Olivine polarized spectra

Figure 7. Spectropolarimetry of the lOpm silicate feature [41] (a) upper panel along the line of sight to the source AFGL 2136. The middle-box shows the polarization spectrum is composed of an emissive component (long-dashed line) and an absorptive component (short-dashed line). Note the change in PA of polarization across the feature (right-hand box) (b) lower panel along the line of sight to the source AFGL 2591. Note that a purely absorptive component is sufficient to account for the polarization spectrum. Note also, the feature at 11.2pm attributed to crystalline olivine.

Figure 1,17 Absorption spectrum of a forsteritic olivine under polarized light. Ordinate axis represents optical density (relative absorption intensity, ///q). From R. G. Burns (1970), Mineralogical Applications of Crystal Field Theory. Reprinted with the permission of Cambridge University Press.

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). 

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