Chemical weathering intensity and rare earth elements release from a chlorite schist profile in a humid tropical area, Bengbis, Southern Cameroon

RésuméUn profil d’altération développé sur chloritoschistes de la zone de Bengbis (Sud Cameroun) a été choisi pour quantifier l’intensité de l’altération et comprendre le comportement des terres rares. Les valeurs de l’indice d’altération mafique combinées aux diagrammes ternaires du système Al – Fe – Mg – Ca – Na – K montrent que l’hydrolyse des feldspaths est proportionnelle à celle des minéraux mafiques (pertes en Mg), bien que l’hydrolyse des plagioclases (Ca, Na) soit plus intense que celle des minéraux ferromagnésiens. Les matériaux d’altération étudiés sont localisés dans le domaine de la kaolinitisation, à l’exception des matériaux nodulaires qui sont légèrement latritiss. La modification du comportement du Mg dans le milieu d’altération s’exprime par les faibles valeurs du rapport Ca/Mg. Le potassium et Be sont lessivés dans le sol en association avec Mg. L’ordre de mobilité des éléments dans l’environnement d’altération étudié est : Ca ≈ Na > Fe2+ ≈ Sr > Mg ≈ Co > Mn > Li > Ba > Rb > P > Cd > Ni > Si > Be > K > Sn. Les enrichissements en K, Cs et Be dans les saprolites sont liés à la présence d’illite. L’accumulation en Cs dans le sol est due à la présence de kaolinite. Le système le plus stable dans le milieu d’altération étudié est : Hf – Nb – W – U. Les saprolites, les matériaux nodulaires et les matériaux argileux meubles superficiels sont appauvris en terres rares par rapport à la roche mère. Les terres rares présentent trois types de comportement le long du profil d’altération, comme l’indiquent les valeurs du rapport (La/Yb)N ((La/Yb)N < 1, (La/Yb)N ~ 1 et (La/Yb)N > 1). Les terres rares légères et les terres rares moyennes s’accumulent dans les matériaux d’altération pour des valeurs de pH comprises entre 5,5 et 5,6 et pour celles de Eh variant entre +60 et +70mV. L’ordre de mobilité de ces éléments dans ces matériaux est le suivant : terres rares moyennes > terres rares lourdes terres rares légères. Ce fait est contre-intuitif, car les terres lourdes sont plus mobiles dans les environnemenst supergènes que les terres rares légères. L’adsorption ou la co-précipitation de ces terres rares sur les oxydes de fer peut principalement contrôler la concentration de ces éléments dans le profil d’altération. Les faibles anomalies en Ce dans les matériaux d’altération de la zone de Bengbis, dues au changement de Ce3+ en Ce4+, sont probablement dues à la présence de faibles quantités de rhabdophane. Les matériaux d’altération étudiés présentent un fractionnement en Gd (Gd/Gd* ~0.70 – 0.84) dues à une intense lixiviation. Ce fait a rarement été signalé dans un environnement d’altération latéritique. Il semble qu’une partie de la distribution et de la remobilisation du gadolinium soit contrôlée par des minéraux mafiques dans les matériaux d’altération étudiés. La distribution et la mobilisation des terres rares sont donc contrôlées par (1) l’adsorption ou la coprécipitation dans les minéraux mafiques et Fe, (2) et légèrement par les minéraux contenant des terres rares tels que le rhabdophane, rencontrés dans les matériaux d’altération étudiés. Abstract An in situ weathering profile overlying chlorite schists in southern Cameroon was chosen to quantify chemical weathering intensity and to study the behaviour of rare earth elements (REE). Mafic index alteration values combined with the ternary diagrams of the Al – Fe – Mg – Ca – Na – K system show that the hydrolysis of feldspars is proportional to that of mafic minerals (losses in Mg), although the hydrolysis of the plagioclases (Ca, Na) is more intense than that of ferromagnesian minerals. The studied materials are localised in the domain of kaolinitisation, except for nodular materials which are slightly lateritised. The change in the behaviour of Mg in the weathering environment is expressed by the low values in Ca/Mg ratio. Potassium and Be are leached in the soil in association with Mg. The order of mobility of the elements in the weathering environment is: Ca ≈  Na > Fe2+ ≈ Sr > Mg ≈ Co > Mn > Li > Ba > Rb > P > Cd > Ni > Si > Be > K > Sn. The enrichments in K, Cs and Be in saprolites are linked to the presence of illite. Cesium accumulation in the soil is due to the presence of kaolinite. The most stable system is: Hf – Nb – W – U. Saprolites, nodular and loose clayey materials are depleted in REE relative to the parent rock. REE exhibit three types of behaviour along the Bengbis profile like indicated by (La/Yb)N ratio values ((La/Yb)N < 1, (La/Yb)N ~ 1 and (La/Yb)N > 1). Light REE and Middle REE accumulate in the weathering materials for pH values ranging between 5.5 and 5.6 and for those of Eh varying between +60 and +70mV. The order of mobility of REE in these horizons is: Middle REE > Heavy REE ≈ Light REE. This fact is counter-intuitive, because Heavy REE are more mobile in supergene environment than Light REE. Adsorption or co-precipitation of LREE onto Fe oxides mainly may control the concentration of these elements in the profile. Weak Ce anomalies in the weathering materials of Bengbis area, due to the change in Ce3+ to Ce4+, are probably due to the presence of low amounts in rhabdophane. The studied weathering materials show a fractionation in Gd (Gd/Gd* ~0.70 – 0.84) due to intense chemical leaching. This fact has been rarely reported in lateritic weathering environment. It appears that, a part of Gd distribution and remobilization is controlled by mafic minerals in the studied weathered materials. REE distribution and mobilization are thus controlled by (1) adsorption or co-precipitation in mafic and Fe minerals, (2) and slightly by REE-bearing minerals such as rhabdophane found in the studied weathering profile. &nbsp

