MINERALOGY AND WEATHERING OF A TRINIDAD ULTISOL DEVELOPED IN PORCELLANITE

Abstract

We sampled a pedon representing the Non Pareil series, which was developed from porcellanite, an unusual soil parent material, and analyzed for selected physical, chemical, and mineralogical properties. The soil is highly weathered with predominantly quartz and smaller amounts of mica in the sand fraction, which constitutes about 40% of the sample. The silt fraction amounts to about 25% of the soil's weight and contains mainly quartz and iron oxide nodules, with substantial amounts of biogenic opal. Opals are most abundant in the 10− to 20-μm separate. Kaolinite comprises about 70% of the crystalline clays of the upper solum. Smectite and illite occur in smaller amounts. The presence of interstratified smectite/illite suggests the progressive formation of the former from the latter. Judging from the amounts of mineral species in the parent rock, we speculate that kaolinite is being formed in the soil by the weathering of smectite and illite and, perhaps, the weathering of cristobalite, which decreases markedly from the parent rock. Weathering and subsequent leaching has markedly redistributed the alkali and alkaline-earth elements, moving most of them out of the solum. Molar weathering ratios based on Zr reflect this substantial weathering and leaching that has taken place to at least 152 cm in depth. The Ti to Zr ratio in the surface horizon, which is one-fifth of its value in the parent rock, suggests weathering and leaching of Ti-bearing minerals in this freely draining, oxidizing, and strongly acid environment.