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 materi al readily maghemitize on the laboratory time scale, has been used as an an alogue for the magnetic mineral of submarine basalts. The ball-milling, in addition to reducing particle site, produces an internal nanocrystalline st ructure. Annealing at high enough temperature (600 degrees C-800 degrees C) removes the nanocrystalline structure but the titanomagnetite crystals car ry a relic of ball-milling induced non-stoichiometry which contributes to m agnetic strain anisotropy. Despite this complication, the coercive force of the annealed material is close to the average coercive force of pillow bas alts and should provide an improved synthetic analogue for laboratory studi es inspired by the magnetism and mineralogy of the submarine crust. The nan ocrystalline 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 tra nsformations produced by annealing, maghemitization and inversion, the spin el component immediately post-transformation has composition and/or concent ration inherited from the pre-transformation spinel, and does not correspon d to equilibrium. Later re-equilibration of the coexisting phases, accelera ted in the laboratory by elevated temperature, leads to diminution of the s pinel component in favour of more stable phases. Unlike the inversion of ma ghemite to haematite, the inversion product of titanomaghemite is not a uni que assemblage of phases. (C) 1999 Elsevier Science B.V. All rights reserve d.