Mg-rich iron oxide spinels from tuffite

Abstract

Powdered crystals of an iron-rich spinel separated from tuffite were examined by chemical analysis, X-ray diffractometry (XRD), magnetometry, scanning electron microscopy and Mossbauer spectroscopy. They are found to contain mainly 48 mass% Fe2O3; 16.9% FeO; 14.3% TiO2; 9% MgO and 3% Al2O3. The magnetization is 29 J T-1 kg-1 at B0 ~ 0.9 T. XRD pattern reveals two cubic phases with α0 = 0.8382(5) and 0.8412(5) nm, respectively. Mossbauer spectra are very complex, but the hyperfine field distribution patterns can be decomposed in two relatively well-resolved Gaussian distributions for the Fe3+ block (averaged isomer shift relative to α-Fe, δ = 0.27 mm s-1) with Bhf = 43.6 T (14.6% of the whole spectral area, WSA) and Bhf = 47.3 (27.7% WSA). A second less resolved distribution block (δ = 0.63 mm s-1) shows two main maxima in the P(Bhf) curve at 35.3 T (25.7% WSA) and 45.2 T (19.7% WSA), respectively. The Mg-rich maghemite (lower α0) appears to be a direct alteration product of a Mg-rich magnetite (higher α0), via an oxidation process of structural Fe2+ to Fe3+ proceeding from the outer to the inner part of the crystal.