Hydroxyl group dynamics in defernite: Raman spectroscopy studies

Abstract

A detailed examination of the altered silicate-carbonate xenolith embedded within the ignimbrite of the Upper Chegem Caldera revealed a new occurrence of a rare carbonate mineral known as defernite, with chemical formula Ca₆[(CO₃)_2-x(Si₂O₇)_x/2](OH)₇[Cl_1-x(H₂O)_x], where x ≈ 0.4. Defernite crystallizes as colorless to white fibrous aggregates, reaching 100–150 μm diameters. Subsequently, Raman investigations of defernite from the Upper Chegem Caldera were conducted to perform a comprehensive structural analysis and compare it with minerals found in other locations. During this examination, band assignments focused on the carbonate ion vibration (CO₃²⁻) with a band at 1085 cm⁻¹ and the hydroxyl group, characterized by a series of strong bands around 3590–3600 cm⁻¹, particularly evident in oriented crystals along the (0 1 0) plane. Experimentation involving the alteration of incident laser light polarization highlighted a reduction in the intensity of carbonate and hydroxyl-related bands and the activation of a band around 3390 cm⁻¹. This phenomenon is explained by the formation of hydrogen bonding between hydroxyl groups and chlorine or molecular water, potentially occupying chlorine positions. Lastly, a temperature-dependent experiment demonstrated the instability of the 3390 cm⁻¹ band, which dissipated with increasing temperature. This insight explains the band’s origin around 3590 cm⁻¹, ascribed to non-degenerate hydroxyl groups as a key marker within the defernite structure.