ROOT-INDUCED IRREVERSIBLE TRANSFORMATION OF A TRIOCTAHEDRAL MICA IN THE RHIZOSPHERE OF RAPE

The ability of the roots of rape (Brassica napus) to promote the transformation and dissolution of a phlogopite mica was studied in the rhizosphere. Rape was cropped for 1 to 32 d on 2-105 mum phlogopite as sole source of both K and Mg. The chemical balance of K and Mg revealed that rape was able to induce a significant release of interlayer-K after 4 d, and even a significant release of octahedral-Mg after 8 d of cropping. After 32 d, the root-induced release amounted to 80 and 21 g kg-1 of total-K and -Mg, respectively. The weathering products sampled in the close proximity of the roots were analysed by X-ray diffractometry (XRD), and by energy dispersive X-ray (EDX) microanalysis of resin-embedded samples prepared for transmission electron microscopy. The XRD analysis of K-saturated, oriented plates showed that part of the vermiculite formed by root activity behaved as a typical hydroxy-aluminous interlayered vermiculite (HIV). The EDX microanalysis revealed that the release of interlayer-K which occurred during the vermiculitization was compensated mostly by Al and Mg originating from the octahedral sheet of the phlogopite. Such crystallographic and crystallochemical changes necessarily involved an acid dissolution of the mica structure, which may be related to the root-induced pH decrease encountered simultaneously in the rhizosphere. Proton excretion by rape roots was thus the probable mechanism involved in the root-induced irreversible transformation of the phlogopite.