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Weathering Assessment of Heavy Minerals in Age Sequences of Australian Sandy Soils

Dep. of Soil Science, Waite Agricultural Research Inst., Univ. of Adelaide, Private Bag Number 1, Glen Osmond 5064, Australia

R. W. Fitzpatrick

CSIRO Division of Soils, Private Bag Number 2, Glen Osmond 5064, Australia

*Corresponding author.

ABSTRACT

This study was conducted to evaluate the distribution, weathering characteristics, and relative degree of mineral alteration of resistant heavy minerals, by semiquantitatively analyzing the surface microtextural features of heavy minerals from six sandy-soil age sequences associated with three different coastal regions in eastern and southern Australia. The 14 pedons analyzed are Psamments and Orthods, the estimated ages of which range from several hundred to 750 000 yr. Zircon, rutile, and sillimanite were identified as the dominant minerals in the nonmagnetic (NM) fraction, and garnet, spinel, epidote, and monazite as major constituents of the slightly magnetic (SM) fraction. The nature and extent of ten microtextural features were measured using scanning electron microscopy (SEM), and analyzed semiquantitatively by assigning nominal numerical values, which were transformed into an ordinal scale. Point-value score (PVS) and value-assessment score (VAS) methods were used to assess the degree of mineral weathering, and tested statistically using the Kruskal-Wallis H statistics. An order of mineral resistance to weathering was established (from more to less resistant): zircon > sillimanite spinel rutile > garnet > epidote > monazite. Mineral weathering patterns in the different soils were complex and depended upon a variety of factors including inherent crystallographic, mineralogical, physical, and chemical properties of the minerals as well as interaction between the various factors that contribute to soil formation. The two methods proved useful in assessing pedogenic weathering trends within and between profiles from the different age sequences and confirmed that, at least in an acid pedochemical environment, zircon is the most reliable mineral on which to base a pedogenetic index. Nevertheless, in old profiles (> 100000 yr BP) zircon also showed evidence of pedochemical weathering.

Received for publication November 27, 1989.