Fe²⁺/Fe³⁺ Intervalence Charge Transfer and Enhanced d-d Absorption in Mixed Valence Iron Minerals at Elevated Temperatures

Abstract

The absorption of light by Fe/Ti and Fe/Fe intervalence charge transfer (IVCT) bands has previously been found in alumnium oxide and Al₂SiO₅ aluminosilicate minerals to decrease markedly at elevated temperatures. A broad survey of the optical absorption spectra at elevated temperatures of various mixed valence iron minerals has been conducted. The systems considered here are cordierite, chloritoid, lazulite, dumortierite, jeremejevite, beryl, osumilite, biotite (mica), pargasite (amphibole) and aegirine (pyroxene). All samples lose significant Fe/Fe IVCT feature intensity at temperature. In beryl, osumilite, biotite, pargasite and aegirine, spin-allowed Fe²⁺ d-d features also decrease in intensity at temperature; in all but beryl, the intensity loss is significant. This trend is consistent with d-d band enhancement via Fe²⁺/Fe³⁺ exchange coupling, which has not previously been identified in the majority of these systems. It is contrasted against the behavior of ordinary spin-allowed Fe²⁺ d-d bands in non-IVCT minerals forsterite (olivine) and elbaite (tourmaline). The depletion of Fe/Fe IVCT and enhanced Fe²⁺ d-d band intensity may both be important mechanisms by which iron-bearing mineral phases become more optically transparent under conditions at depth, which has potential implications for mantle geophysical processes such as radiative conductivity.

        
 

 CIT6859 aegirine (pyroxene). γ orientation spectra
from room temperature to 500 °C    

GRR262 cordierite. β orientation spectra
from room temperature to 1000 °C