Mineralogy of a Chronosequence Formed in New River Alluvium

Abstract

AbstractThe mineralogy of four soils (T0, T1, T2, T3), formed in progressively older New River alluvium, was studied with the assumption that trends could be attributed to age differences. Particle size was determined by pipette; free Fe by citrate‐dithionite (CD) extraction; sand mineralogy by petrographic techniques, heavy liquid separation, and X‐ray diffraction (XRD); and silt and clay mineralogy by XRD and thermal analysis. Clay illuviation and percent CD‐Fe increase with age. An increase in CD‐Fe with depth suggests Fe illuviation. Quartz, feldspar, mica, and amphibole are the major sand‐sized minerals in T0 and T1, with slightly less amphibole in T1. Predominance of quartz and resistant heavy minerals in T2 and T3 suggests a wide time gap between T1 and T2. Trends in silt mineralogy parallel those of sand. Clay mineralogical data show an age progression from mica and vermiculite to hydroxy interlayered vermiculite to kaolinite. Kaolinite appears to form very slowly from other layer silicates, but whole‐soil trends suggest that it may form more rapidly from sand and silt‐sized feldspar. With time, hydroxy interlayered vermiculite becomes the dominant clay‐sized mineral in Ap horizons while kaolinite and vermiculite increase with depth. The depth trend for kaolinite is attributed to eluviation and slow replenishment in the surface, and for the other minerals to decreasing weathering intensity with depth. Whole‐soil data reflect illuviation and reveal a concentration of quartz in the surface with time. Comparison of mineralogical trends suggests that stability is reached sooner in coarser than in finer size fractions.