Iron‐Rich High‐Charge Beidellite in Vertisols and Mollisols of the High Chaouia Region of Morocco

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Abstract
The mineralogy of Vertisols and Mollisols, associated in a hydrotoposequence in the western part of the phosphate plateau of Morocco (High Chaouia), was studied using x‐ray diffraction, interlayer swelling with alkylammonium cations, chemical analysis, and infrared spectroscopy. The swelling behavior of Li‐ and K‐saturated clays demonstrated the beidellitic character of the expandable 2:1 layer silicates that dominate the clay mineralogy of these soils. The beidellite has a heterogeneous layer charge, as measured by the swelling with alkylammonium cations, that ranged from 0.50 to 0.66 charges per half unit cell. The swelling of Li‐saturated clays with alkylammonium cations after heating at 300 °C confirmed the tetrahedral location of the charge. Structural formulas calculated for the fine clays showed that the beidellite is similar in all three soils and confirmed that these clays have a high charge density and are Fe rich. Infrared spectroscopy demonstrated that these beidellites contain substantial amounts of Fe3+ in the octahedral sheet. Kaolinite, chlorite, palygorskite, and quartz were also present, especially in the coarse‐clay fractions and in the subsoil. The presence of this Fe‐rich, high‐charge beidellite in the soils helps explain the behavior of soil K and also offers an explanation of K soil test interpretation problems.