Mineralogical analysis of Chalk solution pipe mineralisation, Aldworth, Berkshire

This report describes the results of laboratory characterisation of mineralisation exposed in Upper Chalk solution features, near Aldworth, Berkshire. X-ray diffraction and scanning electron microscopy reveal that the mineralisation is predominantly formed of gibbsite with poorly crystalline hydrous alumino-silicates (halloysite and possibly allophane). Black microspheres which speckle the surface of the mineralisation are formed of sub micron-sized crystals of Mn oxyhydroxide intergrown with gibbsite. When fractured, the Mn oxyhydroxide microspheres reveal a fibro-radiating morphology and are rich in cobalt.
The mineralisation is thought to have developed from the interaction between acidic, sulphate-bearing groundwater generated in the overlying Tertiary pyrite-bearing sands and clays with the Upper Chalk. Similar mineralisation has been studied by the author at Newhaven, Sussex. The lack of Al(OH)3 polymorphs other than gibbsite suggest that groundwater flow in the solution feature remained unimpeded. The Aldworth features and mineralisation probably formed from pre-existing joints, post-Lambeth Group deposition, under wetter, possibly warmer climatic conditions than today.