HIGH-TEMPERATURE MOSSBAUER-SPECTROSCOPY OF TITANOMAGNETITE AND MAGHEMITE IN BASALTS

Mossbauer spectroscopy of basalt lava samples, exhibiting reversible t hermal magnetization (J(s)-T) curves with Curie temperatures of about 840 K, has revealed considerable amounts of maghemite (gamma-Fe2O3) in many samples. In view of the expected instability of maghemite at tem peratures above 620 K, this reversibility came as a surprise. For furt her studies of the magnetization-temperature relationship of these min erals, we have constructed an elliptical radiation-heated furnace in w hich Mossbauer spectra can be acquired at temperatures between 300 and 900 K. Measurements at different temperatures have been obtained for two types of basalts, one in which the magnetic minerals are nearly pu re magnetite and the other where the room temperature spectrum indicat es a mixture of maghemite and magnetite. The two series show different features of the collapse of the internal magnetic hyperfine field, an d the composition of minerals in the samples changes during the treatm ent, showing maghemite.