Epidote structure

The three polarized spectra of piemontite shown in fig. 4.3 are distinctive with absorption bands of different intensities occurring in each spectrum at approximately 450 nm (22,200 cm-1), 550 mn (18,200 cm-1) and 830 nm (12,000 cm-1) (Bums and Strens, 1967 Smith et al., 1982 Kersten et al., 1987). The molar extinction coefficients of these bands are one order of magnitude higher than those in absorption spectra of other Mn(III) compounds, indicating that Mn3+ ions are situated in an acentric coordination site in the epidote structure. Moreover, extinction coefficients of corresponding bands in the polarized spectra of a suite of manganiferous epidotes vary with composition,... 

The epidote structure illustrated in fig. 4.4 contains three positions of six-fold coordination. The M3 coordination polyhedron is acentric with irregular metal-oxygen and oxygen-oxygen distances due to compression of the site along one... 

Figure 4.4 Crystal structure of epidote. (a) Linkages of the coordination polyhedra viewed along the b axis (b) configuration of the M3 coordination site accommodating Mn3+ ions in the epidote structure. The figure shows the orientations of the crystallographic and indicatrix axes (from Dollase, 1969). Metal-oxygen distances are pm.

Figure 4.5 Energy level diagram for the Mn3+ ion in the M3 site of the epidote structure. Observed transitions refer to the polarized absorption spectra shown in fig. 4.3.

The decrease of molar extinction coefficients of absorption bands in the polarized spectra of piemontites with increasing Mn3+ ion contents ( 4.4.2), which is contrary to the Beer-Lambert law, eq. (3.7), indicates that Mn3+ ions are not located entirely in one site of the epidote structure ( 4.4.2.1). Most of the man-... 

Crystal structure refinements show that Fe3+ and Mn3+ ions are strongly enriched in the very distorted M3 site of the epidote structure (Dollase, 1968 Gabe et al., 1973 Stergiou and Rentzeperis, 1987), but these cations also appear to occupy Ml sites, too, in manganiferous epidotes (Dollase, 1969). These site occupancies correlate with Mossbauer spectral data for Fe3+ (Bancroft et al., 1967 Dollase, 1971 ) and crystal field spectra of Mn3+ in piemontites (Bums and Strens, 1967 Smith et al., 1982) described in 4.4.2 and 5.4.4. EPR measurements of clinozoisites (Vassilikou-Dova and Lehmann, 1987) indicate that Cr3+ and Fe3+ ions occupy the M3 sites. 

Transition metal ions most susceptible to large Jahn-Teller distortions in octahedral coordination in oxide structures are those with 3d4, 3d9 and low-spin 3(f configurations, in which one or three electrons occupy eg orbitals. Thus, the Cr2+ and Mn3+, Cu2+, and Ni3+ ions, respectively, are stabilized in distorted environments, with the result that compounds containing these cations are frequently distorted from type-structures. Conversely, these cations may be stabilized in distorted sites already existing in mineral structures. Examples include Cr2+ in olivine ( 8.6.4) and Mn3+ in epidote, andalusite and alkali amphiboles ( 4.4.2). These features are discussed further in chapter 6. 

In aluminosilicates, too, including epidote and the Al2SiOs polymorphs andalusite, kyanite and sillimanite, Al3+ ions occupy several coordination sites. Again, trivalent cations such as Cr3+, V3+, Mn3+ and Fe3+ may show cation ordering and be relatively enriched in one specific Al3+ site in a crystal structure. 

Variations of extinction coefficients and spectrum profiles with changes in chemical composition of a mineral provide information on cation ordering in the structure. Examples involving Al3+-Mn3+ ordering in epidotes and andalusites are discussed in 4.4.2 and 4.5, and Mn2+-Fe2+ ordering in olivine is illustrated in fig. 4.8. Compositional variations of intensities of absorption bands in polarized spectra of orthopyroxenes described in 5.5.4. (fig. 5.15) have yielded Fe2+/M2 site populations (Goldman and Rossman, 1979), while similar trends in the crystal field spectra of synthetic Mg-Ni olivines described in 5.4.2.4 (fig. 5.12) have yielded site occupancy ratios of Ni2+ ions in the olivineMl and M2 sites (Hu etal., 1990). 

In the case of Cu2+ and Cr2+, compounds of which are susceptible to Jahn-Teller distortions ( 6.3 table 6.1), these cations are predicted to show strong preferences for the most distorted orthopyroxene M2 and amphibole M4 sites. A similar explanation accounts for the observed enrichments of Mn3+ ions in the distorted andalusite Ml, alkali amphibole M2, epidote M3 and, perhaps, epidote Ml sites (table 6.1). The presence of significant amounts of chromium in olivines from the Moon and as inclusions in diamond may be due to the presence of Cr2+ ions, and not Cr3+, in the distorted Ml and M2 sites of the olivine structure (Bums, 1975b), in which Jahn-Teller stability may be attained. A similar factor accounts for the stability and site occupancy of the Cr2+ ion in the orthopyroxene M2 site (table 6.1). 

The Co2+, Ti3 and V3 ions are expected to prefer either distorted or small octahedral sites. Thus, Co2+ should be slightly enriched in the orthopyroxene M2 and cummingtonite M4 sites, favour the pseudo-tetragonally distorted olivine Ml site, and be randomly distributed over the amphibole Ml, M2 and M3 sites. The V3+ and Ti3+ ions are expected to occupy the orthopyroxene Ml and alkali amphibole M2 sites, and to be enriched in distorted epidote M3 sites. As noted earlier, the occurrence and stability of Ti3+ ions in lunar and mete-oritic clinopyroxenes ( 4.4.1) may be explained by the availability of the distorted octahedal Ml site in the calcic clinopyroxene structure. 

Dollase, W. A. (1971) Refinement of the crystal structures of epidote, allanite and hancockite. Amer. Mineral., 56,447-64. 

Stergiou, A. C. Rentzeperis, P. J. (1987) Refinement of the crystal structure of a medium iron epidote. Zeit. Krist., 178,297-305. 

Strens, R. G. J. (1966a) The axial-ratio-inversion effect in Jahn-Teller distorted ML6 octahedra in the epidote and perovskite structures. Mineral. Mag., 35, 777-80. 

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