PALYGORSKITE SMECTITE ASSOCIATION IN A XEROCHREPT OF THE HIGH CHAOUIA REGION OF MOROCCO

The coexistence of smectite and palygorskite was investigated in representative Xerochrept of the High Chaouia region of Morocco This soil developed on an Upper Cretaceous marl under a semiarid climate. X-ray diffraction analysis demonstrated the predominance o smectite and palygorskite. Scanning and transmission electron microscopy confirmed the presence of a fibrous clay. Palygorskite aggregates were found concentrated in the sand and silt fractions of the Bkm, C, and R horizons but these aggregates were not detected in the Ap horizon, probably because of destruction by weathering. Smectite constitutes the major clay phase of the Ap horizon, with a lesser content of palygorskite. In the C and R horizons, clay-sized palygorskite is more abundant than smectite. Microprobe analysis and mineral formula calculation suggest that the palygorskite is high in tetrahedral Al (0.4 per half unit cell) with 53% of the octahedral sites occupied by Al. Cation-exchange capacity measurements and reexpansion after heating with Li suggest that the smectitic phase has a high charge with significant Al substitution in the tetrahedral sheet. The palygorskite was probably inherited from the Cretaceous marl parent material and much of it has been weathered out of the Ap horizon. most or the smectite is probably also inherited, although some may be the product of palygorskite weathering.