Pressure- and temperature-effects on exchange-coupled-pair bands in electronic spectra of some oxygen-based iron-bearing minerals

Abstract

The special properties of spin-allowed transitions of Fe2+, exchanged-coupled in pairs with Fe3+, (ECP-effect) are studied in single crystals of vivianite, phlogopite, biotite, elbaite and schorl under changing temperature and pressure in the ranges 79≤T[K]≤597 and 10-4≤P[GPa]≤8. The two Fe2+dd-transitions, known to be subject to ECP-effects, occur at 11000 to 14000 (II) and 8400 to 9150 cm-1 (III), depending on the structural matrix. With pressure, band energies shift to higher values, while temperature has the opposite effect. Δv is nearly the same in all cases, decrease on temperature, and increase with pressure. Δα/ΔT or Δα/ΔP have similar values for bands II and III in all minerals studied. These observations are interpreted in terms of geometrical and vibrational changes of the octahedra, involved in the pair effects, on changing P and T. They clearly separate the ECP-bands from ordinary dd-transitions and also from IVCT-bands. A unique pressure effect in the spectral range of 17000 to 26000 cm-1 was found in schorl: a band system that immensly gains intensity on pressure. Two explanations are suggested: (a) traces of Ti3+, exchange-coupled to Fe2+ show the above pressure effect typical of ECP, (b) there occurs pressureinduced reduction of Ti4+ in Y-positions, induced by Fe2+ in connected Y-octahedra, whereby OH in trans-configuration of Y-octahedra promote this process.