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Composition of Smectite from Maastrichtian Sediment of the Afikpo Basin, Southeast Nigeria
AGIBE, AN; UZOEGBU, MU; NDUKWE, OS & NDUKWE, VA
Abstract
The sand, silt, shale and clay fractions soils derived from Maastrichtian sediments in the Afikpo Basin,
southeastern Nigeria exhibit unusually high cation-exchange capacities. Smectite was identified by X-ray diffraction in
all sample fractions and was selectively dissolved by treatment with 1HCl and 0.5 NaOH. Structural formulae, based on
chemical analysis of the dissolved material, suggest that the smectite is intermediate in composition between a di- and a
trioctahedral mineral and that the octahedral cation occupancy, together with the number of Mg and Fe atoms per unit
cell, decreases with decrease in particle size of the soil fraction examined. The mean formula for the smectite in the clay
separate is: [(M+= 0.40, Si = 3.33, Al = 0.67), (Al = 1.07, Fe2+= 0.09, Fe3+= 0.41, Mg = 0.82)]O10(OH)2. Similar analysis
of smectite present in clay-size material separate from weathered granitic rock taken from the bases of the soilprofiles
showed that it had close to the full trioctahedral inherited from the weathered granites.The clay mineralogy of the soils
was affected by inherited smectite, probably of hydrothermal of deuteric origin. This mineral is unstable in the soil
environment and, although its alteration products are fairly uniform in composition.
Keywords
Dissolution; Granitic; Octahedral; Smectite; Soil.
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