Palygorskite occurrences and genesis in calcisol and groundwater carbonates of the Tensift Al Haouz area, Central Morocco

Palygorskite has been identified as a major constituent of the clay fraction in carbonate soils of arid and semi-arid regions. This fibrous clay has a great significance as a paleoclimatic indicator and is of fundamental importance for a more complete understanding of conditions under which petrocalcic horizons and groundwater carbonates precipitate. The present study was undertaken in order to evaluate the occurrence, the genesis as well as the micromorphological aspects of palygorskite in calcisol and groundwater carbonates. The samples were analyzed by X-ray Diffraction (XRD), scanning electron microscopy (SEM), and chemical analysis (XRF). Two modes of genesis for palygorskite were identified: (1) In the petrocalcic horizons, the palygorskite is the dominant clay mineral; palygorskite fibers and fiber bundles extend from euhedral or subhedral calcite crystals showing an authigenic origin of palygorskite. The palygorskite is formed in the vadose environment from infiltrating soil water through the intense evaporation of subsurface water after calcite precipitation. (2) In the groundwater carbonates, smectite flakes appear to terminate palygorskite fibers suggesting formation of palygorskite through the alteration of precursor smectite. The increasing aridity would have caused the intense evaporation conducive to the precipitation of calcite and an increase in pH, which lead to dissolution of smectite and formation of palygorskite.