Iron Silica Membrane Structures at the Nanoscale

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iron silica is one of the most abundant mineral forms in Earth’s solid interior. It is a member of the olivine series, which consists of olivine (Fe2SiO4) and its soluble end-members forsterite and fayalite, along with members of the forsterite-fayalite series and the intermediate members in the olivine series, including ash-gray tephroite and pure manganese silicate, knebelite.

The olivine structure is composed of orthorhombic layers where isolated tetrahedral octahedrons are bound to each other by ionic bonds with interstitial cations like calcium, magnesium, and ferrous iron. The tetrahedral sites are symmetrically nonequivalent and each octahedral site has no preference for any of these cations.

Several geochemical environments are thought to be plausible for the formation of iron-silica membranes on early Earth, which may have played an important role in the adsorption, condensation and organization of simple organic molecules. Despite their relevance for prebiotic chemistry, little is known about the structure of these self-organized membranes at the nanoscale.

In this work, we studied the structural and mineralogical properties of model and natural iron-silica membranes using focused ion beam milled sections. These membranes are made of amorphous silica and iron nanoparticles and under geochemically plausible conditions they exhibit a bilayer structure.

Focused ion beam (FIB) milled sections from both model and natural membranes reveal that the silica layer is amorphous and composed of large surface areas with high inter/intraparticle porosities. The internal tube wall is crystalline, consisting of goethite and magnetite platelets, as shown by ELNES. Elemental mapping of the membranes revealed a spatial correlation between Si, O and Fe. However, this did not correspond to a zone of iron-silicates, but rather to iron nanoparticles over the amorphous silica support.

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