Reversed Partial Thermo-Magnetic Remanence in Natural and Synthetic Titano-Magnetites

Abstract

Summary Some natural titano-magnetites from Indian lavas show reverse partial thermo-remanent magnetization (reverse PTRM) on test in the laboratory. After oxidation in air at 45OoC, the PTRM becomes normal. Synthetic specimens of titano-magnetite of similar bulk composition showed normal PTRM. However, after heating in air for 10 minutes at 450 C the PTRM became reverse: after an hour's heating it was again normal. So the results for both the natural and synthetic titano-magnetites suggest that the phenomenon of reverse PTRM is associated with a state of partial oxidation : unoxidized or fully oxidized material shows normal PTRM. The PTRM curves suggest that the reverse PTRM is due to the existence of two magnetic components with a weak magnetic interaction, 0.1 to 1-0Oe for cooling in the Earth's field. The high-temperature component A, with a Curie point between 400 and 500 C, is thought to consist of nearly fully oxidized titano-magnetite. The low-temperature component B, with a Curie point around 250 C might be some metastable state of partial oxidation. The failure to identify the low-temperature phase, using an optical microscope, suggests that the size of the regions of different magnetic properties may be below the limit of optical resolution. This phenomenon of reverse PTRM in partially oxidized titanomagnetite seems rather similar to that found in certain ilmenite-hematite minerals and in pyrrhotite.