The 2/1 clay minerals (smectites) are the principal secondary components in the inicial levels of pyroxenitc weathering of the Santa Fé ultrabasic massif. Detailed mineralógical studies (RX, DTA, optical microscopy and scanning electron microscopy) on the principal minerals from the pyroxenites-pyroxene and trioctahedral micas - in several weathering horizons indicate that the smectites have different origins. Beginning with the first stages of weathering, the pyroxenes hydrolyze, giving origin to the 2/1 structures by neogenesis. These ferriferous smectites (nontronites) are the principal minerals in the clay fraction of the fine saprolite. On the other hand, the trioctahedral micas, namely mixed layer mica-vermiculite structures, are highly stable in the surface environment and preserve their original shape, even in the upper lateritic horizons. Chemically, a loss of Si and Mg and a gain of Al occur, whereas mineralogically the analysis by scanning electron microscopy reveals an incipient transformation in to smectite. These 2/1 clay minerals grow on the rims of the micaceous crystals or in the zones that serve as preferencial conduits for altering solutions. Such smectites appear in the coarsest fraction of the saprolite associated with micaceous lamellae that have been morphologically modified by the formation of this secondary mineral.
