Quartz nanoparticles in 2461-2495 million years old banded iron formation from Dales Gorge, Hamersley, Western Australia

Abstract

The 2461-2495 million year old banded iron formation (BIF) from Dales Gorge Formation, Hamersley, Western Australia contains hematite, magnetite, Fe (II)-silicate, quartz, carbonates and apatite [1]. Massive hematite were observed to contain quartz nanoparticles with size ranged from <100 nm to 400 nm. Electron dispersive spectroscopic (EDS) measurements indicated their chemical compositions are close to quartz. Fourier infrared probe also showed a Si-O composition. Some quartz crystals were observed on the surface of massive aggregates of hematite (Figure 1), more quartz nanoparticles could be found inside the etching cavities bigger than quartz in hematite aggregates with a few etching cavities contain two quartz nanoparticles. All those quartz nanoparticles have euhedral faces. Those quartz nanoparticles are common in the anhedral hematite aggregates, but different from quartz crystals in the BIF assemblage which were bigger. Quartz nanoparticles are the only mineral that can be observed in the etching cavities, and they were the only mineral observed in hematite. This implied their formation was short after the precipitation of hematite, but earlier than magnetite, iron-silicates and carbonates. Those particles on the surface of hematite appeared tightly fixed in the etching cavities indicated undisturbed hydrological condition, while the crystals in bigger etching cavities implied a poppling hydrodynamic condition. We suggest that those quartz nanoparticles fell on the just precipitated hematite iron-gel in the shallow oxidized sea environment with an exogenesis. It implied that hematite was the only chemical phase at the early stage of banded iron deposition.