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Berthierine. an iron-rich trioctahedral 1:1 layer silicate (d(001) = 7.04 angstrom), is a major component in the A and C horizons of the Jaeger series. an Arctic desert soil, at Ellef Rinenes Island. Northwest Territories, Heating the powder sample to 300 degrees C displaces the 060 reflection from 1.548 to 1.513 angstrom, thus indicating the ffansformation of the trioctahedral berthierine structure to a dioctahedral one owing to the oxidation of Fe2+ to Fe3+. Judging from the d value of 060 and the relative intensities of other reflections of the unheated sample. berthierine in the soil is a mixture of monoclinic and orthorhombic polytypes having an intermediate ferrous-ferric composition. Chemical data suppoft the proposed intermediate composition. Althoueh berthierine is highly susceptible to weathering. the severely restricted weathering environment in the Arctic region is the main reason for the rare survival of berthierine in the Jaeger soil.