STUDIES ON THE BASALTIC SOILS OF NORTHERN IRELAND

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Abstract
Summary: The fine sand separates (0.02–0.2 mm.) from five basaltic soil profiles have considerable cation‐exchange capacities (C.E.C.) ranging from 16.5 to 31.4 m.e. per cent. Only a small part of the C.E.E. is due to clay minerals (diameter > 2μ) cemented into aggregates of fine‐sand dimensions by free sesquioxides.Mineralogical studies of the fractions obtained, after separation by the bromoform technique, show all heavy mineral groups (s.g. > 2.85) to have a high content of pyroxenes (principally augite) followed by the opaque minerals (mainly magnetite). The heavy groups have small C.E.C., i.e. < 7.5 m.e. per cent., as expected from the mineralogical composition.In the light fractions (s.g. < 2.85) which account for 57–80 per cent. of the total fine‐sand separates, quartz predominates in fractions from surface horizons of the profiles. In the lower horizons where quartz is less abundant, greenish‐yellow particles (pseudo‐aggregates) predominate and comprise over 45 per cent. of the light fractions. Whilst zeolites occur only in negligible amounts and feldspars do not represent more than 10 per cent. of the light fractions, yet the C.E.C. of these fractions are considerable, ranging from 41.7 to 72.4 m.e. per cent. Refractive‐index measurement and colour indicate that the pseudo‐aggregates are mainly chlorites. They must be individual particles, since prolonged reduction failed to cause any further disruption.The calculated values for the C.E.C. of the pseudo‐aggregates range from 90 to 110 m.e. per cent.; the same order of magnitude as that for vermiculite and montmorillonite.X‐ray photographs of the isolated pseudo‐aggregates from three fine sands show that the particles are of two types: (a) principally vermiculitic under good drainage conditions, (b) principally montmorillonitic where the drainage conditions are poor. Chlorites could not be detected, but there was a small amount of kaolinite and some amorphous material.It is suggested that the pseudo‐aggregates are individual particles representing an intermediate stage in the weathering of ferromagnesian rocks to the clay minerals, vermiculite or montmorillonite.The high C.E.C. of the fine sands are due to (i) a small contribution from clay minerals (diameter < 2μ) present in the fine sands as the result of aggregation and cementation by free sesquioxides, and (ii) a major contribution from the pseudo‐aggregates, which have C.E.C. between 90 and 11O m.e. per cent.