Weathering and mineralogical evolution in a high Alpine soil chronosequence: A combined approach using SEM–EDX, cathodoluminescence and Nomarski DIC microscopy

Abstract

Physical and chemical weathering of primary minerals of granitic till in the proglacial area of Morteratsch (Switzerland) was investigated using cathodoluminescence (CL), Nomarski differential interference contrast (DIC) microscopy and scanning electron microscope (SEM–EDX). The investigated time-span ranges from 0 to 140 years of sediment exposure. For the very early stage of weathering or soil formation only little information is available. The main aim of our investigation was consequently to see whether weathering of primary minerals can be detected in such a short time-span using for the first time for soils well-established methods as CL and Nomarski DIC microscopy in geo- and material science such. For that purpose, the fine earth fraction (<2 mm) of topsoil samples was investigated. Some physical weathering had taken place within 140 years. The delamination of biotite seems to increase with time. SEM and CL analyses also demonstrate early weathering of quartz by evidencing edge pits and structural bonds – such as Si-O-Si in quartz – that start to break and to transform into free radicals. K-feldspar and plagioclase contain Fe. When using Fe3+ as reference point (680–700 nm) to standardise the CL spectra, the Al-O⁻-Al defects of K-feldspar exhibit a relative decrease with time; this was not the case for plagioclase. The CL measurements showed that the investigated apatite contained REE (rare earth elements) in the crystal structure. However, none of the other techniques (DIC, SEM–EDX) was helpful in detecting any specific weathering features for apatite. In the time span of 140 years, epidote weathering was evidenced using XRD in a previous investigation and here using DIC microscopy (morphologic changes). Several mineral changes could be traced within a very short weathering sequence using the applied techniques. These changes include physical (e.g. biotite), chemical or crystal structure (K-feldspar, biotite) features. Such an analytical combination is promising, therefore, for the detection of chemical, physical and mineralogical characteristics and changes in very young glacial sediments.