Determination of quantitative cation distribution in orthopyroxenes from electronic absorption spectra

Abstract

Electronic and Mössbauer absorption spectra and electron microprobe data are correlated for iron-bearing orthopyroxenes. The correlation provides a means of quantitatively determining the distribution of Fe²⁺ between the M(1) and M(2) sites of orthopyroxene crystals from electronic spectra and electron microprobe analysis. The electronic spectra are used to analyze the changes in the Fe²⁺ distribution produced during heating experiments and confirm earlier results from Mössbauer spectra. Two components of the spin-allowed transition of Fe²⁺ in the M(1) site are identified at about 13,000 cm⁻¹ and 8,500 cm⁻¹ in γ. Molar absorptivity (ɛ) values for all spin-allowed Fe²⁺ absorption bands in the near-infrared region are determined. The M(2) Fe²⁺ band at ∼5,000 cm⁻¹ in β is the analytically most useful for site occupancy determinations. It remains linear with concentration (ɛ=9.65) over the entire compositional range. The band at ∼10,500 cm⁻¹ in α is the most sensitive to M(2) Fe²⁺ concentration (ɛ=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined.