Determination of Quantitative Cation Distribution
in Orthopyroxenes From Electronic Absorption Spectra

Don S. Goldman, George R. Rossman
 Division of Geological and Planetary Sciences
California Institute of Technoogy
Pasadena, CA  91125


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 gamma. Molar absorptivity (epsilon) values for all spin-allowed Fe2+ absorption bands in the  near-infrared region are de­termined. The Fe2+ band at ~5,000 cm-1 in beta is the analytically most useful for site occupancy determinations. Itremains linear with concentration (epsilon = 9.65) over the entire compositional range. The band at ~10,500 cm-1 in alpha is the most sensitive to M(2) Fe2+concentration (epsilon = 40.8), but deviates from linearity at high iron concentrations. The origins of spin-forbidden transitions in the visible region are examined.

last updated: 5-Oct-2020