Clay mineral formation in soils of two different chronosequences in the Swiss Alps

Two chronosequences of soils developed from postglacial tills, at two different sites in the Swiss Alps, were studied with respect to their soil mineralogy and chemistry. The age of these soils ranged between 0 and 11,500 years. The parent material for both chronosequences was similar, but differences were found in the clay fraction. Within the considered time span, podzols developed at both sites, however at one site already after about 3000 years and at the other a few years later. Especially trioctahedral minerals were strongly weathered within the time span of about 11,000 years. Smectite and regularly interstratified dioctahedral mica/smectite could be found in the most weathered horizons at one site. The origin of smectite could be traced back to both chlorite and trioctahedral mica which supports the fact that smectite is the end product of chlorite alteration and regularly interstratified mica/smectite (or even smectite) the end product of mica weathering in strongly leached and acidified horizons. At the other site, smectite was already present in the parent material, most presumably due to hydrothermal formation. During the first 500 years of soil formation, a kind of retrograde podzolisation was observed leading to the elimination of this smectite. Within the same time, increased amounts of dioctahedral minerals were formed and after 11,000 years, a small proportion of pedogenic smectite was discernible, again. Main clay mineral transformations occurred at both sites within the first 3000 years of soil development. Weathering and transformation reactions were most advanced in the E horizon being almost complete after 3000 years at one site. The presence of significant amounts of chlorite and/or a minimal content of mica of ca. 5% seems, however, to be compulsory in order to detect low charge expandable minerals already after a 3000-year duration of soil formation in the Alps, otherwise smectite formation is retarded.