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

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 T_K 600 and 10^–4 P_GPa 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 ⁵ E⁵ T₂. 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 ⁶A_1g⁴A_1g, ⁴E_g, ⁴T_2g and ⁴T_1g 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 ⁵ E⁵ T₂ 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.

Monika Koch-MüllerEmail: Phone: +49-331-288-1492/1402Fax: +49-331-288-1492/1402