Electronic absorption spectroscopy of natural (Fe2+, Fe3+)-bearing spinels of spinel s.s.-hercynite and gahnite-hercynite solid solutions at different temperatures and high-pressures

Abstract

Natural Fe2+, Fe3+-bearing spinel solid solutions from the spinel s.s.-hercynite and gahnite-hercynite series were analyzed and studied by electronic absorption spectroscopy in the spectral range 30000–3500 cm−1 in the temperature and pressure ranges 77 ≤ TK ≤ 600 and 10−4 ≤ PGPa ≤11.0. Two crystals were light-violet in color (type I) and six green or bluish-green (type II). The spectra of both types of spinels are dominated by an UV-absorption edge near 28000 to 24000 cm−1, depending on the iron contents, and a very intense band system in the NIR centered around 5000 cm−1, which is caused by spin-allowed dd-transition of tetrahedral Fe2+, derived from 5E→5T2. The strong band is in all spinels studied, split into four sub-bands, which can only be observed in very thin platelets. Between the UV-edge and the high-energy wing of the NIR-band there occur a number of very weak bands in type I spinels while the green type II spinels show some of these with significantly enhanced intensity. The intensity of the very weak bands is nearly independent from temperature. Such bands are attributed to spin-forbidden electronic transitions of IVFe2+. Temperature and pressure dependence of the intensity enhanced bands of spinels type II indicate that they are caused by IVFe2+ and VIFe3+. They are attributed to spin-forbidden transitions 6A1g→4A1g, 4Eg, → 4T2g and →4T1g of VIFe3+, the two latter being strongly intensified by exchange-coupling interaction with adjacent IVFe2+. The pressure dependence of IVFe2+dd-band system in the NIR caused by spin-allowed 5E→5T2 transition noticeably differs from that of octahedral Fe2+, an effect which is attributed to a dynamic Jahn-Teller effect of IVFe2+ in the spinel structure.