Identify key serpentines antigorite, lizardite and chrysotile with various compositions and crystallographic orientations using micro-Raman spectroscopy

Abstract

Serpentines are geologically important minerals, and antigorite (Atg), lizardite (Lz) and chrysotile (Ctl) are the three key varieties. Their quick and accurate identification with micro-Raman spectroscopy requires to consider the effects of different crystallographic orientations and different chemical compositions. By collecting from existing literatures all Raman spectroscopic data and compositional data acquired from the same or identical Atg, Lz and Ctl samples, we critically examined the compositional effects for the first time, and found that some compositional parameters like the Al₂O₃ and Cr₂O₃ contents have significant impacts on the Raman features. Taking into account the effects of both compositional difference and crystallographic orientation difference, we propose two identification schemes for Atg, Lz and Ctl: the first one uses those weak but characteristic Raman peaks at 1200–1000 cm⁻¹, and the second one uses those intense and unanimously-observed Raman peaks at ∼688, 378 and 229 cm⁻¹. As for the first identification scheme, no peak at 1200–1000 cm⁻¹ suggests the presence of Lz; a single peak at ∼1045, at ∼1070, or at ∼1106 cm⁻¹ indicates the presence of Atg, Lz, or Ctl, respectively; two Raman peaks at ∼1040 and 1070 cm⁻¹ implies the presence of Lz; the occasionally observed one single peak at ∼1040 cm⁻¹ may imply the presence of either Atg or Lz, which can be sought out by resorting to the peak position ratio R_∼1045/688 (Atg having R_∼1045/688 > ∼1.521 whereas Lz attaining R_∼1045/688 < ∼1.521). As for the second identification scheme, Atg can be readily separated from Lz and Ctl by using the exact wavenumbers of the Raman peaks at ∼688 and ∼378 cm⁻¹, and Lz and Ctl can be further discriminated by using the exact wavenumbers of the Raman peaks at ∼378 and ∼229 cm⁻¹. Under most circumstances, both identification schemes do not require the information of crystallographic orientation or composition, and can be conveniently applied to identify the serpentines of Atg, Lz and Ctl.