Contribution of image analysis to the study of weathering process in alkaline basalt (Middle Atlas, Morocco)

Quaternary alkaline basalts of Middle Atlas, are weathered into spheroidal volumes organized into weathering cover. In the profile studied, the study of transformations from a fresh core basalt to the most weathered rinds, ha s been analysed using BESI images (backscattred electron image analysis). A part from the microscopic analysis of each weatherted basalt, proportions of primary minerals, clays and pore space has been quantified in different stages of weathering. Indeed, ill fresh core basalt, weathering is characte rized by a first dissolution of the glass, followed by feldspar transformat ions in twinning plans, cleavages and in micropores inherited from the magm atic and cristallographic history of the rock. In most basaltic weathered rinds, pore spaces given by the dissolution of t he glass, were filled by a mixture of clays and iron products. In these sam ples, feldspars are completely dissolved giving important porosity occupied by small quantity of halloysite and kaolinite. The olivine and pyroxene tr ansformations are accompanied by weak pore space formation. At the bottom o f the soil, this porosity decreases in response to filling by later seconda ry products which come from the upper. part of the soil by weathering solut ions. (C) 2000 Elsevier Science B.V. All rights reserved.