Crystallinity of soil kaolinites in relation to clay particle‐size and soil age

SUMMARY The crystallinity of soil kaolinites as a function of clay particle-size and soil age was investigated in soil chronosequences of the Shingle House Creek and Hawkesbury River alluvial terraces in south-eastern Australia. The youngest soils (late Holocene) in each sequence are texturally uniform Entisols containing kaolinite and illite. The oldest soils (Pleistocene to late Tertiary) are Ustalfs with strong textural differentiation and are predominantly kaolinitic. With increasing age, textural B horizons are increasingly enriched in kaolinite and in particles of fine clay (< 0.2 μm) size. In two sub-fractions of the fine clay (0.2-0.06 μm; < 0.03 μm), no corresponding changes were observed in the crystallinity of kaolinites (as measured by the index, Ck) with age. However, values of Ck were significantly higher in the coarse clay (2-0.2 μm) than for both fine clay fractions in all except the Ultic Paleustalf of the oldest, possibly late Tertiary, terrace of the Hawkesbury River sequence. In this soil, Ck values are low in all three clay-size fractions. In these sequences, the effects of both clay particle-size and soil age were identified in the crystallinity of kaolinites. Disorder as a result of pedogenesis, however, was associated only with the most prolonged weathering and the strongest soil textural differentiation.