Micromorphology and chemical weathering of a K-rich trachyandesite and an associated sedimentary cover (Parkes, SE Australia)

The micromorphology, composition and sequential formation of weathering products of a K-rich trachyandesite and an associated sedimentary cover at Parkes, SE Australia are described. This has been achieved using a combination of mineralogical (optical, X-ray microdiffraction and SEM-EDXA microanalysis) and chemical (X-ray Fluorescence) techniques. The weathering profiles have developed in a low-altitude, relatively flat landscape setting. The following processes are dominant: (a) weathering of primary minerals notably feldspars and micas to transient 'poorly crystalline' oxyhydroxide patches, smectite and ultimately kaolinite; (b) weathering of sediments in a zone of intense solution activity and redox reactions resulting in development of mottles and ferruginous nodules; and (c) greater thickness (up to 350 cm) and compositional differences between the soil over weathered sediments and in situ weathered bedrock. The dominant profile microsystems evolved under a past humid climate later modified by the prevailing semi-arid to arid conditions.