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, Fe 2+ = 0.09, Fe 3+ = 0.41, Mg = 0.82)] O 10 (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
Highlights
Stratigraphic Setting: The Santonian deformational process resulted in the fragmentation of the lower Benue trough (Table 1) into the Abakiliki syncline (Kogbe, 1976)
The smectite was completely dissolved by the acid treatment, leaving feldspar as residue in the case of the coarse clay.Structural formulae (Table 4), calculated from the elements dissolved by HCl and NaOH, suggest that the composition of the tetraoctahedral sheet was similar to that of the smectite in the soil samples
Smectite is unstable under the conditions prevailing in the soil
Summary
Formation in the Afikpo Basin, southeast Nigeria, were analyzed. The Amuvi (AMU) sample showed some mottling in the AMU3 indicative of drainage impedence, whereas Obotme(OBO) samples appeared to be freely drained. Silt and clay fractions were separated by the usual sieving and sedimentation procedures following dispersion of the soils using a Rapidis 150 ultrasonic vibrator, according to method of Genrich and Bremner (1972). Noncrystalline material was dissolved by boiling DCB-treated samples in 0.5 NaOH (Hashimoto and Jackson, 1960). X-ray powder diffraction (XRD) analysis was carried out ondeferrated, NaOH-treated samples of sands, silts and clay using a Philips X-ray diffractometer and Nifiltered CuKα Radiation at 40 kV and 20 ma. Previously treated with DCB and NaOH to remove free oxides and amorphous silicates, was carried out by digesting 100 mg of sample (Ba-saturated) in 50 ml of 1 HCl on a water bath at 80oC for 24 hr. The residues were boiled in 0.5 NaOH as before
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