Microstructure and microchemistry of iron ooliths

Samples of iron ooliths from Israel, Lorraine, Scotland and chamosite pellets from Loch Etive, Scotland were examined by scanning electron microscope and Electron probe x-ray microanalyzer. The ultrastructure of the samples examined consists of three main elements: flakes, platelets and subspherical nanograins. The flakes are the most abundant element, more or less parallel to each other and tangential to the oolith. They are believed to represent polycristalline aggregates of goethite or chamosite. Microchemical variation of Fe, Al and Si, confirm the presence of clay minerals, allumogoethite and amorphous silica in goethite ooliths, the later tentatively equated to the subspherical nanograins. Ti crystals are present in all ooliths, calcite crystals in chamosite ooliths and pellets. The recent chamosite pellets from Loch Etive are chemically less homogenous than the ancient ooliths.