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The Red Latosols of Sri Lanka: II. Mineralogy and Weathering¹

ABSTRACT

Soil samples from the principal soil series of the Red Latosols of Sri Lanka were subjected to (i) mineralogical analysis of the fine sand fractions by density, magnetic and optical methods, and (ii) mineralogical analysis of the clay and silt fractions by X-ray diffraction, DTA, chemical, and electron optical methods in order to characterize these soils and elucidate their genesis.

Mineralogical analysis of the fine sand fractions confirmed that the parent materials were uniform with depth and within series, but differed among series. The near-complete absence of easily weatherable minerals indicates intense weathering either before deposition or during soil formation.

The fine (< 0.2 µm) clays of all three Red Latosol soil series consist almost exclusively of well-crystallized kaolinite. The remainder is made up of free iron oxides and a small amount of amorphous components. Similarly the coarse (2–0.2 µm) clays are dominated by kaolinite, but they also contain minor amounts of other minerals, chiefly mica and mixed layer minerals with anatase and possibly some interstratified materials being present as traces. Coarse clays from the Wilpattu series contain, in addition, a clay mineral that is probably a vermiculite-smectite intergrade. The mica in the surface horizons appears to be stabilized against weathering, possibly by phytocyling of K. Free iron oxides are present mainly as discrete particles of hematite. Small amounts of amorphous materials also occur.

¹ Contribution from Land Use Div., Irrigation Dep., Colombo, Sri Lanka, and Univ. of Alberta, Edmonton, Alberta, Canada. Alberta Institute of Pedology publication T-76-4.

² Scientist, Land Use Div., Irrigation Dep., Colombo, Sri Lanka, and Associate Professor, Dep. of Soil Science, Univ. of Alberta, Edmonton, Alberta, Canada, T6G 2E3, respectively

Received for publication July 7, 1975. Accepted for publication July 23, 1976.

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