Raman Spectroscopy and Lattice Dynamics Calculations of Two Fe2+ Silicate Minerals: Pellyite and Grandidierite

Raman measurements and lattice dynamics (LD) calculations were performed on the Fe2+- silicate minerals, pellyite (Ba2Ca(Fe2+,Mg)2Si6O27) and grandidierite ((Mg,Fe2+)Al3(BO3)SiO4O2), to determine vibrational assignments to spectral features, with special interest in Fe2+-site contributions. This interest was motivated by borosilicate glass redox studies, where Fe2+-correlated Raman features were observed near 410 cm−1. Pellyite and grandidierite contain Fe2+O4 tetrahedra and Fe2+O5 sites, respectively, that link to silicate tetrahedral networks, and were used as Fe site standards in the borosilicate glass studies. LD of pellyite indicate observed modes at frequencies greater than 800 cm−1 can be assigned to localized Si-O stretch and O-Si-O bend displacements, from 500 to 800 cm−1 to SiO4 tetrahedral deformations, and below 500 cm−1 to Fe2+O4 and SiO4 tetrahedral network deformations. The spectra and LD findings for grandidierite show localized Si-O and B-O stretch modes above 750 cm−1, with mixed longer-range motions below 750 cm−1. For pellyite, significant vibrational mode contributions from Fe2+O4 tetrahedra can be correlated to two prominent peaks at 386 and 425 cm−1, near the frequency of the Fe2+-related peak observed in the borosilicate glass study. However, for grandidierite, no such FeO5-site mode correlations can be made.