Aspects of mineral transformation during weathering of volcanic materials : the microscopic and submicroscopic level

A.G. Jongmans

    Research output: Thesisinternal PhD, WU


    <p>Mineral transformation at the earth surface is a complex process. In volcanic ejecta, such transformations tend to be fairly rapid. Many weathering studies on volcanic materials have been carried out at different scales of observations, mostly using bulk samples. However, to get a proper understanding of the mechanisms of weathering of primary minerals and formation secondary minerals it is necessary to obtain data of undisturbed material at the scale of observation that micromorphology and submicroscopy deal with. Weathering studies at the micrometre scale with help of micromorphology showed the heterogenous character of mineral weathering and the co- existence of different secondary minerals.<p>The main objective of papers in this thesis was to characterize and explain alteration of primary minerals and formation of secondary minerals at the particle level in volcanic soils in relation to (micro) environmental conditions.<br/>Thin sections of volcanic soils were studied by micromorphology, and relevant features were characterized chemically and mineralogically by submicroscopical methods performed on (un)disturbed samples isolated from thin sections.<br/>Mineral transformations were studied both on sites in the temperate humid zone and in the humid tropics.<p>In a chronosequence of Quaternary terraces of the Allier in France, micromorphological and sub microscopical analyses showed:<br/>- Alteration of basaltic particles leads to clay formation whereas weathering of granite fragments contributes to the sand fractions.<br/>- A relative increase in the contents of Ti, Al, and Fe and a decrease of K, Na, Ca, and Si occur in weathering rinds of basalt pebbles. Differences in weathering intensity are predominantly a function of chemical composition of the basalt rather than a function of time.<br/>- A mass balance calculation carried out on an isovolumetric, altered basalt pebble with a fresh core, and on an enclosed, genetically related neoformed clay coating showed that all elements, except Fe were leached from all weathering rinds. Only Si, Al and some Ca were found in the clay coating, and part of the Al was derived from an external source.<br/>- Isotropic and anisotropic coatings occur in a Paleosol in an older terrace. Micromorphological observations demonstrated that such coatings are genetically related. The isotropic coating consists of allophanic material with minor amounts of 2:1 phyllosilicates, whereas the anisotropic types consist of 2:1 phyllosilicates only. Both types result from recombination of trachytic weathering products under restricted leaching conditions during coating formation.<br/>- Micromorphological observation demonstrated three types of coatings in two Planosols in two older terraces. Isotropic and anisotropic, translucent materials occur locally within one coating, suggesting a genetic similarity. These coatings are due to secondary mineral formation, The third type, anisotropic dusty clay coatings clearly resulted from clay illuviation. Cluster analyses reveal that the coatings of the same type were chemically more alike than different coatings in the same profile. About 83% of the grouped samples were classified correctly as either isotropic and anisotropic weathering coatings or anisotropic illuviation coatings.<p>A technique is described to isolate undisturbed microparts of pedofeatures from thin sections. Such microparts can subsequently be analysed by Transmission Electron Microscopy. This technique allows performance of micromorphological, mineralogical and chemical analyses on one undisturbed micro sample at micrometre to nanometre scales.<p>Isotropic coatings in the C horizon of a young Hapludand in Guadeloupe and in the C and R horizons of an old Hapludand in Costa Rica, both developed on andesitic volcanic materials were allophanic. Isotropic coatings present in the Bw horizons of both soils contain allophane and imogolite. The Al/Si molar ratios in the coatings in the Bw horizons are higher than those in the coatings of the C and R horizons. Anisotropic coatings are wholly gibbsitic and occur only in the Bw horizon of the older Costa Rican Hapludand. The gibbsitic coatings show a gradual transition to isotropic coatings and both types looks alike in plane polarized light suggesting a genetic relationship. The allophane coatings resulted from initial weathering of the parent materials, whereas the gibbsite coatings represent the ultimate stage of secondary mineral formation. The differences in chemical and mineralogical composition of the coatings are thought to be the result of different leaching conditions at the macro and micro scale.<p>2:1 Phyllosilicates in Hapludands on Holocene andesitic beach ridges in Costa Rica occur as clay pseudomorphs after primary minerals. They are inherited from hydrothermally altered parent material from which the beach ridges were derived, and are not due to post depositional soil formation. Weathering and biological activity affect the clay pseudomorphs which leads to clay- sized particles consisting of 2:1 phyllosilicates. They are incorporated in the allophanic groundmass that results from actual soil formation in the Hapludands.<p>Isotropic coatings also found in a West Java (Indonesia) Oxisol on andesitic volcanic parent materials are probably due to weathering of airborne ash additions. The coatings recrystallize to anisotropic coatings suggesting a neoformed genesis. Both types of coatings appear to be common in three andesitic catenas in Indonesia. The amount of coatings and the crystallinity tend to increase as the dry season become more pronounced. The anisotropic coatings can easily be confused with illuviation coatings.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • van Breemen, N., Promotor
    • Buurman, P., Promotor, External person
    Award date2 Sep 1994
    Place of PublicationS.l.
    Print ISBNs9789054852797
    Publication statusPublished - 1994


    • volcanic soils
    • andepts
    • andosols
    • soil formation
    • physical properties
    • mechanical properties
    • natural resources
    • volcanic rocks
    • basalt
    • volcanic lava
    • pedology
    • soil micromorphology


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