Electronic intervalence Fe²⁺ + Ti⁴⁺ → Fe³⁺ + Ti³⁺ charge-transfer transition in ilmenite

Abstract

The optical absorption spectrum of natural ilmenite was measured on a narrow translucent rim zone of a thin polished slide of thickness ca. 0.004 mm, mounted with epoxy resin upon a supporting glass plate. The color of the material in the zone is dark red. At conoscopic observation between crossed Nicols, it displays a very dark, vague, off-centered conoscopic figure. Unpolarized optical absorption spectrum of ilmenite, measured in the transparent zone, consists of a very intense high-energy absorption edge which covers a major part of the visible range and causes the dark-red color of ilmenite. On the edge tail, which extends far to the NIR range, there is an intense, broad, asymmetric, vaguely structured absorption band with maximum at around 7000 cm⁻¹. On the account of high iron and titanium contents (0.82 and 0.86 apfu, respectively), it may confidently be attributed to Fe²⁺/Ti⁴⁺ IVCT transition, though its energy is unusually low compared to other Fe²⁺, Ti⁴⁺-bearing minerals studied so far. This most probably proved that the effect of lowering of energy of IVCT bands with increasing number of ion pairs, established by Amthauer and Rossman (Phys Chem Miner 11:37–51 1984), on Fe²⁺/Fe³⁺ IVCT bands also takes place in case of electronic Fe²⁺/Ti⁴⁺ IVCT transition in ilmenite.