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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