The crystal chemistry of the humite minerals: FE2+-Ti4+ charge transfer and structural allocation of TI4+ in chondrodite and clinohumite
- Autor(en)
- Klaus Langer, Alexej N Platonov, Stanislav S Matsyuk, Manfred Wildner
- Abstrakt
Single crystals of the humite-group minerals, with general crystal chemical formula 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 A°. 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.
- Organisation(en)
- Institut für Mineralogie und Kristallographie
- Externe Organisation(en)
- Johann Wolfgang Goethe-Universität Frankfurt am Main, National Academy of Sciences of Ukraine (NASU)
- Journal
- European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
- Band
- 14
- Seiten
- 1027-1032
- Anzahl der Seiten
- 6
- ISSN
- 0935-1221
- Publikationsdatum
- 2002
- Peer-reviewed
- Ja
- ÖFOS 2012
- 1051 Geologie, Mineralogie
- Link zum Portal
- https://ucrisportal.univie.ac.at/de/publications/bd5ee58f-b822-41d3-8c1b-270d3413ba64