Mineralogical, micromorphological and geochemical transformations in the initial steps of the weathering process of charnockite from the Caparaó Range, southeastern Brazil

- The hydrolysis of charnockite occurred in different stages.
- The alteration occured in accordance with the order of stability of minerals.
- For plagioclase and orthopyroxene the alteration occurs in a single stage.
- For the alkali feldspar and amphibole the alteration happens in two steps.
- Formation of an isalteritic horizon.

X-ray diffraction (XRD), X-ray Fluorescence (XRF), optical microscopy, Scanning Electron Microscopy coupled with Energy Dispersive Spectrometry (SEM-EDS) and Electron Probe micro-analyser (EPMA) and Wavelength-Dispersive Spectroscopy (WDS) were conducted on charnockite from the Caparaó Suite and its alteration cortex to determine the mineralogical, micromorphological and geochemical transformations resulting from the weathering process. The hydrolysis of the charnockite occurred in different stages, in accordance with the order of stability of the minerals with respect to weathering: andesine/orthopyroxene, pargasite and alkali feldspar. The rock modifications had begun with the formation of a layer of incipient alteration due to the percolation of weathering solutions first in the pressure relief fractures and then in cleavage and mineral edges. The iron exuded from ferromagnesian minerals precipitated in the intermineral and intramineral discontinuities. The layer of incipient alteration evolves into an inner cortex where the plagioclase changes into gibbsite by direct alitisation, the ferromagnesian minerals initiate the formation of goethitic boxworks with kaolinitic cores, and the alkali feldspar initiates indirect transformation into gibbsite, forming an intermediate phase of illite and kaolinite. In the outer cortex, mostly traces of alkali feldspar remain, and they are surrounded by goethite and gibbsite as alteromorphics, characterising the formation of the isalteritic horizon that occurs along the slope and explains the bauxitization process at the Caparaó Range, SE Brazil.