Low-temperature magnetism of synthetic Fe-Ti oxide assemblages

摘要

Low-T magnetometry (<300 K) is increasingly used to identify magnetic mineral assemblages. The present work aims at establishing a robust data basis for the characterization of polycrystalline synthetic Fe-Ti oxide assemblages from low-T magnetic measurements. The two-phase samples contained ilmenite-hematite solid solutions (ilmenitess) with compositions near to the ilmenite end-member (X_(llM> 0.69), that coexisted either with titanomagnetites or with pseudobrookite_(ss) of intermediate to Ti-rich compositions (X_(Usp)> 0.6; X_(Fpb) > 0.57). Single-phase samples of ilmenite_(ss )and titanomagnetite were also prepared and measured to underpin the results on the two-phase assemblages. We have concentrated on in-phase and out-of-phase AC susceptibility and saturation remanence and some hysteresis measurements. The temperature-dependent susceptibility and remanence curves reveal diagnostic, composition dependent signals for both ilmenitess and titanomagnetite. In particular, titanomagnetite is distinguished by frequency-dependent signals in the in-phase and out-of-phase susceptibility curves. We have focused on critical transition temperatures for ilmenitess such as Curie (T_C) and Neel temperatures (T_N), the spin glass transition temperature (T_G) and the blocking temperature (T_B). We observe a continuous decrease of T_C with increasing X_(llm), down to about 0.93 and a slight increase of T_N for X_(llm) between 0.95 and 1.0. T_G is constant for X_(llm)between 0.76 and 0.95 and absent for 0.98 < X_(llm)< 1.0, where antiferromagnetic behavior occurs. TB is only slightly lower than T_C for 0.84 < X_(llm) < 0.94, indicating a very restricted "superparamagnetic" field that vanishes for higher X_(llm) values. The new data for ilmenitess allow the construction of a revised, more physically appealing low-temperature phase diagram for this system. We observe two very good linear correlations between the peak temperature in the X-T curves and X_(nm), one for the range 0.69-0.92 and one for 0.95-1.00, which can be useful for simple estimates of ilmenite_(ss) compositions.