Saprolithology applied to pedology: Mineral alteration in soil-saprolite profiles along a climate gradient in Triunfo Massif (NE Brazil)

Abstract

Saprolites, along with bedrock, are the main parent materials of soils worldwide. In Northeast Brazil, the climate-geomorphology relationship in specific regions, called “humid islands”, affects saprolite weathering. This regional peculiarity generates a climatic gradient that influences the weathering of crystalline rocks and the genesis of soil-saprolite systems representative of other regoliths found on a global scale. In this study, we combined saprolithology and pedology (soil mineralogy and soil geochemistry) to better understand the weathering process of soil-saprolite profiles derived from syenite along the Triunfo Massif – NE Brazil. We collected soil and saprolite samples to determine the primary and secondary mineralogical assemblies and describe possible pathways of alteration for these minerals using X-ray diffractometry, optical microscopy, and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. We also determined the weathering degrees of soil horizons and saprolite layers based on mineralogical (Xd and Ip) and chemical indexes (chemical index of alteration, chemical index of weathering, and plagioclase index of alteration). The results showed that the climatic gradient influenced the weathering of minerals, such as K-feldspars, plagioclases, biotite, and hornblende and the pathways of alteration of these primary minerals. We also found that the types of secondary minerals formed are rather similar. The wetter climate at the Massif summit resulted in higher weathering index values and favored pathways of alteration associated to the formation of iron oxides and 1:1 secondary phyllosilicates. The drier climate at the Massif base favored other pathways of alteration associated to the formation of 2:1 secondary phyllosilicates. In addition, micromorphological and geochemical evidence demonstrated an inversion in the classical sequence of mineral stability and biotite was more altered than hornblende placed side by side, possibly due to the influence of structural and geochemical factors.