Pyrite Containing Grey Clay Soils and Deep Sands, Swan Coastal Plain, Western Australia

The morphology, chemical and mineralogical properties of contrasting acid sulphate soils in Perth, Western Australia were investigated. The pale coloured deep sand comprising the Bassendean Dune soil has distinct A, E, B and C horizons whereas in some low lying areas clayey soils of the alluvial Guildford Formation occur. Adjacent to rivers estuarine sediments are present. All three types of soil contain subsoil horizons that are saturated and are consequently pyritic. Individual pyrite crystals are present in all horizons except for the A horizon of the deep sands and range in size from 0.07 to 1.5 μm. Single crystals of pyrite in the clayey alluvium range from 0.65-1.2 μm. Framboidal pyrite is present in the B horizon of sands ranges from 5.8-10.0 μm whereas the size of framboidal pyrite in clay soils ranges from 3.3-8.6 μm. An estuarine mud contained calcium carbonate in shell fragments, so that when this material is exposed to the air by a lowering of the water table, acidification caused by oxidation of pyrite may be neutralised by dissolution of calcite. This can not occur in the sandy soils which do not contain carbonate minerals so that soil pH is reduced to very low values when air enters the soil profile.