Pedogenic differences between two adjacent basalt-derived soil profiles 2. Mineralogical characteristics

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Abstract
The mineralogical composition of a black vertisol (Arcadia form) and a red alfisol (Shortlands form) profile, derived from Jurassic basalt and situated 35 m apart, has been determined by means of X-ray diffraction analysis. The non-clay fractions of the least-weathered zones (C2) contained plagioclase (labradorite) and pyroxene (both orh and mkl) as dominant components, with labradorite slightly more prevalent in the Short- lands saprolite. The C1 horizon of the Arcadia pedon was devoid of mafic minerals and composed of quartz and calcite, while the equivalent horizon of the Shortlands profile still contained pyroxene and feldspar, although in smaller proportions and associated with quartz. In both solums, quartz was dominant. Hematite accounted for slightly more than half of the secondary iron phases throughout the two solums. Smectite constituted the primary alteration product in both profiles. In the Arcadia soil, smectite remained the dominant phyllosilicate phase. In the Shortlands profile, however, smectite was progressively transformed into kaolinite with an increasing degree of weathering and constituted about half of the clay fraction in the top horizon. The transformation of smectite to kaolinite proceeded predominantly via the dissolution-precipitation pathway. The swelling behaviour of smectite after the application of the Greene-Kelly test indicated that layer charge arose predominantly from tetrahedral substitutions in all samples. Dodecylamine intercalation and K-saturation pointed to high interlayer charge densities, characteristic of high charge smectite or low charge vermiculite. Within the Arcadia pedon, changes in chemical composition of the clay fractions were not reflected in the clay suite. In the Shortlands profile, the magnitude of trends in clay mineral composition was only partially reflected in its chemistry.