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