Amorphous inorganic materials and mineralogy of tropical mountainous topsoils in the Atlantic Forest southeastern Brazil

Inorganic amorphous materials influence soil physical and chemical properties, with the potential to reflect the environmental conditions, contributing to processes such as nutrient cycling, water retention capacity, and carbon capture and stabilization. This study aimed to evaluate the content and composition of amorphous inorganic material and the mineralogy of the silt-clay fraction in mountainous topsoils of the Atlantic Forest, associating these attributes with indicators of soil weathering. Topsoil samples were collected from two sites in Serra dos Órgãos, RJ (continental slope - BM and oceanic slope - SB), and one site in Serra do Caparaó, ES (SM). Mineralogy was quantified using powder X-ray diffraction (XRD), while total chemical composition was determined by X-ray fluorescence (XRF). The amorphous material content was quantified using the internal standard method, and its composition was assessed through mass balance calculations. Molar ratios indicate the direct dissolution of K-feldspar and plagioclase, forming gibbsite as the main weathering reaction at all sites. Amorphous material constitutes the dominant fraction in all topsoils, ranging between 27 % and 44 %, composed mainly of forms of SiO₂ (100–230 g kg⁻¹) and Fe₂O₃ (30–70 g kg⁻¹). Analyses of molar compositions between primary and secondary minerals and variations in the composition of the amorphous fraction revealed the following trend in weathering degree: SM > SB > BM. The relative SiO₂ content in the amorphous fraction was explained by the rate of feldspar decomposition into gibbsite, which acts as an indicator of the weathering process at the sampling sites.