Mineral alteration and genesis of Al–rich soils derived from conglomerate deposits in Cabo Basin, NE Brazil

In the Cabo Basin, NE Brazil, conglomerates have given rise to soils with attributes that are rarely observed in humid tropical climates and remain relatively unknown within the scientific community. These soils have high levels of Al in the exchangeable phase, diminishing their agricultural and environmental potential in an underdeveloped region of great socioeconomic importance. This paper presents the mineralogy and genesis of soils derived from conglomerates on a slope in the Cabo Basin, with the aim of meeting the demand for studies on Al–rich soils. Three soils were classified, morphologically described and sampled for the accomplishment of chemical, physical and mineralogical analyses, including the main exchangeable cations content, mineralogy of the sand, silt and clay fraction, differentiation of the Fe and Al forms, mineral micromorphology and chemical composition of mineral species. In addition, chemical and physical attributes were used in multivariate analyses. The soil mineralogy is associated with the weathering evolution of three mineralogical zones identified in the Cr horizons. Pathways of alteration followed by feldspars and micas resulted in kaolinite, smectite and hydroxy–Al interlayered smectite formation (argilification). The susceptibility of the conglomerate to weathering, constant releases of Al from the structure of aluminous minerals, low contents of exchangeable bases and consequent low pH values (<5.3) were associated with high levels of exchangeable Al. At the highest position on the slope, a C/Cr transitional horizon provided evidence of the pedogenesis advance on the Cr horizon. Pedogenetic processes then resulted in morphological, physical and mineralogical differentiation between the A and Bt horizons. In addition, a cluster analysis showed similarities between attributes of the Bt and Cr horizons due to their genetic relationships.