Sources and formation of iron minerals in eastern Mediterranean coastal sandy soils - a HRTEM and clay mineral study

Pedogenic iron minerals and their magnetic and color properties have been studied mainly as reflectors of environmental conditions, especially in Chinese and European loess/palaeosol sequences. The suggested sources of iron for the formation of pedogenic iron oxide minerals were inherited iron oxide minerals, primary iron-bearing minerals and phyllosilicates. Reddening of soils is caused by the addition of neoformed pedogenic hematite from the source minerals under favorable conditions. The sources and pedogenic processes of reddening in red Mediterranean soils have been sporadically studied, but not in the abundant sandy coastal plain soils. The current study attempts to fill two gaps: 1. exploring reddening of coastal sandy soils in relation to iron sources, accumulation of dust and soil evolution. The sandy soils were compared to modern dust and to mountainous soils developed on carbonates under the same climatic regime; 2. revealing the role of clay transformation in the release and secretion of iron. For this purpose, mineralogical compositions of bulk samples and clay fraction, as well as bulk chemical composition, were determined. High-resolution transmission electronic microscopy (HRTEM), coupled with energy dispersive spectrometry (EDS) and selected area electron diffraction (SAED), was used for visual observation, point chemical analysis, and crystal structure designation of nanoscale soil particles. The study documents hematite and goethite as the dominant iron minerals in dust, beach sand and coastal sandy soils, as well as in mountainous soils in Israel. The hematite content is higher than that of goethite in all types of samples, except in Calcaric Cambisols, where goethite is dominant. Iron-bearing minerals in dust are transformed in the soil environment to form pedogenic hematite, goethite and ferrihydrite; the scarcity of the latter indicates that the transformation processes are rather rapid. Ilmenite is found, apparently for the first time, as an important source for iron. Dust clays start transformation in the sandy soil environment immediately after deposition, during which adsorbed and structural iron is released and used to form pedogenic iron minerals. The pedogenic processes of iron mineral formation are similar in mountainous and coastal plain soils but are more intense in the latter due to the higher permeability of the sandy substrate.