Hydroxy-Al polymers block the frayed edge sites of illitic minerals in acid soils: studies in southwestern Japan at various weathering stages

The transformation of hydroxy-interlayered vermiculite (HIV) to vermiculite in humid temperate soils may involve the gradual alteration of illitic minerals. However, it is difficult to detect such minor and progressive changes using conventional methods. We measured the amount of the frayed edge site (i.e. the weathering front of illitic minerals) in soil clays using Radiocaesium Interception Potential (RIP) methodology, and elucidated the effect of hydroxy-Al polymers on the frayed edge site that may be occluded within HIV structures in the acidic soils of southwestern Japan. X-ray diffraction patterns showed the progressive transformation of HIV to vermiculite, or further, to smectite in soil clays located in upper horizons and therefore subjected to more intensive podzolization. In this process, the amount of hydroxy-Al polymers (represented by hot-citrate Al) gradually decreased, while the vermiculitic charges increased (represented by Cs-fixing capacity). However, the amount of the frayed edge site (represented by the RIP) firstly increased but then reversed and decreased in the upper layers of podzolic soils. After hot-citrate extraction to remove the hydroxy-Al polymers from HIV, the amount of the frayed edge site increased in HIV-dominated clays, whereas there was a negligible increase in vermiculitic- or smectitic-clays, thus indicating the blockage effect of hydroxy-Al polymers on the frayed edge site. The sequential increase followed by the decrease that we documented in the amount of the frayed edge site along with the HIV-vermiculite-smectite transformation, suggested that the weathering front of illitic minerals was exposed as the HIV layers lose hydroxy-Al polymers. Thereafter, the charges in the exposed frayed edge site might decline under the very acidic conditions of the upper layers of podzolic soils.