Abstract
A single soil (dystrochrept) profile developed from weathered gneisses (Gambasca, Po valley, Dora-Maira Massif, SW Alps), was characterized by petro- graphic observations, X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) coupled with microchemical analyses. Di- and trioc- tahedral micas, corresponding to the white mica and to the biotite relics found in petrographic sections, are phengite and more or less K-altered biotite observed by HRTEM, while a third 10A phase, with illitic chemical composition, corresponds to the material surrounding primary micas. Kaolinite and halloysite were commonly observed by HRTEM: free or lying in contact with biotite and phengite (kaolinite) and as hollow tubes or spirals (halloysite). A few chlorite particles were found inti- mately mixed with biotite (Mg- chlorite) and phengite (Al- , Mg- chlorite). XRD diagram decomposition shows disordered and ordered (1:1) interlayered mica- ver- miculite whilst HRTEM data established different vermiculitization stages of bio-
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