ORIENTED OVERGROWTH OF AC ICULAR MAGHEMITE CRYSTALS ON QUARTZ

ABSTRACT: Maghemite with acicular morphology occurs as an oriented growth on fresh quartz surfaces inthe magnetic sandy fraction of the B horizon of a Typic Rhodoxeralf from southern Portugal. The maghemite was characterized by M6ssbauer spectroscopy, X-ray diffraction and electron microscopy (scanning and transmission), and on the basis of a structural similarity between quartz and maghemite, a mechanism has been proposed for the formation of the oriented overgrowth. Correlation with an interpretative chemical model is discussed, assuming active sites over quartz surfaces. Infrared absorption data agree with the proposed models. Maghemite crystals with an unusual needle-like morphology and clustered in a star-like pattern were identified in the sand fraction of B horizons of two Alifsols and one Ultisol from southern Portugal (Abreu & Robert, 1985, 1987). This maghemite was associated with hematite and quartz with minor amounts of mica and kaolinite. The morphology ofmaghemite, both synthetic and natural (in soils), is reported to be sub-rounded (Taylor & Schwertmann, 1974; Schwertmann & Taylor, 1977). However, crystals with acicular morphology similar to that reported in soils by Abreu & Robert (1985) wer