Properties of soil kaolinites from south‐western Australia

Abstract

SUMMARY The clay fraction of soils from south‐western Australia is dominated by kaolinite. These soil kaolinites give broad basal reflections (median value for WHH 001 =0.41° 2θ) and have poorly ordered structures. The median value of the Hughes & Brown crystallinity index for 35 soil kaolinites is 5.4 compared to 42.0 for mineral standard kaolinites. The median values for surface area and cation exchange capacity of soil kaolinites are 35 m 2 g −1 and 56.7 mmol c kg −1 , respectively, compared to 10 m 2 g −1 and 4.8 mmol c kg‐ l for standard kaolinites. The dehydroxylation temperature of soil kaolinites is about 50 K lower than for standard kaolinites and decreases with increasing Fe content. Soil kaolinite crystals are much smaller (c. 0.10 pm) and are irregular subrounded to rounded plates in comparison to the larger (c. 0.28 pm), perfectly hexagonal euhedral plates of Georgia kaolinite. Analytical electron microscopy of single soil kaolinite crystals indicates that the Fe content varies with an average value of c. 2.5% Fe, 2 O 3 . X‐ray diffraction line broadening data indicate that crystal size decreases as the Fe content increases. Crystal order also decreases with increasing Fe content. The P sorption maximum of soil kaolinite ranges from 486–654 μg P g −l and is thus about six times larger than for the clay fraction of Georgia kaolinite.