HIGH-TEMPERATURE STABILITY OF MAGHEMITE IN PARTIALLY OXIDIZED BASALT LAVA

Mossbauer spectroscopy of basalt lava samples, exhibiting reversible thermal magnetization (J(s)-T curves with Curie temperatures of about 580-degrees-C, has revealed considerable amounts of maghemite (gamma-Fe2O3) in many samples. In view of the expected instability of maghemite at temperatures above 350-degrees-C, this reversibility is rather surprising. Here we report Mossbauer studies on heated lava samples, showing high content of maghemite. The samples were kept at 600-degrees-C in oxidizing, reducing, and inactive atmospheres, respectively, for different lengths of time, and then analyzed with Mossbauer spectroscopy at room temperature. The Mossbauer spectra showed that maghemite is stable in the oxidizing atmosphere for at least several hours. In the inactive atmosphere a considerable amount of maghemite still exists after two hours heating. In the reducing atmosphere maghemite had transformed to magnetite after only 30 minutes.