Fe2+-Ti4+ charge-transfer in dumortierite

Single crystals of several varieties of dumortierite, an orthorhombic, pseudohexagonal borosilicate mineral with spectacular colour and pleochroism, were studied by electronic absorption spectroscopy and electron-microprobe analysis in order to elucidate the origin of colour and pleochroism: (a) deep blue dumortierite from Sahatany, Madagascar, (b) red magnesiodumortierite from Dora-Maira, Western Alps, and (c) deep violet dumortierite from Arizona, USA. The Fe and Ti contents are, respectively, 0.020 and 0.008 (a), 0.010 and 0.200 (b) and 0.055 and 0.079 (c) atoms per formula unit. The electronic 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, ΔV_1/2 = 5000 cm⁻¹, absorption band which is strictly polarized II [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 shoulder near 20000 cm⁻¹ and could only be fitted by the two component bands that are typical of the blue (a) and red (b) varieties. The strict polarization II [001], the large half-band width and intensity are typical of metal-metal charge-transfer (MM CT) excitation. The evaluation of band intensities as well as the relation between colour and crystal chemistry suggest that heteronuclear Fe²⁺-Ti⁴⁺ CT is the excitation mechanism in all three varieties (a), (b) and (c). On the basis of the peculiarities of the crystal structure and band polarization, we propose that the excitation occurs between Fe²⁺ and Ti⁴⁺ substituting for Al in the face-sharing M(1) octahedra forming straight chains II [001], in which the metal positions are partly vacant and the metal ions next to such voids are off-centred, such 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 types of Fe-Ti dimers [- □ - F²⁺ - Ti⁴⁺ - □ -] and [- M - Fe²⁺ - Ti⁴⁺ - □ -], the latter with reduced off-centring but still longer Fe-Ti distance compared to other Fe-Ti CT-systems in face-sharing octahedra with 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 disordering, including the voids, in the M(1) chains.