Abstract

The initial stages of the superficial geochemical weathering of anorthosites have been investigated in a deposit at Château-Richer, Québec. The parent rock consists almost exclusively of andesine and is weathered to a thickness of about 120 cm. A till overburden indicates that the profile was truncated by the Wisconsinian ice. The rock is not extensively disintegrated, but the chemical weathering is rather intense. The Parker diagram shows a strong decrease of the weathering index from the parent rock to the weathering zone. While this is not the case for other crystalline rocks in the area, it may be accounted for by the fragility of the plagioclase.The < 2 μm fraction, which amounts to 2% of the soil, presents an evolution that is influenced by the site, especially the drainage conditions. At the base of the profile, drainage is poor and 2:1 minerals, smectite and vermiculite, are present in addition to 40% kaolinite. The 1:1 mineral increases to 60% near the contact with the till, while the amount of 2:1 minerals decreases because of the better internal drainage conditions.

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