Weathering of anorthosite saprolite: Influence on soil formation in the Borborema province (northeastern Brazil)

Abstract

Anorthosites are plutonic rocks of great interest in earth and planetary sciences, characterized by their distinctive composition, with more than 90% consisting of plagioclase feldspar. These rocks are of particular relevance due to their distinctive mineral composition and notable interest in terrestrial and planetary bodies, including the moon. Despite the abundance of research on the weathering of anorthosites, there is a paucity of studies that have examined the soil developed on this rock. In this study, regolith profiles from the Anorthositic Complex of Passira, located in the northeastern region of Brazil, were selected for analysis. Analyses were conducted, encompassing physical, morphological, chemical, mineralogical, and micromorphological analyses. The results provided insights into the porosity system of the saprolite that markedly influenced the pedogenetic processes and the formation of clay minerals. A novel observation regarding anorthosite weathering in a tropical region was reported: the saprolite with greater preservation of the lithic material and fewer microstructural fracturing favored the formation of 2:1 clay minerals due to greater silica retention in the soil solution, resulting in a formation of Cambisol with shrink-swell protovertic features. Conversely, saprolites exhibiting greater porosity and augmented microstructural fracturing promoted the formation of 1:1 clay minerals due to enhanced silica leaching, resulting in the formation of an Lixisol.