Fragipan formation in argillic brown earths (Fragiudalfs) of the Milfield Plain, north-east England. III. Micromorphological, SEM and EDXRA studies of fragipan degradation and the development of glossic features

The micromorphology of eluvial and glossic areas with weak soi1, strength from the upper fragipan, and the upper parts of grey polygonal zones, in some argillic brown earths (Glossic Fragiudalfs) from north-east England shows unstable void walls stripped of clay with remnants of ferri-argillans, pale grey grainy clay coatings and darker grainy coatings. These occur alongside loose surface residues of skeleton grains and thin grey fine silt coatings of quartz and muscovite. SEM and EDAX studies of coating surfaces, and optical microscopy, SEM and EDXRA of very thin polished sections, show that some grainy clay coatings form as alteration rims of ferri-argillans through localized waterlogging, iron oxide loss and micro-erosion leading to micropitting and disoriented fabrics. The inclusion of coarse clay to fine silt-sized quartz, feldspar and muscovite in other grainy coatings suggests either alteration of impure ferri-argillans or accumulation of degradational products derived from elsewhere on void walls. This is more certainly the case for dark grainy coatings in layered compound illuviation coatings adjacent to glossic areas. These fragipans are degrading from the top downwards by processes which are partly a consequence of the effects that the fragipan has on water percolation and root penetration. The destabilization of void walls, the degradation of ferri-argillans and the remobilization of clay to form glossic features did not begin until seasonal waterlogging in parts of the upper pan was sufficient to mobilize iron.