Evidence by transmission electron microscopy of weathering microsystems in soils developed from crystalline rocks

Abstract

AbstractHigh-resolution electron microscopy and microanalytical studies were performed on clay and bulk soil samples developed on various igneous and metamorphic rocks from Galicia in the Coruna province in NW Spain. Two mineralogical microsystems can be distinguished showing different stages of weathering. For feldspars, exsolution and mass transformation lead to the delineation of parallel domains, and to formation of gel or paracrystalline minerals. For micas, alteration starts with a physical breakdown, i.e. exfoliation and crystal microdivision; individualization of monolayers is followed by gel formation. A solid phase diffusion phenomenon can explain the formation of 1 : 1 phyllosilicates inside 2 : 1 phyllosilicates or within the continuum of the original crystals. Evidence of such weathering stages in crystalline rocks can be related to the occurrence of specific climatic conditions between temperate and tropical areas.