Silica redistribution during weathering of alkaline volcanic rocks drives pedogenic quartz

Quartz in the Colli Albani soils is demonstrated to be a pedogenic mineral rather than an inherited component of the volcanic parent material. Its complete absence from the underlying leucite-bearing pyroclastic deposits, together with distinctive microtextures, provides compelling evidence for in situ precipitation during pedogenesis. Mineralogical relationships, leaching experiments and silica mass-balance calculations identify leucite and analcime as the principal sources of dissolved silica released during weathering. Approximately 84% of the silica released by feldspathoid destabilization was retained within the soil profile through precipitation of quartz and kaolin-group minerals, whereas only a minor fraction was lost in solution. Batch-leaching experiments further indicate that dissolution of the feldspathoid–zeolite assemblage accounts for nearly all dissolved silica mobilized from quartz-free soils, suggesting that any transient amorphous silica reservoir made only a limited contribution. These results demonstrate that extensive quartz neoformation can be sustained entirely by silica derived from feldspathoid weathering indicating that its development is controlled primarily by parent-material mineralogy rather than by total alkali content. These findings suggest that pedogenic quartz may represent a widespread, but largely overlooked, weathering product in alkaline igneous terrains, highlighting the fundamental role of primary mineral assemblages on silica redistribution during soil formation.