A previous classification scheme for clay minerals is revised and extended; the underlying principles are analyzed in detail.

Some minerals which have been referred to as corrensite are considered together with some related minerals. It is suggested that the name corrensite be retained for the original material so described—namely, a I:1 regular interstratification of chlorite and swelling chlorite. The possible constitution of swelling chlorite, its difference from vermiculite, and its relationship to labile chlorites produced synthetically are all discussed.

Fresh samples of some Scottish basic igneous rocks contain two-component interstratified minerals consisting mainly of layers of chlorite, swelling chlorite and saponite in different combinations. The differential thermal curves are characteristic of swelling chlorite.

The chemical method for preparing pseudo-chlorite from montmorillonite suffers from the disadvantage that the magnesium hydroxide is soluble in the ammonium chloride formed. The electrolytic method described overcomes this difficulty and, in addition, gives preparations with up to five brucite layers between the montmorillonite sheets. Ions other than magnesium may also be used. A simple technique for overcoming difficulties associated with preferred orientation in the Brindley powder X-ray camera is described.

Analyses of shales, mudstones and associated strata from the British Coal Measures reflect the general similarity of the clay mineralogy of the rocks studied. Variations in the Na+:K+ ratio appear to be due to variations in the Na+:K+ ratio in illite. It is suggested that degraded illite supplied to the site of sedimentation had a higher Na+:K+ ratio than the illite now present in the shales and that post-depositional changes have caused a fall in this ratio. Under conditions of rapid sedimentation such changes tend to be inhibited by rapid burial. In the seat earths, produced by very slow sedimentation, post-depositional changes continued to an advanced stage and resulted in low Na+:K+ ratio in the final sedimentary rock.

Montmorillonite is found to be a predominant clay mineral in the < 2µ fraction of the A2 horizon of podzol profiles in Fennoscandia, even when these are only 300 years old. In fractions between 2µ and 20µ vermiculite and hydrobiotite predominate, but are rarely detected in the fraction coarser than 20µ. Vermiculite and hydrobiotite are also found in the finer fractions of the B horizon, but are only rarely detected in the C horizon where their presence is apparently due to contamination with interglacial or older products of weathering. The presence of kaolinite in some profiles is explicable in the same way.