Clay fractions and glauconite (clays) lose 30-80% of the 3 9 Ar produced by the nuclear transformation 3 9 K(n, p) 3 9 Ar during irradiation in a reactor. As a result it has not been possible to date clays by the 4 0 Ar- 3 9 Ar step-heating technique. New observations on 3 9 Ar loss in clays are presented, and the transport behavior of the 3 9 Ar recoils in the elementary clay particle was simulated by the Monte-Carlo method. A new procedure for dating clays by the 4 0 Ar- 3 9 Ar step-heating technique is proposed. The use of an aliquot of the same clay, along with its K-Ar age, is suggested as a specific monitor of the irradiated sample, enabling stepwise 4 0 Ar- 3 9 Ar analysis. This specific monitor monitors, without changes in the experimental setup, the integral of 3 9 Ar production from 3 9 K during irradiation and its retention in the solid substrate. The new procedure was successfully demonstrated on the geochronological standard glauconite GL-O.