Relationship between magensium and iron content in some olivine and raman spectroscopy

Abstract

In the past, Raman spectroscopy was considered to be inconvenient analytical method because of its high running cost and time consuming. Recently, this technique has become one of the most famous non-destructive analytical methods for materials in many scientific interests (e.g. gemology, geology and material sciences). Raman Effect is an inelastic scattering light wave activated by the monochromatic light within structure of material. The Raman scattering usually has wavelength that differs from the origin; the different values of wavelength are specific pattern depending on structure of material. All Raman spectra used under this study were obtained by Renishaw laser Raman spectroscope (Model 1000) at the Gem and Jewelry Institute of Thailand (Public organization). Green Ar/Ar laser source is used to generate the laser with wavelength of 514.5 nm. Olivine is one of the most common rock-forming mineral in silicate group, originated particularly in the earth's upper mantle. Naturally, it consists mainly of 2 abundant solid-solution members; Forsterite (Mg[subscript 2]SiO[subscript 4]) and Fayalite (Fe[subscript 2]SiO[subscript 4]). Twenty single crystals and seven batches of very fine -grained natural olivine, which contain end-member contents ranging from Fo[subscript 91.5] to Fo[subscript 50.5], are available for this study. The relationship between the end-member contents of the studied olivine and their Raman spectra was observed. Raman peak positions significantly show linear downshift against increasing in Iron contents. This phenomenon can be explained by substitution of larger Fe cations along M1 and M2 sites in the olivine structure yielding the shorter bond length and stronger bond strength. These lead to the downshift of both internal and lattice vibration modes of waves in crystal. Consequently, the non-destructive Raman spectroscopic technique is possibly applied to both qualitative and semi-quantitative analyses for olivine within wide range of Fo content. The most suitable peaks for this purpose are assigned to the SiO[subscript 4] internal mode (1000-800 cm[superscript -1]) because most of the spectrum peaks in this range are usually present. In addition, grain size of olivine does not effect positions of Raman peaks, but it would only broaden the peak shape.