Phase Evolution in the Fe-3 O-4 -Fe-2 TiO4 Pseudo-Binary System and Its Implications for Remanent Magnetization in Martian Minerals

Abstract

Titanomagnetites offer a rich system to explore the role of fine microstructure on magnetic properties. They are important minerals in basalts, and are commonly found on the moon and Mars. Here magnetic measurements were used to monitor decomposition and phase evolution in the pseudo-binary Fe 2 TiO 4 -Fe 3 O 4 solid solution system. The phases appearing in the decomposition are a strongly magnetic magnetite and a weakly magnetic Ti-rich spinel. For the 40, 50, and 60 at% Fe 2 TiO 4 compounds (balance Fe 3 O 4 ) explored here, a metastable solid solution is nonmagnetic at temperatures where decomposition kinetics can be monitored in reasonable experimental times. The magnetization of magnetite formed by the decomposition offers a direct measure of the volume fraction transformed. Time-dependent magnetization measurements were used to monitor the kinetics of decomposition and compared to models for spinodal decomposition and nucleation and growth kinetics for compositions outside the spinodes. The fine microstructure resulting from spinodal decomposition and exchange bias mechanisms for coupling, may be important in understanding the remnant state of these minerals on Mars