Gillespite

Figure 3.3 Polarized absorption spectra of the tetragonal mineral gillespite, BaFe2+Si4OI0, measured with light polarized parallel and perpendicular to the c crystallographic axis (see 3.8.1).-E c spectrum ..E-Lc spectrum.

The crystal structures of transition metal compounds and minerals have either cubic or lower symmetries. The cations may occur in regular octahedral (or tetrahedral) sites or be present in distorted coordination polyhedra in the crystal structures. When cations are located in low-symmetry coordination environments in non-cubic minerals, different absorption spectrum profiles may result when linearly polarized light is transmitted through single crystals of the anisotropic phases. Such polarization dependence of absorption bands is illustrated by the spectra ofFe2+ in gillespite (fig. 3.3) and of Fe3+in yellow sapphire (fig. 3.16). 

Figure 3.19 The crystal structure of gillespite projected down the c axis. Large spheres Ba2+ small spheres Fe2+ in square planar coordination tetrahedra linked [Si04].

Group theoretical interpretation of gillespite polarized spectra... 

Figure 3.20 Vibrational modes, u, to u7, of atoms in a square planar site such as the [FeOJ coordination environment in gillespite (from Hitchman, 1985). Note the acentric a2u, p2u ar>d eu modes that facilitate electronic transitions within Fe2+ ions by vibronic coupling.

Fe2+ many green ferromagnesian silicates such as peridot, diopside and actinolite, gillespite,... 

Hazen, R. M. Burnham, C. W. (1974) The crystal structure of gillespite I and II A structure determination at high pressure. Amer. Mineral., 59,1166-76. [Correction and addendum Amer. Mineral., 60,937-38 (1975).]... 

Huggins, F. E., Mao, H.-K. Virgo, D. (1976) Gillespite at high pressure results of a detailed Mossbauer study. Ann. Rept. Geophys. Lab., Yearb. 75,756-8. 

Mackay, D. J., McMeeking, R. F. Hitchman, M. A. (1979) Magnetic anisotropy and electronic structure of gillespite, a mineral containing planar, four co-ordinate, high-spin iron(II). J. Chem. Soc., Dalton, pp. 299-305. 

Strens, R. G. J. (1966b) Pressure-induced spin-pairing in gillespite BaFeSi4O10. Chem. Comm., p. 777. 

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