Weathering and soil genesis from the Nasinu Basalt, South-East Viti Levu, Fiji

Detailed studies of basalt weathering in the South Pacific Islands are relatively rare. In order to address this gap, this study investigated weathering changes and soil formation for the predominantly fine-grained feldspar-phyric basalt flows (6.4 Ma) in the Nasinu area, about 10 km north-north-east of Suva, Fiji. In the hot humid environment locally, these rocks have been subjected to rapid and deep weathering. Three weathering profiles (Nasinu S1, S2 and S3 at approximately 60 m elevation) show features of strong kaolinitisation. Aluminium and iron enrichment in the horizons of the Nasinu weathering profiles is marked, with the presence of kaolinite, gibbsite, goethite, haematite and magnetite (grains) confirming substantial mineralogical change. Suggested mechanisms for these changes are presented. Using Soil Taxonomy, the Nasinu soil pedon (JBS-1) developed on the weathered basalt is a Typic Palehumult, very fine, mixed, isohyperthermic. A comparison is made with the weathering pattern of the Nakobalevu basalt located only about 25 km from this location.

Clay stability in clay-dominated soil systems

Seven samples from a chronosequence of soils developed in historically created polders on the Atlantic coast (Marais Poitevin, Vendée, France) were investigated in order to illustrate the rate of mineralogical change in a clay-dominated system. The oldest polder was constructed in 1665, the last one in 1912; thus the time span of soil evolution is from 80 to 330 years. All the samples had more than 50% clay (<2 μm). The most reactive, fine clay sub-fraction (<0.1 μm) was investigated in detail by X-ray diffraction and chemical analysis. The observed mineralogical changes with increasing age followed the schematic reaction:smectite + mica = illite + mixed-layer minerals.The progress of reaction in time appears to be non-linear. This reaction seems to occur in a chemically constant system, and the mineralogical change can be seen as a readjustment of species to a given chemical composition.

Geochemistry and mineralogy of Triassic sandstones and implications for groundwater composition

Borehole samples from the Wildmoor Sandstone Formation have been analysed for whole-rock major and trace elements and the mineralogy determined by X-ray diffraction and conventional thin sections. The forty-two samples studied were obtained from boreholes drilled in a long established effluent site and from control boreholes away from the site. Elements are associated with one or more of calcite, quartz, K-feldspar, and clay plus oxide. The most important mineralogical change due to current infiltration is progressive loss of calcite. Background element concentrations are established and only Pb, and in one sample Cu and Zn, are found in higher concentrations representing anthropogenic additions to the system.