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The crystal chemistry of the humite minerals

¹ Institut für Angewandte Geowissenschaften, Technische Universität, Ernst-Reuter-Platz 1, D-10623 Berlin, Germany
² Institute of Geochemistry, Mineralogy and Ore Formation, Academy of Science of Ukraine, Prospekt Palladina 34, 03680 Kyiv, Ukraine
³ Institut für Mineralogie und Kristallographie, Universität Wien, Geozentrum, Althanstraße 14, A-1090 Wien, Austria

Single crystals of the humite-group minerals, with general crystal chemical formula where M is predominantly Mg eventually substituted by Fe²⁺, are studied by electron microprobe analysis, X-ray diffraction and polarised electronic absorption spectroscopy, in the present paper two chondrodites (n = 2) and four clinohumites (n = 4). The aim was to elucidate colour and pleochroism of such minerals and to evaluate their local crystal chemical properties, esp. the structural allocation of Ti⁴⁺.

The dominating features of all spectra are: (i) a slightly polarised absorption edge in the UV at energies > 35000 cm^–1; (ii) a strong and broad band at 23300 cm^–1 with band widths near 6000 cm^–1 and strongly polarised with E||X in all specimen of the two minerals; and (iii) a complex low energy band system in the NIR (11600 cm^–1 in E||X, 9500 cm^–1 in E||Z and 7700 cm^–1 in E||Y) which corresponds in all details to the dd-band system in olivines caused by Fe²⁺ in (M1) and (M2). Such spectral properties explain the observed colour and pleochroism, X golden yellow to orange, Y and Z light yellow to almost colourless. The band properties of (ii) are typical of excitation of metal-metal charge transfer, MM-CT, the band energy is consistent with that expected for MM-CT in Fe²⁺Ti⁴⁺ pairs at a distance near 3.2 Å. The analysis of the relation between optical and crystallographic vectors in the minerals studied suggests that the FeTi-CT interaction occurs predominantly along the polyhedral units M2₅M3M3M2₅ in the structures of both chondrodite and clinohumite as it is the case for the iron-rich clinohumite of Platonov et al. (2001). The evaluation of all information obtained suggests that Ti⁴⁺ is allocated in the M3 positions of the low-titanium chrondrodites and clinohumites studied.

Key-words: chondrodite, clinohumite, Fe-Ti charge transfer, Ti allocation, electronic absorption spectroscopy..