Composition and origin of smectite in soils derived from basalt

Abstract--The sand and silt fractions of soils derived from Tertiary basalt in northeastern Ireland exhibit unusually high cation-exchange capacities. Smectite was identified by X-ray powder diffraction in all size fractions and was selectively dissolved by treatment with 1 N HCI and 0.5 N NaOH. Structural formulae, based on chemical analysis of the dissolved material, suggest that he smectite is intermediate in compo-sition between adi- and a trioctahedral mineral and that the octahedral cation occupancy, together with the numbers of Mg and Fe atoms per unit cell, decreases with decrease in particle size of the soil fraction examined. The mean formula for the smectite in the clay separates i: M§ Fe3+0.4,Mgo.s2)O,0(OH)v Similar analysis of smectite present in clay-size material separated from weath-ered basalt rock taken from the bases of the soil profiles howed that it had close to the full trioctahedral occupancy. It is suggested that the soil smectite isa weathering product of the trioctahedrai smectite in-herited from the weathered basalt