Structural Interpretation of Polarized Absorption Spectra of the Al-Fe-Mn-Cr Epidotes

R. G. Burns1 and R. G. J. Strens2
Department of Mineralogy and Petrology, Downing Place, Cambridge
1Present address: Department of Chemistry, Victoria University of Wellington, Wellington, New Zealand.
2Present address: Department of Earth Sciences, The University, Leeds 2.

Summary: Measurements have been made of the magnetic susceptibility and of the polarized absorption spectra of epidotes of the Al-Fe, Al-Fe-Mn, and Al-Cr series. Magnetic susceptibility measurements at room temperature by the Guoy method indicate that Fe3+, Mn3+, and Cr3+ are in ‘high-spin’ electronic states in the epidote structure, and the electronic transitions observed in the absorption spectra have been assigned on this basis. The polarized spectra, measured over the range 4000 to 22 000 Å by a polarizing microscope using a universal stage technique, showed each epidote group to be distinctly pleochroic. This pleochroism correlates with the presence of ions in distorted coordination sites.

Extinction coefficients of Fe3+ in epidote are higher than those for other Fe (III) compounds, indicating that Fe3+ ions occupy the non-centrosymmetric (Al, Fe) site. The low value (13200 cm−1 at 0·155 Fe3+ and 12700 cm−1 at 0·915 Fe3+) of the crystal-field splitting parameter, Δ, implies that the site is easily expanded to accommodate ions larger than Al3+ There is also evidence that increasing Fe3+ content causes distortion of the (Al,Fe)O6 octahedron, suggesting a possible mechanism for the zoisite ⇌ clinozoisite transition.

The spectra of Al-Mu-Fe epidotes have been interpreted in terms of intense contributions from Mn3+ ions in the non-centrosymmetric (Al, Fe) site, and weak contributions from Mn3+ ions in the centrosymmetrie AlOH site. Values of Δ vary from 13730 cm−1 (0·082 Mn3+) to 13400 cm−1 (0·747 Mn3+). Both (Al, Fc) and AlOH sites have strong tetragonal distortions (c/a ≈ 0·95), leading to the observed large Jahn-Teller splitting of 12000 cm−1.

The Al-Cr epidote spectra show weak absorption bands, attributed to Cr3+ ions in a centrosymmetric site of approximately orthorhombic symmetry (i.e. the AlO site). The value of Δ is about 16300 cm−1.

Review of all available data on the distribution of Fe3+, Mn3+, and Cr3+ between the sites gives the following site preferences: Fe3+, Al > AlO > AlOH; Mn3+, Al ≥ AlOH ≫ AlO ; Cr3+, AlO ≫ Al, AlOH.

Mineralogical Magazine; June 1967 v. 36; no. 278; p. 204-226; DOI: 10.1180/minmag.1967.036.278.04
© 1967, The Mineralogical Society
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