Abstract
Palygorskite forms crusts or cutans on the surfaces of soil peds from two regions in Australia, Alice Springs (Northern Territory) and Eastern Riverina (New South Wales). Electron microscope examination of replicas prepared from cutans showed the palygorskite fibers to be arranged in parallel orientation within thick mats. Fibers were occasionally observed on top of coarse-grained skeleton grains. X-ray diffraction showed that, while the cutans contain almost exclusively palygorskite, the mineral is absent from within the soil peds. The chemical composition of crusts, determined by electron microprobe analysis, corresponds to that of Mg-rich palygorskite. Chemical examination of transects through crusts indicates that while the palygorskite content of the Northern Territory crusts is fairly constant, the mineralogically pure palygorskite in the periphery of the Riverina crust is becoming gradually intermixed with illite within the crust.
The equilibrium constant of a pure Australian palygorskite (Mt. Flinders) was determined by dissolution in distilled water under air and nitrogen. The standard free energy of formation (aΔFf°) is — 1131.5 kcal/mole. The information from the solubility experiments was used to construct stability diagrams showing the stability of palygorskite as a function of pH, pMg, and pSi. The composition of 1:1 water extracts from soil horizons containing palygorskite coatings, plotted onto one of these diagrams, falls within the stability field for palygorskite. It is concluded that soil water with a suitable composition, and not only lake water, can satisfy the conditions required for the formation of palygorskite, and that the pedogenic formation of that mineral is therefore feasible.
