Bauxitisation of anorthosites from Central Brazil

• Published in: Geoderma Vol. 167; pp. 319 - 327
• Main Authors: de Oliveira, Fábio Soares, Varajão, Angélica Fortes Drummond Chicarino, Varajão, César Augusto Chicarino, Boulangé, Bruno, Gomes, Newton Souza
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-11-2011; Elsevier
• Subjects: Agronomy. Soil science and plant productions; Alitisation; Aluminum base alloys; Anorthosite; Barro Alto Complex; Bauxite; Bayer process; Biological and medical sciences; Brazil; Earth sciences; Earth, ocean, space; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; Geochemistry; Isalteritic transformation; Minerals; Plagioclase; Soil and rock geochemistry; Soils; Surficial geology; Transformations; Weathering; South America; Brazil; Goias Brazil; Barro Alto Complex; Bauxite; Alitisation; Isalteritic transformation; Anorthosite; oxides; X-ray diffraction; SEM data; feldspar; bytownite; drill cores; Optical microscopy; silicates; framework silicates; soils; fractures; scanning electron microscopy; electron probe; bauxite; weathering; igneous rocks; plagioclase; micromorphology; hydroxides; sedimentary rocks; X-ray fluorescence; gibbsite; anorthosite; plutonic rocks; hydrolysis; ultramafics
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2011.09.006

Petrological studies using X-ray diffraction (XRD), X-ray fluorescence (XRF), optical microscopy, scanning electron microscopy (SEM-EDS) and electron microprobe analyzer (WDS) showed the mineralogical, micromorphological and geochemical transformations due to the bauxitisation of anorthosite from the Barro Alto Stratiform Mafic–Ultramafic Complex (Central Brazil). The hydrolytic alteration of the anorthosite occurred in two different stages in accordance with the order of stability of the minerals to the weathering: firstly the bytownite and secondly the ferromagnesian minerals. The weathering solutions, benefited from the existing network of fractures, percolated the weakness zones of the minerals characterising the microsystem of contact in which cores of plagioclase and ferromagnesian minerals were formed. In the first stage, the plagioclases are altered directly to gibbsite, and at an early stage, during the change process, the gibbsite crystals surround the primary ferromagnesian minerals that are totally or partially preserved. The transformation is isalteritic and is responsible for formation of porous alteromorphs consisting of septa of coarse gibbsite and fine gibbsite. As the weathering process advances, the ferromagnesian minerals directly enter to goethite. The boxworks of gibbsite and goethite characterise a primary plasmic microsystem.
► The alteration of the anorthosite in Barro Alto occurred by alitisation process. ► The plagioclases are altered directly for gibbsite. ► The ferromagnesian minerals are altered directly for goethite. ► The box works indicated an pseudomorphs of primary minerals. ► The bauxitisation is characterised by an isalteritic transformation.