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
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