Mineralogical evolution of supergenic alteration in alkaline rocks of Lages, southern Brazil

The District of Lages (SC Brazil) is the southernmost known bauxite deposit of Brazil. Mineralogical and chemical changes of phonolites show that nepheline is the first mineral to change, releasing sodium first and potassium and silicon later. The alteration of nepheline forms dissolution cavities which highly increase rock porosity. In some cases gibbsite partially or entirely fills the cavities. Globular material is found side by side with large crystals of gibbsite on the corroded K-feldspar. The advanced alteration of feldspar converges to gibbsite in the final stage but locally shows an intermediate silico-aluminous phase. Pyroxenes are altered to goethite along the cleavage and parting planes at first and are almost completely replaced in more advanced stages of alteration. Later veinlets of white halloysite and lithiophorite run accross the entire weathering profile.Formoso M.L.L., Nahon Daniel, Novikoff André, Valeton Ida. Mineralogical evolution of supergenic alteration in alkaline rocks of Lages, southern Brazil. In: Proceedings of the 9th international Clay Conference, Strasbourg, 1989. Vol I : Clay-organic interactions. Clay minerals in soils. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1990. pp. 165-173. (Sciences Géologiques. Mémoire, 85