Direct evidence of basic aluminium sulphate minerals in an S-imapcted Andosol.

The formation of basic aluminium sulphate (BAS) minerals [(K,Na)nAlx(OH)y(SO4)z] has often been invoked to explain sulphate retention in soils. These minerals have not yet, however, been directly observed in the soil. We extracted the clay fractions of Andosols intercepting large inputs of volcanogenic sulphur dioxide and acids (HCl, HF), simply by dispersing clays with Na+-resins in deionized water without any other chemical treatment. Clay fractions concentrate 39–63% of total sulphur content of soil. Transmission electron microscopy coupled with energy-dispersive analysis revealed the presence of BAS particles, appearing as nodules and spheres. These particles have an equivalent diameter smaller than 0.2 µm. They have an Al:S ratio close to 2.2 and 3.8 and are possibly amorphous aluminite or basaluminite, respectively. They seem to have been formed in microenvironments enriched in sulphate, but also in fluoride anions. Their formation seems to have been enhanced by the combination of large inputs of acids and SO2 and an effective Al supply from weathering of volcanic glass