Humidity related magnetite alteration in an experimental setup

Abstract

SUMMARY Low-temperature oxidation (LTO) of magnetite is an alteration process which occurs under normal atmospheric conditions, causing maghemitization. The use of magnetic properties as palaeoclimate proxies requires improved understanding of how humidity and temperature affect such processes. We exposed natural magnetite, with grain size ranging from <1 to ∼30 μm, to different humidity conditions at room temperature and 70 °C for 1 yr. Changes in room temperature setups were very minor, but in all 70 °C setups alteration was detected by magnetic and mineralogical properties. Lowering of the Verwey transition temperature (Tv) turned out to be the most sensitive indicator of LTO, and also lattice constants correlate well with the shift of Tv. Thermomagnetic curves and XRD-results indicate that LTO affects the entire volume of the particles rather than only surface layers. The sample exposed to high relative humidity (rH) >90  per cent at 70 °C showed the strongest degree of LTO with an increase of the oxidation degree by ∼3 per cent according to Tv, and it was the only setup where partial alteration to hematite was indicated by Mössbauer analysis. The sample with extremely dry conditions (rH of ∼5 per cent) at 70 °C, and the sample that was exposed to cycles of high and low humidity in 2-weeks alternation at 70 °C, both revealed a smaller degree of LTO. The smallest change of the high temperature setups was observed for the sample with intermediate rH of ∼13 per cent. The results suggest a non-linear sensitivity of magnetite alteration to humidity conditions, high humidity strongly favours alteration, but alteration is strongly reduced when extreme humidity alternates with dry conditions, suggesting an importance of seasonality in natural weathering.