Crystal field spectra and jahn teller effect of Mn³⁺ in clinopyroxene and clinoamphiboles from India

Abstract

Blanfordite (I), winchite (II), and juddite (III), all showing vivid colors and pleochroism, from highly oxidized parageneses of Indian gondites were studied by microprobe, Mössbauer, and microscope-spectrophotometric techniques and by X-ray structure refinements. The compositions of the Mn-bearing minerals were close to diopsideacmite (I) and magnesio-arfvedsonite to magnesio-riebeckite (II and III). Transition metal ions are located inM(1)-octahedra (I) or predominantlyM(2)-octahedra (II, III).

Mössbauer spectra of⁵⁷Fe(IS, ΔE _Q) are typical of octahedral Fe³⁺ only. Polarized absorption spectra in the UV/VIS/NIR ranges explain color and pleochroism of the minerals. The position of the UV-“edge” is correlated with Fe³⁺-contents of the minerals, except for judditeE ∥Z, where the edge shows an unusual low energy position. This is most likely due to Mie-scattering of submicroscopic inclusions of braunite with nearly uniform dimensions. In the VIS range, the spectra are dominated by a complex band system between 15,000 and 20,000 cm⁻¹.

Energies and ɛ-values of component bands are compatible with those of Mn³⁺ d-d transitions in other Mn³⁺-bearing silicates. The polarization behavior of component bands can best be explained by aC ₂(C2″) symmetry of the crystal field. The Jahn-Teller splitting (<9,000 cm⁻¹) of the⁵ E _g ground state of Mn³⁺ inO _h crystal fields is appreciably smaller than in other Mn³⁺-silicates. Crystal field parameters 10Dq, (I) 13,650, (II) ca. 11,640, and (III) 11,925 cm⁻¹, are near to that in piemontite. The crystal field stabilization energy of Mn³⁺, (I) 146, (II) ca. 140, (III) 142${{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {\text{g}}}} \right. \kern-\nulldelimiterspace} {\text{g}}}{\text{ - atom}}_{{\text{Mn}}^{{\text{3 + }}} }$, is appreciably smaller than that found in other Mn³⁺-silicates (piemontites and manganian andalusites, viridines and kanonaite).