Structural transformations of clay minerals in soils of a climosequence in an Italian Alpine environment

Clays of a soil sequence with five profiles in the Val Genova (northern Italy) along an elevation gradient with climate ranging from moderate to Alpine were investigated with XRD using several diagnostic treatments. Smectites developed in the surface horizons of podzolic soils either from chlorite through the removal of hydroxy interlayers or from mica, which weathers in a first step to regularly or irregularly interstratified clay minerals. Citrate treatment allowed the detection of low-charge expandable minerals in the Bhs or Bs horizons. Therefore, the reduction of the charge of 2:1 clay minerals occurred before the removal of hydroxy polymers by fulvic acids and low-weight organic acids. Due to the more intense podzolization process near the tree line, the d(060) region showed a temporal evolution of trioctahedral to dioctahedral mineral structures in the well developed Podzols. The pedogenic smectites of the E or Bhs horizons generally included one or several populations with various charges. In most cases, smectite was a heterogeneous mixture of montmorillonite and interstratified beidellite-montmorillonite. A pure beidellite phase could not be detected. The soils near the tree line, where weathering processes were most intense, had two main components: one with a charge >0.75, representing vermiculite-like minerals, and the other with a charge near 0.25, representing smectite. The charges of the beidellitic component and montmorillonite were almost equal. The higher the weathering state of the investigated soils, the lower was the layer charge of smectites.