DETERMINATION OF QUANTITATIVE CATION DISTRIBUTION IN ORTHO-PYROXENES FROM ELECTRONIC ABSORPTION-SPECTRA

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 (e{open}) 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 (e{open}=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 (e{open}=40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined. © 1979 Springer-Verlag.