molecular structure and Completely homogeneous;2)The frequency doubling or combined frequency absorption of the molecular group stretching vibration and bending vibration is absorbed in almost all silicate-based gem minerals.The absorption peak of"water"is widely present in the near-infrared spectrum of silicate gem minerals,and its existence form can be divided into water molecules H₂O,hydroxyl groups OH^-and M-OH.The absorption frequency of H₂O is slightly higher than the absorption frequency of OH^-,much higher than the absorption frequency of M-OH,and the full width at half maximum of the absorption peak of H₂O is generally wider than that of OH^-and M-OH.The absorption peak of Si-O in the near-infrared of silicate gem minerals is relatively weak,wherein the absorption frequency of Si-O_t is around 4250cm^-1;the absorption frequency of Si-O_b-Si is around 4860cm^-1.The near-infrared spectrum of the island-shaped silicate gem mineral has a broad absorption band of Fe²⁺electronic transition and crystal field change caused by charge transfer at 7800cm^-1 and 5800cm^-1,which is common in manganese garnet and olivine.other types of island-like silicate gem minerals such as zircon（ZrSiO₄）,topaz（Al₂[SiO₄]（F,OH）₂）and kyanite（Al₂[SiO₄]O）mainly shows the stretching vibration absorption of OH^-in the near-infrared spectrum;The absorption peak of H₂O in the near-infrared spectrum of the ring-shaped silicate gem mineral is stronger than the absorption peak of other molecules,indicating that the annular silicon-oxygen backbone tunnel contains a large amount of adsorbed water;Chain silicate in the near-infrared spectrum of gem minerals,the double-stranded gem minerals exhibit more M-OH absorption peaks in the 4650cm^-1-4300cm^-11 band,while the single-chain gem minerals mainly exhibit the absorption of OH^-near 7020cm^-1.The absorption of H₂O near 5253cm^-1;The near-infrared spectrum of layered silicate gem minerals mainly shows the absorption peak of“water”.The absorption peak of“water”can be divided into interlayer water H₂O and structural water OH^-,such as serpentine7285cm^-1 position absorption belongs to the absorption of interlayer water H₂O,and the absorption of 5085cm^-1 belongs to the absorption of structural water OH^-;The near-infrared spectroscopy absorption of the framework silicate gem minerals is less,mainly manifested by the absorption of H₂O near 7200cm^-1 and around 5200cm^-1,and there is also an absorption peak of B-O at a position near 4860 cm^-1.