Original paper

Characterization of clay minerals in a dystrochrept developed in a gneiss (Dora-Maira Massif, Western Alps). Complementarity between X-ray diffraction and transmission electron microscopy

Alessandria, Bruno; Aquilano, Dino; Amouric, Marc; Grauby, Olivier


A single soil (dystrochrept) profile developed from weathered gneisses (Gambasca, Po valley, Dora-Maira Massif, SW Alps), was characterized by petrographic observations, X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) coupled with microchemical analyses. Di- and trioctahedral 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 10Å 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 intimately mixed with biotite (Mg–chlorite) and phengite (Al–, Mg–chlorite). XRD diagram decomposition shows disordered and ordered (1:1) interlayered mica–vermiculite whilst HRTEM data established different vermiculitization stages of biotite.