Identifying Polymorph of Serpentine using Microscopic Raman Spectroscopy

Abstract

Serpentine is a phyllosilicate mineral having several polymorphs. Because the crystal structure is dependent on thermodynamic environment where it was produced, it has been desired for the polymorphs to be distinguished for geoscientific investigation. In the present study, we achieved to unambiguously determine the serpentine’s polymorphs with microscopic Raman spectroscopy. Among various Raman-active vibrational modes, we focused on two vibration mode because layer structure of phyllosilicate layers alters with the crystal structure. With the peak frequencies determined with sub-pixel accuracy, the polymorphs were clearly separated in two-dimensional plot of the peak frequencies. Because microscopic Raman spectroscopy is nondestructive method which requires neither large single crystal nor any pretreatment of samples, our it is applicable to small (＜1 mm), fragile, rare, and/or heterogeneous samples. It is thought to be useful for geoscientific analysis of the mantle, meteorite surface, and identification of hydrous minerals in sample return missions of asteroids.