TRANSFORMATION OF TITANOMAGNETITE TO TITANOMAGHEMITE - A SLOW, 2-STEP, OXIDATION-ORDERING PROCESS IN MORE

Magnetic iron oxides in a sequence of pillow basalts that were dredged from the Atlantic Ocean floor have been studied to characterize titan omaghemite and to define the processes of maghemitization. Distances f rom the spreading ridge and ages (in parentheses) of the samples are 0 -10 (0-1), 160 (9), 450 (26), and 900 km (70 Ma). Iron titanium oxides occur as 1 to 10 mu m-sized dendritic and cruciform-shaped crystals w ith identical appearances in all samples and with no signs of change o r significant heterogeneity in composition or structure as observed by TEM and AEM. Parameters change progressively from the youngest to the oldest, e.g., Curie temperature = 180 to 360 degrees C; lattice param eter = 8.466 to 8.361 Angstrom; number of octahedral cations per cell from Rietveld refinement = 14.8 to 12.1; mean hyperfine (internal) fie lds at 300 K from Mossbauer data = 37 to 45 T. The large Ti contents ( Uv(60) to Uv(70)) are nearly constant. SAED patterns show superstructu re reflections only for the oldest sample. The youngest sample has par ameters corresponding to nearly unoxidized titanomagnetite, whereas th e oldest is near-end-member titanomaghemite. Intermediate samples are partially altered but display no superstructure reflections, implying a lack of significant ordering of vacancies. The data therefore show t hat the process of (titano)maghemitization has two distinctly differen t components: (1) oxidation and loss of Fe, with creation of disordere d vacancies, and (2) ordering of vacancies. The data collectively impl y a process dominated by solid state diffusion of Fe from the crystals , oxidation of Fe, and creation of vacancies wherein the O closest-pac ked framework is preserved, in sharp contrast to a model of addition o f O or to dissolution and neocrystallization.