Relationships between magnetic parameters, chemical composition and clay minerals of topsoils near Coimbra, central Portugal

Abstract

Magnetic measurements, mineralogical and geochemical studies were carried out on surface soil samples in order to find possible relationships and to obtain environmental implications. The samples were taken over a square grid (500×500 m) near the city of Coimbra, in central Portugal. Mass specific magnetic susceptibility ranges between 12.50 and 710.11×10−8 m3 kg−1 and isothermal magnetic remanence at 1 tesla values range between 253 and 18 174×10−3 Am−1. Chemical analysis by atomic absorption spectrometry shows that the concentration of various toxic elements was higher than the mean background values for world soils. Higher values of susceptibility and toxic elements content were reported near roads and rivers. Urban pollution and road traffic emission seem to be the main influence for these values. A semi-quantitative X-ray diffraction study has been carried out on a representative set of subsamples, using peak areas. Illite (average 52 %), kaolinite (average 55 %), chlorite (average 6 %) and irregular illite-smectite mixed-layers (average 9 %) are the major clay minerals groups identified. Mineral composition of total fraction confirms the presence of magnetite/maghemite. The clay minerals results point to a contrast in the behavior of the main clay minerals: illite, chlorite, and kaolinite (also, smectite in some samples), which are generally in agreement with the magnetic and geochemical data. The results showed that magnetic measurements are a sensitive, fast, inexpensive and robust method, which can be advantageously applied for studying soils affected by urban and road pollution.