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A Plinthaquult of the Aripo Savannas, North Trinidad II. Mineralogy and Genesis¹

ABSTRACT

The Aripo Savanna Fine Sand, a Plinthaquult, has developed on water-transported erosion products derived from the Northern Range in Trinidad. The profile consists of a surface-bleached layer of 25 cm thickness composed of silt and fine sand, underlain by an intensely red- and yellow-mottled argillic layer. The first 10 cm of this layer has ochreous diffuse mottling and is very rich in Fe. Deeper, the mottles are more distinct and red in color on an almost grey matrix. On exposure, these mottles (plinthite) harden irreversibly to nodules. Chemical and mineralogical properties of nodular material thus formed ranging from 1 to 30 years of age were examined. With time, the nodules become firmer and richer in Fe. The intensity of red coloration within the nodules is directly related to the amount of Fe present. All nodules are very low in Mn; Ti is inversely related to Fe but Ca is evenly distributed throughout all nodular materials regardless of age. No crystalline Fe minerals occur, but lepidocrocite and goethite are present in the ochreous mottles in a thin horizon above the nodule-producing layer.

The soil is derived from micaceous (muscovite) parent materials, but mica content is low; kaolinite derived from mica weathering is the dominant mineral. There are also identifiable amounts of montmorillonite or vermiculite-type mineral thought to be derived from weathering of mica and representing intermediates in the formation of kaolinite. Cation exchange capacity and surface area values are in accordance with this mineral assemblage. The silt and sand fractions are almost pure quartz. Absence of feldspars in the silt fraction is confirmed by only trace amounts of Ca and Na found by chemical analysis. The data indicate that this soil is perhaps the most weathered of Trinidadian soils.

¹ Contribution of the Dept. of Soil Science, Univ. of West Indies, St. Augustine, Trinidad, and Agronomy Dept., University of Illinois, Urbana.

² Senior Lecturer in Soil Science, U.W.I., and Associate Professor of Soil Mineralogy, U.I., respectively.

Received for publication January 2, 1969. Accepted for publication May 12, 1969.