Exchange properties and mineralogy of some soils derived from lavas of lower old red sandstone (devonian) age. II. mineralogy

Abstract

The mineralogy of some soils developed on tills derived from basalt and andesite of Lower Old Red Sandstone age has been investigated by X-ray diffraction and optical and scanning electron microscopy. All particle-size fractions as well as some unweathered Lower Old Red Sandstone lavas are rich in layer silicate minerals. The lavas contain abundant saponite of hydrothermal or late-stage deuteric origin. This mineral occurs in rõck fragments which are found in the coarse sand and as aggregated grains in the fine sand and silt and is responsible for the high cation-exchange capacity of these fractions. As well as saponite, the clays contain kaolinite, illite and a variety of dioctahedral interstratified minerals. Scanning electron microscope observations show that plagioclase felspar grains in the fine sand fraction commonly bear regularly shaped etch pits which seem to indicate weathering by solution. The exchangeable calcium and magnesium in the soils derive mainly from decomposition of plagioclase felspar and saponite, respectively, the increasing magnesium content down the profile reflecting the higher content of saponite in the lower horizons. In all probability the exchangeable potassium which is concentrated in the clay fraction is preferentially sorbed at exchange sites on particle edges and is not associated with any particular clay mineral. Clay-mineral aggregates in lava-derived soils may be common, but they can originate in three different ways, viz,, pedogenic cementation, weathering of primary minerals, and inheritance from parent rock.