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X-ray mineralogical examination of the 2 20/~m fraction of oriented samples from a highly calcereous swamp soil and neighbouring Mio-Pliocene strata in AI-Hasa showed a wide range in mineralogy: attapulgite, illite, montmorillonite, chlorite and kaolinite, in decreasing order. Powder X-ray diffraction revealed an abundance of attapulgite, calcite, quartz and in a few samples, traces of dolomite, Fe-oxides, feldspars and gypsum.The soils are believed to have been formed by deposition of highly calcareous wind-borne silt particles in an already formed swamp. The surface and subsurface horizons are underlain by an organic residue layer, 30-50 cm thick, containing 14-36~ organic matter. The whole profile sits on hard, impervious, sedimentary layers that mineralogically resemble some strata of the outcrop.Attapulgite and illite are the most abundant and common clay minerals in the soils and strata, but are most pronounced in the latter. Attapulgite not only occurs in A1-Hasa, but is widely distributed in the Arabian Peninsula; from east of the Mediterranean through the deserts of Syria and Iraq down to eastern Saudi Arabia, where it occurs only in association with limestone parent material. In this area of extreme aridity (A1-Hasa), the origin of the minerals and their relative abundance is believed to be associated with the source of sediment since minimum alteration seems to have taken place after deposition.