Chamosite in the Recent Sediments of Loch Etive, Scotland

ABSTRACT Recent, organic-rich, sandy muds in Loch Etive, a hydrographically restricted sea loch in western Scotland, contain fine sand-sized fecal pellets consisting largely of chamosite. The chamosite is very fine-grained and is arranged tangentially around the pellets and around sand grains, and also appears to replace some detrital silicates. X-ray diffraction analyses indicate that the chamosite is poorly ordered but distinguishable from detrital chlorite. The chamosite contains 38.1 percent Fe2O3, 10.1 percent Al2O3, 32.0 percent SiO2 and 4.7 percent MgO. The calculated structural formula indicates that the octahedral sites are almost entirely filled with iron. The chamosite in Loch Etive is forming in an environment rather different from other reported Recent chamosites. The environment of deposition is not therefore critical; diagenetic reactions within the Recent sediment, controlled by sedimentation rate, organic productivity of the waters, and organic content of the sediments play a far more important role. Winnowing and sorting of the sediment in Loch Etive would concentrate the chamosite pellets into Fe-rich layers which would then resemble many ancient oolitic ironstones.