The mineralogy, geochemistry and surface area of mudrocks from the London Clay Formation of southern England

This report describes the results of mineralogical and geochemical analysis of a suite of mudstones from the London Clay Formation of southern and south-eastern England. The work was carried out as part of the ongoing ‘Ground Movements: Shrink/Swell’ project under the Physical Hazards Programme. The first part of the report gives an introduction to the geology of the London Clay Formation and a summary of previous mineralogical studies of these rocks. A summary of analytical methods employed (X-ray diffraction analysis, X-ray fluorescence spectrometry and surface area determinations) is then provided and the results discussed with reference to their likely effect on the engineering performance of the London Clay Formation. The study has generally confirmed the findings of previous workers with typical non-clay mineral assemblages composed of quartz, feldspar (albite and K-feldspar), carbonates (dolomite, siderite and calcite), ‘mica’, pyrite, gypsum and goethite while clay mineral assemblages are generally formed of smectite, illite, kaolinite and chlorite. However, the wide geographic and stratigraphic distribution of the analysed samples has provided important new information which will aid not only interpretation of the engineering behaviour of these rocks but also their diagenetic and geological histories. The engineering properties of the London Clay Formation are heavily influenced by its clay mineralogy and the proportion of clay minerals present. This study has shown that while the composition of the clay mineral assemblages is similar across the London Clay Formation outcrop, the proportion of clay material increases from west to east. The most clay-rich samples are found in the London Basin and particularly around the Thames estuary, central Essex and just west of London in eastern Berkshire/Surrey. These clay-rich samples also contain the most smectite and are therefore likely to undergo the greatest shrink-swell volume changes during wetting and drying and provide the most problems in terms of their engineering behaviour. It would appear most likely that the smectite in the London Clay Formation was contributed by the reworking of ash-rich soils while kaolinite was sourced from tropical weathering of granites to the west. A rare occurrence of a pure interlayered kaolinite/smectite clay assemblage in a sample from Studland Bay, Dorset is likely to have developed due to the acidic, tropical weathering of a volcanic-ash or smectite-rich deposit. The very common presence of pyrite, together with gypsum in the London Clay Formation means that concrete engineering sited in these rocks may require sulphate-resistant compositions to avoid acid attack and thaumasite formation