Fe2+-Ti4+ charge transfer between face-sharing octahedra : polarized absorption spectra and crystal chemistry of ellenbergerite

Abstract

The new high-pressure mineral ellenbergerite, [Mg,(Ti,Zr),⃞]2 Mg6Al6Si8O28(OH)10 is hexagonal and strongly pleochroic with ω colourless, ε purple to lilac with strong zonation of colour intensity. Electron probe analyses and polarised absorption spectra were obtained on 27 spots in 5 crystals from 3 samples. Ti and Fe contents vary between 0.18 and 0.59, 0.02 and 0.11 atom pfu, respectively. The E⊥c spectra are featureless, with an UV edge near 34 000 cm-1. The E//c spectra show a single band with a maximum in the range 18 500-19 500 cm-1, a half-width of 6 500 cm-1 and an intensity which is linearly correlated with the [Fe]. [Ti] concentration product. Crystals taken from different samples define different correlation lines, probably reflecting variations of the Fe2+/Fe3+ ratio between samples. The band is assigned to Fe2+-Ti4+ charge transfer between face-sharing octahedra of the single chain along the 63 axis. The oscillator strength is 480. 10-4. The uniqueness of the band implies complete partitioning of Ti into the single chain and ordering of Mg(Fe), Ti(Zr), and vacancies within this chain, with Ti off-centred toward the vacancy. A very weak band or shoulder present near 12 500 cm-1 in the E//c spectra of the crystals supposed to be the more Fe3+-rich is tentatively assigned to Fe2+-Fe3+ charge transfer in the same sites because the E//c spectrum of a new Mg-phosphate with ellenbergerite structure (ω colourless, ε blue-green, Ti = 0.005, Fe = 0.28 atom pfu) shows a unique band at 12 400 cm-1 (half-width 5 000 cm-1) which must be assigned to Fe2+-Fe3+ charge transfer between face-sharing octahedra of the single chain. Comparison is made with the structurally related dumortierite, suggesting that Fe and/or Ti are also off-centred in the octahedral single chains of this mineral.