Fe2+-Ti4+ charge-transfer in dumortierite

Single crystals of several varieties of dumortierite, an orthorhombic, pseu dohexagonal borosilicate mineral with spectacular colour and pleochroism, w ere studied by electronic absorption spectroscopy and electron-microprobe a nalysis in order to elucidate the origin of colour and pleochroism: (a) dee p blue dumortierite from Sahatany, Madagascar, (b) red magnesiodumortierite from Dora-Maira, Western Alps, and (c) deep violet dumortierite from Arizo na, USA. The Fe and Ti contents are, respectively, 0.020 and 0.008 (a), 0.0 10 and 0.200 (b) and 0.055 and 0.079 (c) atoms per formula unit. The electr onic absorption spectra are dominated by a weakly polarized absorption edge in the UV near 35000 cm(-1) (4.34 eV) and by a very strong and broad, Delt a v(1/2) = 5000 cm(-1), absorption band which is strictly polarized paralle l to [001] and has its maximum at 16500 cm(-1) in (a) and (c), or at 20300 cm(-1) in (b). In the violet crystals (c), the band has a pronounced should er near 20000 cm(-1) and could only be fitted by the two component bands th at are typical of the blue (a) and red (b) varieties. The strict polarizati on parallel to [001], the large half-band width and intensity are typical o f metal-metal charge-transfer (MM CT) excitation. The evaluation of band in tensities as well as the relation between colour and crystal chemistry sugg est that heteronuclear Fe2+-Ti4+ CT is the excitation mechanism in all thre e varieties (a), (b) and (c). On the basis of the peculiarities of the crys tal structure and band polarization, we propose that the excitation occurs between Fe2+ and Ti4+ substituting for Al in the face-sharing M(1) octahedr a forming straight chains parallel to [001], in which the metal positions a re partly vacant and the metal ions next to such voids are off-centred, suc h that MM distances in such chains are unusually long for face-connection. Thus, the bands at 20300 and 16300 cm(-1) are assigned to the two extreme t ypes of Fe-Ti dimers [- square - Fe2+ - Ti4+ - square -] and [- M - Fe2+ - Ti4+ - square -], the latter with reduced off-centring but still longer Fe- Ti distance compared to other Fe-Ti CT-systems in face-sharing octahedra wi th CT-energies near 13000 cm(-1) (corundum, hibonite). The violet colour of (c) is then caused by a mixture of the two types of dimers due to disorder ing, including the voids, in the M(1) chains.