Secondary sulfates of Itaquaquecetuba, State of São Paulo

Abstract

Iron sulfides, pyrite and marcasite, occur as cement in the Itaquaquecetuba Formation rocks. Exposure of these minerals to oxidizing conditions causes the formation of acid solutions, which attack feldspars, micas and other materials. Iron, aluminum, calcium, magnesium, potassium, and sodium sulfates precipitate from these solutions. Mineralogical and chemical studies allowed the identification of sulfates melanterite, rozenite, coquimbite, metavoltine, alunogen, epsomite, and gypsum, as well as minerals of the halotrichite, copiapite and alunite groups. Several mineralogical transformations took place after sample collection, including the formation of materials not observed in the Itaquaquecetuba outcrops, such as römerite, paracoquimbite, and an amorphous iron sulfate. The habit of the minerals and aggregates - crusts, saccharoidal or fibrous aggregates and stalactites - reflects rapid crystallization. The approximate sequence of formation of the Itaquaquecetuba sulfates is: 1) hydrated iron (II) sulfates; 2) hydrated sulfates of iron (II), iron (III) and other cations; 3) hydrated hydroxysulfates of iron (III) and other cations. After this stage, two divergent lineages are apparent, one with the formation of hydrated iron (III) sulfates and the other with the formation of hydroxysulfates of iron (III) and possibly other cations. The final stage of alteration produces goethite.