Location of natural trace elements in silty soils using particle-size fractionation

In silty material, 80% of the particles range from 2 to 50 μm. Particle-size fractionation within this size range allows determination of the main mineral bearers of natural trace elements (TE) in silty soil. Three silty soils were studied: at Château-Thierry in the Paris Basin, at Muret in the Aquitaine Basin, and at Thann in the Alsace plain. These soils mainly consist of quartz, alkaline feldspars, plagioclase, micas and chlorite, the quantities of which vary with particle-size as shown by XRD and chemical analyses of major elements. Quartz content increases with increasing particle size, whereas feldspars, micas and chlorites occur mainly in the 2–10, 0.2–2 and < 0.2 μm particle-size fractions, respectively. Cu, Co, Cr, Ni, Zn, Pb and major-element contents were measured; TE values showed a negative correlation with Si and a positive one with Al, related to the absence of TE in quartz. Negative correlations of TE with Na and Ca are interpreted as the absence of TE in alkaline feldspars and plagioclase. TE values correlate with Mg, suggesting that they are present in chlorite and trioctahedral micas. Correlations of TE with K are more complex, K-bearing minerals being K-feldspar and micas. In the < 2 μm fractions, K is lost while TE contents increase. Micas were thus considered to be one of the main TE bearers. As correlation does not pass through zero, other K-bearing minerals such as K-feldspar were considered as TE free. Generally speaking, TE contents increase as particle-size decreases.