Isotropic Smectite Coatings and Spheroids in Holocene Volcanic Deposits under Temperate Climate, France

Isotropic clay coatings and spheroids were observed in Holocene, excessively drained, basaltic scoria deposits underlying a Hapludand at the Puy de la Vache volcano in the Massif Central in France. The morphology, chemistry, and mineralogy of these neoformed clay fabrics were studied by optical microscopy and analytical scanning and transmission electron microscopy. Optical microscopy of samples taken at 1‐ to 3‐m depth in the deposit showed thin, isotropic coatings and spheroids inside vesicles of the scoria fragments. In addition, isotropic coatings, occasionally up to 200 µm thick, covered scoria fragments, forming bridges between them. The isotropic coatings indicated a post‐depositional, pedogenic origin. The internal fabric of coatings and spheroids were similar, suggesting a common genetic relationship. Although isotropic behavior of coatings generally points to an amorphous mineralogical composition, analytical electron microscopy showed the presence of small (<1 µm) crinkly dioctahedral phyllosilicate particles, forming a honeycomb pattern. Periodically restricted drainage conditions in the numerous meso‐ and microvesicular pores of the scoriae were assumed to cause local and temporary stagnation of Si‐rich soil solutions and, consequently, to favor smectite formation despite the overall high permeability. Since scoria fragments at 3‐m depth were unaltered, formation of the secondary clay coatings was ascribed to co‐precipitation of leached elements liberated by weathering in the overlying Hapludand. The small size and random orientation patterns of the smectite clay particles and their occasional association with amorphous compounds explained the isotropic properties of the clay coatings when observed under the polarizing microscope.