The studies of weathering of ultrabasic rocks are of great interest both from a geochemical point of view and related to the study of ore deposits. Geochemically, the low Al content leads us to consider these rocks as belonging to a specific sifemic series, whose composition is leaded by Si, Mg and Fe; surface weathering in this case is dominated by the dynamic properties of the pair Si-Mg instead of Si-Al. Here the purpose was to describe the main features of the initial stages of weathering of the serpentinized Barro Alto ultrabasic rocks, dominated by the properties of the original rocks. Following that line of reasoning residual phases of well drained and incipently developped profiles have been studied. Mineralogically, the saprolite in contact with the partially weathered rocks is heterogeneous. Olivine nuclei of the original rock have been completely removed and in their place we find either cavities or amorphous material. In due time this material may evolue with loss of silicon and goethite formation. Pyroxenes are almost completely weathered, giving rise to talc and releasing the iron which remains on the proflle forming either amorphous products or goethite. Quartz and chlorite, though not occurring in the serpentinized rock, are abundant in this horizon, and are possibly derived from the action of hypogenic solutions. In the higher horizons above the saprolite, chlorite slowly hegins its weathering process giving rise to intergraded chlorite-smectites, with a slight expansion. Antigorites from the serpentine groups became pratically absent, and talc remains as a stable surface product. Goethite becomes the main mineral in these horizons. Chromite and the other ore minerals are almost intouched by weathering and occur as residual concentrates of the higher horizons. Geochemically the profile evolues with iron (III) enrichment and concomitant loss of Si and most of the Mg, which is still, found . only in the higher levels as magnesian chlorites. A high Ni concentration is present throughout but never attains the structure of an autonomous mineral. It behaves, therefore, as a typical trace-element. Its occurence in the lowest weathering horizons is related to the amorphous compunds that are in the saprolite and capture Ni by adsortion and do not release it on account of the prevailing physico-chemical conditions.
