Comparaison des résultats analytiques obtenus après fusion alcaline au métaborate avec ceux obtenus après fusion alcaline au tétraborate dans les niveaux d'altération des grès de l'Inkisi (secteur de Brazzaville, République du Congo) : applications dans l

In comparing the analytical results obtained by ICP-AES after alkali fusion with lithium metaborate and with lithium tetraborate in the analysis of samples from a weathering profile of Inkisi sandstone (south of Brazzaville, Republic of Congo), we notice important differences, notably in the upper levels of the profile. Those differences are either positive or negative. For Some element, like Al, differences are very small. That can due to the precision of the analysis. On the other hand, other elements, like Si, present important differences, sometimes positive, sometimes negative. In order to evaluate these fluctuations, we have defined a coefficient of variation Ki, as follows : Ki(%) = 100 * ((oxidei (tf)-oxidei (mf)) / oxidei (mf)) (with tf: result obtained after tetraborate alkaline fusion and mf : result obtained after metaborate alkaline fusion for the i oxide analyzed). The Ki curves in function of the depth established for three chemical elements (K, Na, Si) permit to locate on the weathering profile the levels more fusible to tetraborate (positive Ki) and those more fusible to metaborate (negative Ki) for each element. So, by this way, we can know if a chemical element released from a primary mineral has integrated more or less refractory secondary mineral structures. By comparing these curves with mineralogical evolution along the weathering profile, as showed by X ray diagrams, we have obtained the following results: -Sodium and potassium Ki curves can be correlated with the weathering level respectively of albite and potassium feldspars. -The silicon Ki curve can be connected with the type of secondary minerals formed successively along the weathering profile. So, the Ki coefficient associated with other analytical methods like X ray diffraction, can be used in weathering studies. According to our knowledge, it is the first time that such geochemical approach is used in weathering profile study.