Abstract

Single crystals of the humite-group minerals, with general crystal chemical formula

\(n{\cdot}[M_{2}SiO_{4}]{\cdot}[M_{1-x}Ti_{x}(F,OH)_{2-2x}O_{2x}]\)
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 cm−1 with band widths near 6000 cm−1 and strongly polarised with EX in all specimen of the two minerals; and (iii) a complex low energy band system in the NIR (11600 cm−1 in EX, 9500 cm−1 in EZ and 7700 cm−1 in EY) 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.

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