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Petroplinthite or indurated lateritic concretions have been studied micromorphologically and also with the scanning electron microscope (SEM). It has been shown that the laterite is formed from a kaolinite mass by the absolute enrichment of iron oxyhydrates. In thin‐sections, the iron forms a vacuolar network; the vacuoles are composed of the original kaolinite and the network is formed by closely crystallizing goethite crystals. The goethite forms a strong, rigid framework giving the form and the hardness of the petroplinthite.SEM photographs of amorphous manganese and manganite are given. These are very localized occurrences. The few quartz grains in these laterites are etched and on the surface secondary quartz has formed.

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Different minerals characterize the three fractions in the different weathering zones. Halloysite in the clay fraction especially in the α and γ zones is due to a physical comminution of silt‐sized tubes. Strong peaks for halloysite (XRD) in the silt and sand fractions in these zones is shown (SEM) to be from partly altered feldspar fragments. Gibbsite is present mainly in the fine‐silt size and its presence in the sand in the α zone is due to cementation and aggregate formation. Aggregation of kaolinite also results in its being present in the silt and sand fractions.Estimates of the minerals in the three fractions are shown diagramatically as depth functions. XRD diffractograms of the three fractions and TEM micrographs of the clay fraction are also presented. It is concluded that the morphologically identified weathering zones also differ in their mineralogical properties.

Volcanic ash soils were studied with the scanning electron microscope to observe the morphology and occurrence of aUophane, imogolite and halloysite. Allophane and imogolite in general were randomly distributed indicating a process of dissolution and recrystallization. Halloysite is typically associated with feldspars showing that it is formed by direct alteration.The imogolite fibres studied with the SEM show them to be much thicker than currently reported in the literature and it is thought that pretreatment of the clays for TEM measurements results in a partial destruction. Halloysite consists typically of short rigid tubes and thus differs from imogolite which tends to curl.