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

Two chronosequences of soils developed from postglacial tills, at two diffe rent sites in the Swiss Alps, were studied with respect to their soil miner alogy and chemistry. The age of these soils ranged between 0 and 11,500 yea rs. The parent material for both chrono sequences was similar, but differen ces were found in the clay fraction. Within the considered time span, podzo ls developed at both sites, however at one site already after about 3000 ye ars and at the other a few years later. Especially trioctahedral minerals w ere 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 t raced back to both chlorite and trioctahedral mica which supports the fact that smectite is the end product of chlorite alteration and regularly inter stratified mica/smectite (or even smectite) the end product of mica weather ing in strongly leached and acidified horizons. At the other site, smectite was already present in the parent material, most presumably due to hydroth ermal formation. During the first 500 years of soil formation, a kind of re trograde podzolisation was observed leading to the elimination of this smec tite. Within the same time, increased amounts of dioctahedral minerals were formed and after 11,000 years, a small proportion of pedogenic smectite wa s discernible, again. Main clay mineral transformations occurred at both si tes within the first 3000 years of soil development. Weathering and transfo rmation reactions were most advanced in the E horizon being almost complete after 3000 years at one site. The presence of significant amounts of chlor ite and/or a minimal content of mica of ca. 5% seems, however, to be compul sory 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. (C) 2001 Elsevier Science B.V. All rights reserved.