J. R. O’Neil C. Bonnot-Courtois Norbert Clauer 1982 <p>The effect of progressive natural weathering on the isotopic (Rb-Sr, K-Ar, δD,<span>&nbsp;</span><i>δ</i>¹⁸<i>O</i>) and chemical (REE, H₂O⁺) compositions of biotite has been studied on a suite of migmatitic biotites from the Chad Republic. During the early stages of weathering the Rb-Sr system is strongly affected, the hydrogen and oxygen isotope compositions change markedly, the minerals are depleted in light REE, the water content increases by a factor of two, and the K-Ar system is relatively little disturbed. During intensive weathering the K-Ar system is more strongly disturbed than the Rb-Sr system.</p><p>Most of the isotopic and chemical modifications take place under nonequilibrium conditions and occur<span>&nbsp;</span><i>before</i><span>&nbsp;</span>newly formed kaolinite and/or smectite can be detected. These observations suggest that</p><ul class="list"><li class="react-xocs-list-item"><span class="list-label">1.</span><p>(a) “protominerals” may form within the biotite structure during the initial period of weathering, and</p></li><li class="react-xocs-list-item"><span class="list-label">2.</span><p>(b) only when chemical equilibrium is approached in the weathering profile are new minerals able to form.</p></li></ul> application/pdf 10.1016/0016-7037(82)90115-6 en Elsevier The effect of natural weathering on the chemical and isotopic compositions of biotites article