Alteration meteorique des biotites en algerie

Abstract

The authors compared the weathering of biotite on different morphological surfaces of the granitic outcroprs of Beni-Toufout (northeast Algeria), using the polarizing microscope, scanning electron microscope and microprobe, and mineralogical analysis. On the youngest surfaces, the optical and crystallographic characters of biotites are preserved, except on edges where folding appears accompanied by loss of pleochroism and birefringence and by ferruginisation. At this stage biotites are still composed of 10-Å minerals with interstratified ones at the edges. In the next stage of weathering, folding affects the whole mineral. Meta-halloysite appears at this stage. Then the weathering proceeds by developmental of “brushes” at ends of minerals and by development of folding, creating large cavities in the mineral, some filled with argillans and other with kaolinite. Mineralogical analysis shows an increase of kaolinite but 10-Å vermiculite and interstratified minerals are still present. The next step consists of a progressive increase of kaolinite and ferruginistation. Cavitation of minerals has become important, but small flakes of unweathered biotite can still be found. In the extreme stage of weathering, the mineral is colorless gray under crossed nicols; its flakes are outlined by ferruginisation. Internal cavities are partially filled with argillans. Mineralogical analysis shows only kaolinite and ferruginous compounds. In this extreme weathering, two subtypes of weathered biotites were observed, one characterized by total kaolinisation with a weak internal cavitation and the second by a very strong internal cavitation and a dominance of ferruginous compounds. The weathering of biotites in Beni-Toufout is intermediate between the temperate forms and those from the tropics. As in Europe, weathering is progressive with a vermiculite stage present but much less developed. As in the tropics, weathering leads to a kaolinisation of biotites.