Relationships between quartz weathering and soil type (Entisol, Inceptisol and Alfisol) in Sierra Nevada (southeast Spain)

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Quartz was studied in a soil development sequence of Entisol-Inceptisol-Alfisol in a Mediterranean mountain climate on a metamorphic quartz-rich substrate, using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and backscattered electron (BSE) and electron microprobes (EMP), with fine sand (250–50 μm) as the selected particle-size fraction. The quartz in the soil is inherited mainly from the parent rock. However, a study of the surface characteristics of the quartz grains and measurement of non-Si elements (Al3+, Ti4+, Fe3+, Na+, K+, Mg2+ and Ca2+), both near-edge and in the centre of grains, showed that the quartz is altered substantially in the progression parent rock < Entisol < Inceptisol < Alfisol, and in the A, AC and Bt horizons rather than in the C horizon. In all profiles and horizons the content of non-Si elements was greater in the near-edge zones than in the centre of the grain. This pedochemical process thus operates mainly at the surface of the grain and results in a variety of pits, fissures and other corrosion features (hole or corrosion gulf, opening of inter-granular surfaces, areal alteration, smoothing of edges of conchoidal fractures, and deepening of incisions of conchoidal fractures). Estimation of the W index, which assesses the intensity of the weathering process from the size and density of the etch pits, revealed that many of the quartz grains had alterations somewhat similar to the quartzes of tropical soils. This study casts doubt on the paradigm of quartz inalterability in soils of little to medium pedological change in temperate zones.

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