Thermal infrared emission spectroscopy of the pyroxene mineral series

Abstract

The thermal infrared emissivity spectra of coarse particulate samples of compositions in the pyroxene series display reststrahlen features (absorptions) that distinguish not only orthorhombic from monoclinic structures, but also major end‐members within the two structural groups, as well as minerals within solid solution series. The exact number of reststrahlen features observed and their positions are dependent on mineral structure and cation occupancy of the M1 and M2 sites. End‐member quadrilateral pyroxenes (Mg2Si2O6‐Fe2Si2O6‐Ca[Mg, Fe]Si2O6) are easily distinguished from each other and from minerals in the nonquadrilateral series (NaFeSi2O6‐Na[Al, Fe]Si2O6‐LiAlSi2O6). Furthermore, among quadrilateral pyroxenes, variations in Mg/(Mg + Fe) are linearly correlated with several band locations, as are variations in Ca content in high‐Ca clinopyroxenes. In both quadrilateral and nonquadrilateral compositions, Christiansen feature positions are also diagnostic. No correlations with minor constituents (of the order of 0.05 atoms per formula unit) were observed. The detailed spectral characteristics of pyroxenes and their variability as a function of structure and cation occupancy are presented here with determinative curves for the identification of pyroxene composition. These data have important implications for the interpretation of spectral data from both laboratory and remote sensing instruments because they should permit a more detailed determination of pyroxene composition in measured unknown pure mineral and bulk compositions dominated by surface scattering, i.e., all particulates greater than ∼65 μm, and solid samples.