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Geochemical and mineralogical properties of soil can be used to assess their agricultural productivity potential. Ten soils from 4 geological formations in the south-eastern Nigeria were studied. Soil samples were taken from typical A and B horizons from each soil. The soil properties assessed were the free forms of Fe and Al, particle size distribution, total elements, and clay mineralogy. The soils are deep and highly weathered. Crystalline Fe and Al were more dominant than amorphous or less crystalline forms. The geochemical properties of the soils are dominated by Si[O.sub.2], [Al.sub.2] [O.sub.3], and [Fe.sub.2][O.sub.3], whereas MgO, Ti[O.sub.2], and Zr[O.sub.2] occur in moderate to low quantities. Other basic elements such as [Na.sub.2]O, [K.sub.2]O, and CaO are very low in the soils, with an insignificant role in their genesis. The low contents of these elements in the soil may be the cause of their low nutrient release and storage in the soil. Ferritisation, laterisation, and kaolinitisation are the major pedogenic processes in the soil. The dominant clay mineral in all the soils was kaolinite; other minerals present though less common were illite, vermiculite, and smectites. Based on elemental contents and clay mineralogy, the soils can be grouped into 3 distinct groups: those with only kaolinite, illite, and vermiculites; and those that also contain smectite. These groupings can indicate the agricultural fertility potential. Additional keywords: Fe oxide, A1 oxide, pedogenic processes, clay mineralogy, agricultural productivity., Introduction The extensive soil systems and landscape of south-eastern Nigeria are underlain by sedimentary materials of different ages and different formations. The geological formations were described by Ofoegbu (1985) as [...]

Studies of mineral distribution in soils provide vital information for understanding the genesis of the soil. We studied the soils formed on the floodplain of the River Niger to determine the occurrence, distribution, and weathering transformations of minerals in a soil chronosequence in eastern Nigeria. Five soil profiles representing 5 depositional stages were studied. The soils have aquic moisture regimes and an isohyperthermic temperature regime by Soil Taxonomy. Gleysation due to poor drainage is very dominant. The soils are low in pH, organic matter, and exchangeable cations. Plinthisation and ferralitisation resulting in high values of [Fe.sub.2][O.sub.3] and [Al.sub.2][O.sub.3] were observed in the coarse sand, while in the fine sand fractions, quartz and feldspar grains have accumulated with mica being next in abundance. The occurrence of expansible minerals and kaolinite in the clay fractions is as a result of transformation of mica and feldspars giving rise to these minerals. We postulate that the origin and abundance of [K.sub.2]O and MgO in the clay fractions were from the breakdown of the structural units of the expansible minerals, micas and feldspars. Illite undergoes a transformation process to expansible minerals, while kaolinitisation is the major process in the clay fractions. Principal component analysis shows that 23 mineral variables which relate with kaolinite and other silicate clays can be reduced to 5 principal components. Additional keywords: feldspars, mineral transformation, principal component, expansible clay minerals, Fe-precipitation., Introduction Studies on the mineral and elemental distribution within soils provide vital information for the assessment of their genesis and behaviour. Teveldal et al. (1990) reported that, in some soils, [...]

In the Oriola depression of Southern Portugal near the town of Evora, field examinations revealed the presence of Tertiary sedimentary deposits that had the consistency of duricrusts. These duricrusts, occurring close to the land surface, were examined in the field as well as in the laboratory, with the objective of establishing their composition and formation. Micromorphological examinations showed that the duricrusts were composed of clasts of fine sand to fine gravel sizes cemented together by a matrix dominated by either palygorskite or carbonate. The matrix: clasts ratio varied from about 1: 3 when the cement was carbonate, to 3: 1 or higher when the matrix was palygorskite. Occasionally the ratio in the latter was even higher. The clasts consisted of quartz, feldspar, and some Mg-rich metamorphic minerals. The palygorskite matrix fibres were arranged in mats within which they had parallel orientation. The mats or `domains', which had a length of 15-25 µm and a width of about 40 µm, showed random orientation. In analogy to the term `calcrete' the term `palycrete' is used for the palygorskite duricrusts. The palycrete, of an average thickness of 0.5 m, frequently rested directly on the Hercynian basement rocks, and was covered by a recent solum. In the B horizons of the solum, the palygorskite appeared to undergo recent weathering and transformation into smectite. The duricrusts were proposed to have formed by authigenic calcite or palygorskite, which filled the interstices between clastic particles that had been deposited on Hercynian basement rocks following their peneplanation in the early Tertiary. The proposed environment of deposition and formation was that of an intermittent playa-lake in a semi-arid, seasonal climate, where strong evaporative processes had been active. Alteration and weathering of the mafic minerals contained in some of the clasts, in addition to interstitial solutions rich in Si and Mg, had created the chemical environment required for palygorskite neoformation. From the state of weathering--disintegration of the palygorskite in the soils formed on the palycrete, it is inferred that the environment for palygorskite neoformation had ceased to exist after palycrete formation. Additional keywords: Mediterranean, Palaeosols, Rana, Tertiary, Serra de Portel., Introduction Duricrusts are among the most common surface phenomena known that affected ancient and modern landscapes. Essentially, duricrusts are composed of coarse-grained mineral material cemented together by matrix materials such [...]