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 Mössbauer 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. Mössbauer 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 B_hf = 43.6 T (14.6% of the whole spectral area, WSA) and B_hf = 47.3 (27.7% WSA). A second less resolved distribution block (δ = 0.63 mm s^-1) shows two main maxima in the P(B_hf) 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.