ISOTROPIC SMECTITE COATINGS AND SPHEROIDS IN HOLOCENE VOLCANIC DEPOSITS UNDER TEMPERATE CLIMATE, FRANCE

Isotropic clay coatings and spheroids were observed in Holocene, exces sively drained, basaltic scoria deposits underlying a Hapludand at the Puy de la Vache volcano in the Massif Central in France. The morpholo gy, chemistry, and mineralogy of these neoformed clay fabrics were stu died by optical microscopy and analytical scanning and transmission el ectron microscopy. Optical microscopy of samples taken at 1- to 3-m de pth in the deposit shelved thin, isotropic coatings and spheroids insi de vesicles of the scoria fragments. In addition, isotropic coatings, occasionally up to 200 mu m thick, covered scoria fragments, forming b ridges between them. The isotropic coatings indicated a post-depositio nal, pedogenic origin. The internal fabric of coatings and spheroids w ere similar, suggesting a common genetic relationship. Although isotro pic behavior of coatings generally points to an amorphous mineralogica l composition, analytical electron microscopy showed the presence of s mall (<1 mu m) crinkly dioctahedral phyllosilicate particles, forming a honeycomb pattern. Periodically restricted drainage conditions in th e 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 pe rmeability. Since scoria fragments at 3-m depth were unaltered, format ion 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 pa rticles and their occasional association with amorphous compounds expl ained the isotropic properties of the clay coatings when observed unde r the polarizing microscope.