The crystal chemistry of the humite minerals:: Fe2+-Ti4+ charge transfer and structural allocation of Ti4+ in chondrodite and clinohumite

Single crystals of the humite-group minerals, with general crystal chemical formula n(.)[M2SiO4](.)[M1-xTix(F,OH)(2-2x)O-2x] 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 em I with band widths near 6000 cm(-1) and strongly polarised with E I X in all specimen of the two minerals; and (iii) a complex low energy band system in the NIR (11600 cm(-1) in E parallel to X, 9500 cm(-1) in E parallel to Z and 7700 cm(-1) in E parallel to 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 Angstrom. 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 M2(5)M3M3M2(5) 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.