Si-magnetite nano-precipitates in silician magnetite from banded iron formation: Z-contrast imaging and ab initio study

Abstract

Si-bearing magnetite or silician magnetite is common in low- and high-temperature rocks. However, details about possible Fe-silicate or Si-Fe-oxide discrete phases/nano-precipitates were not available due to the limitations of conventional high-resolution TEM. Combining Z-contrast imaging and ab initio calculation using density functional theory (DFT) method, we have derived both composition and crystals structure of the discrete nano-precipitates within host magnetite. The nano-precipitates of Si-magnetite with composition of [¨0.5Fe2+ 0.5]VI[Fe3+]VISiIVO4 or g-Fe1.5SiO4 occur in silician magnetite from a banded iron formation from Western Australia. In the Si-magnetite precipitates, Si replaces Fe3+ in tetrahedral sites of the magnetite structure and vacancies are introduced in the octahedral Fe2+ sites. The Si-magnetite precipitates distribute along {111} of the host magnetite. Widths of the precipitates are even multiples of d111 of magnetite, such as 2d111, 4d111, and 6d111. Ordering of the vacancies in the Si-magnetite will result in symmetry of P4332, which is a subgroup of Fd3m for magnetite. Stacking of Si-magnetite and magnetite (111) layers along the [111] direction also occur in magnetite. The nanoprecipitates result from exsolution of Si-magnetite from the host silician magnetite at low temperature. The occurrence of the thin nano-precipitates within the magnetite host results from the minimization of interfacial energy between the precipitate and the host magnetite. Relatively high concentrations of aqueous silica and Fe-silicate complex species in pore fluid might enhance the incorporation of Si into the silician magnetite during crystallization of the magnetite.