The magnetism and microstructure of pulverized titanomagnetite, Fe2.4Ti0.6O4:: the effect of annealing, maghemitization and inversion

Titanomagnetite, Fe2.4Ti0.6O4, pulverized in a ball-mill to make the material readily maghemitize on the laboratory time scale, has been used as an analogue for the magnetic mineral of submarine basalts. The ball-milling, in addition to reducing particle site, produces an internal nanocrystalline structure. Annealing at high enough temperature (600 degrees C-800 degrees C) removes the nanocrystalline structure but the titanomagnetite crystals carry a relic of ball-milling induced non-stoichiometry which contributes to magnetic strain anisotropy. Despite this complication, the coercive force of the annealed material is close to the average coercive force of pillow basalts and should provide an improved synthetic analogue for laboratory studies inspired by the magnetism and mineralogy of the submarine crust. The nanocrystalline structure persists in maghemitized pulverized titanomagnetite, and the magnetization process parameters are consistent with an increasing importance of thermal fluctuations as maghemitization proceeds. In the transformations produced by annealing, maghemitization and inversion, the spinel component immediately post-transformation has composition and/or concentration inherited from the pre-transformation spinel, and does not correspond to equilibrium. Later re-equilibration of the coexisting phases, accelerated in the laboratory by elevated temperature, leads to diminution of the spinel component in favour of more stable phases. Unlike the inversion of maghemite to haematite, the inversion product of titanomaghemite is not a unique assemblage of phases. (C) 1999 Elsevier Science B.V. All rights reserved.