Iddingsitisation of olivine and kaolinitisation of biotite in two contrasting tephra-derived soils along the Cameroon Volcanic Line (CVL)

Abstract

Mineral transformations in soils are important processes in assessing pedogenesis. A micromorphological study, complemented by mineralogical and physicochemical analyses, was conducted for two contrasting soils developed on tephra of Mt. Manengouba, i.e. an Andosol and a Cambisol, in order to compare weathering trends based on olivine and biotite alteration. The contrasting character of the two soils is portrayed in their various properties, and is also observed in the field. The majority of the investigated soil properties proved to be effective in capturing differences between the two soils. Iddingsite, formed by pellicular alteration of olivine, occurs in both soils. Its presence is an inherited feature, as recorded by identical occurrences of affected olivine crystals as isolated grains in the groundmass of the soils and as phenocrysts in volcanic rock fragments within the Andosol profile. Kaolinitisation of biotite, producing parallel linear alteration patterns, is limited to the Cambisol and is attributed to pedogenic processes, recording a more advanced stage of weathering than for the Andosol profile. Kaolinitisation is thus more indicative in assessing pedogenesis compared with iddingsitisation in these soils.