The crystal chemistry of the humite minerals: Fe2+-Ti4+ charge transfer and structural allocation of Ti4+ in chondrodite and clinohumite
Langer, Klaus; Platonov, Alexej N.; Matsyuk, Stanislavs.; Wildner, Manfred
European Journal of Mineralogy Volume 14 Number 6 (2002), p. 1027 - 1032
published: Dec 1, 2002
ArtNo. ESP147051406005, Price: 29.00 €
Single crystals of the humite-group minerals, with general crystal chemical formula n·[M2SiO4]·[M1-xTix(F,OH)2-2xO2x] 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 atenergies > 35000cm-1; (ii) a strong and broad band at 23300cm-1 with bandwidths near 6000cm-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.