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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 [¨0.5Fe 2+  0.5] VI [Fe 3+ ]VISiIVO 4  or g-Fe 1.5 SiO 4  occur in silician magnetite from a banded iron formation from Western Australia. In the Si-magnetite precipitates, Si replaces Fe 3+  in tetrahedral sites of the magnetite structure and vacancies are introduced in the octahedral Fe 2+  sites. The Si-magnetite precipitates distribute along {111} of the host magnetite. Widths of the precipitates are even multiples of d 111  of magnetite, such as 2d 111 , 4d 111 , and 6d 111 . 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.

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