A swelling hematite/layer-silicate complex in weathered granite

Abstract

A naturally occurring hematitic iron oxide/layer-silicate complex has been found in red mottled patches of a deeply weathered granite in north-east Scotland. X-ray diffraction shows a basal spacing of 36 Å—also observable by high resolution electron microscopy—which expands to 40 Å with glycerol and contracts to 33·5 Å on heating. Selected area electron diffraction reveals a composite hematite/layer-silicate pattern with the a-axis of hematite parallel to the b-axis of the silicate. The IR spectrum of the complex clearly shows the contribution made by each of the components. The silicate, with bands due to OH stretching at 3602 cm⁻¹, OH deformation at 855 cm⁻¹, and Si-O stretching at 1085, 1035, 540 and 471 cm⁻¹ resembles ferruginous pyrophyllite, while the hematite, with a perpendicular band at 647 cm⁻¹, in-plane bands at 519, 438, 400, 302 and 227 cm⁻¹ and a characteristic pattern of relative band intensities, is similar to a platy form of soil hematite. Electron microprobe analysis of individual particles gives the complex an (Fe + Al): Si ratio of 6:1, which is consistent with a structure made up of twelve octahedral sheets terminated on both sides by a silicate sheet. It seems likely that the complex developed from a siliceous ferrihydrite which became progressively more organized with geological time.