Low-temperature magnetism of synthetic Fe-Ti oxide assemblages

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 (ilmenite(ss)) with compositions near to the ilmenite end-member (X-Ilm > 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 ilmenite(ss) 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 ilmenite(ss) 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-Ilm down to about 0.93 and a slight increase of T-N for X-Ilm between 0.95 and 1.0. T-G is constant for X-Ilm between 0.76 and 0.95 and absent for 0.98 < X-Ilm < 1.0, where antiferromagnetic behavior occurs. T-B is only slightly lower than T-C for 0.84 < X-Ilm < 0.94, indicating a very restricted superparamagnetic field that vanishes for higher X-Ilm values. The new data for ilmenite(ss) 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 chi'-T curves and X-Ilm, 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.