Mineralogical and physical properties of lateritic pallid zone materials developed from granite and dolerite

• Published in: Geoderma 1990-08, Vol.47 (1), p.33-57
• Main Authors: McCrea, A.F., Anand, R.R., Gilkes, R.J.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Soils; Surficial geology
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/0016-7061(90)90046-C

We investigated the influence of parent rock type on the properties of the pallid zones of lateritic soils which have developed from granite and meta-quartz dolerites at Jarrahdale, Western Australia. A major finding is that the clay matrix of pallid zone materials derived from both parent rocks has very similar fabrics and physical and mineralogical properties. Differences in fabric and other properties between pallid zones formed from the two rock types are therefore simply due to the higher quartz contents of the granite pallid zone. The clay matrix mostly consists of tubular halloysite (0.1–2.5 μm) and platy kaolinite (0.05–1 μm) crystals arranged in a randomly oriented fabric. Halloysite crystals are partly unrolled in those materials where dehydration has altered 10 Å halloysite to 7 Å halloysite. The average bulk densities of pallid zone materials derived from granite and dolerite are 1.74 g/cm 3 and 1.31 g/cm 3, respectively, with the difference being due to the high quartz content of granite materials. The particle size distribution curves for pallid zone materials derived from granite and dolerite coincide when calculated on a coarse sand-free basis. The water contents of saturated pallid zone materials increased linearly with clay content, being greater for pallid zones derived from dolerite. The materials are highly porous with up to 50% porosity and with pores that are mostly 0.01–0.1 μm as shown by mercury injection porosimetry and soil water retention characteristics. Consequently, most water is retained at tensions greater than pF 4.2 (wilting point). Water retention curves for pallid zones materials derived from granite and dolerite coincide when calculated as percentages of the total water retained at saturation. The pore size distributions for pallid zone materials are consistent with the crystal sizes and microfabric that were determined using optical and electron microscopy. The strength of pallid zone materials increases linearly with bulk density and is thus systematically greater for the materials derived from granite.