Mineralogical, geochemical and geomechanical characterization of lateritic and alluvial clayey mixture products from Monatele - Ebebda, as building materials

This work focuses on the mineralogical and chemical analysis of two alluvial clays from the Sanaga River, four lateritic clays from Monatele and Ebebda regions in southern Cameroon and, on the physicomechanical characterization of these clayey mixtures. The exploitation of XRD patterns reveals that quartz, muscovite, kaolinite corundum and rutile are the main minerals in alluvial clays, associated with goethite, hematite and ilmenite in lateritic materials. The main oxides are SiO2, Al2O3 and Fe2O3 in lateritic clays. The low proportion of fluxing oxides (Na2O + K2O + CaO + MgO < 2.50 wt%) causes insufficient sintering during firing. Their particle size distribution is suitable for roofing tiles, lightweight blocks and solid bricks. Geotechnical characterization was carried out on five representative mixtures of lateritic and alluvial clay fired at five different temperatures, 900, 950, 1000, 1050 and 1100°C. The water absorption values are suitable for bricks (< 25%) at all the tested temperatures and for roofing tiles (< 20%) except at 900°C for the mixtures of one site at Monatele and at 1050°C for the mixture with 80 wt% of lateritic clay site at Ebebda. The flexural strength values are suitable for bricks (> 2.0 MPa) in all the sites except for two mixtures at Monatele, for tiles (> 6.5 MPa) in, one site at Ebebda and for the specimens with 80 and 60 wt% lateritic clays in one site at Monatele at 950 and 1050°C. The linear shrinkage values are high, causing deformations and microcracking on the produced bricks which require an addition of degreasers in order to reduce their plasticity before being used as building materials.Keywords: Monatele-Ebebda (Centre Cameroon), lateritic and alluvial clays, mineralogy, géochemistry, geomechanical characterizatio

Characteristics of Kaolinitic Raw Materials from the Lokoundje River (Kribi, Cameroon) for Ceramic Applications

Eight kaolinitic materials from the Lokoundje River at Kribi were sampled and investigated for their physical, chemical, mineralogical and thermal characteristics in order to evaluate their potential suitability as raw materials in ceramics. The Lokoundje kaolinitic materials are clayey to silty clayey and are predominantly composed of kaolinite and quartz. The alkali (Na2O + K2O) content ranges between 1 and 2.5 wt.%; these low values do not favor vitrification of the ceramics but may be improved through flux amendment. The presence of goethite in some samples limits their utilization in white ceramics. The minerals content, color, metallic sound, cohesion, linear shrinkage, flexural strength, bulk density, water absorption and microstructure were determined. The XRD data reveal that kaolinite and goethite were transformed, respectively, into mullite and hematite. The colors of the fired products are characteristic of their mineral assemblage. The metallic sound is indicative of low vitrification which is confirmed by the presence of cracks due to low flux contents. The cohesion is good to very good, due to the abundance of kaolinite. The physicomechanical properties increase with temperature as well as densification. The geochemical data show that the Lokoundje alluvial clays are suitable for the manufacture of white stoneware tiles

Mineralogical, geochemical, and geotechnical features of lateritic soils from termite mounds in two contrasting savannah areas (central Cameroon) as raw materials for brick making

Termite mound soils (TMS) from humid savannah (HS) and dry savannah (DS) were evaluated as raw materials for compressed earth bricks (CEB) and fired bricks. Mineralogy and major elements geochemistry were performed by X-Ray Diffraction and X-Ray Fluorescence, respectively. Physico-mechanical characteristics of unfired and fired bricks at 900, 950, 1000, 1050, and 1100 °C after 7 curing days were evaluated. The studied TMS are made up of quartz, muscovite, anatase, kaolinite, hematite, and goethite. Illite is present in humid savannah while in DS gibbsite appears. These materials are rich in SiO2 (58.96–61.79 wt%), Al2O3 (16.93–18.78 wt%), and Fe2O3 (7.41–10.33 wt%). The TMS from both HS and DS are sandy clay. Those from DS are silty (13%) than those from HS (<5.7%). Termite mound materials in DS are moderately plastic, while those in HS are highly plastic. Flexural strength values vary between 2.20 and 2.38 MPa for unfired bricks and between 2.41 and 3.26 MPa for fired bricks, respectively at 1100 and 1050 °C. Compressive strength values are ranged from 2.01 to 3.50 MPa for unfired bricks and 2.44 (1100 °C) to 11.08 MPa (1050 °C), with the best values in the DS area. Water absorption and linear shrinkage values are less than 25% and 5%, respectively, in the studied fired and unfired bricks. The physical and mechanical properties of unfired and fired bricks show that the studied TMS can be used for dense brick manufacturing. Materials from dry savannah exhibit better characteristics as construction materials due to relatively high weathering intensity leading to a spread-out particle size distribution, sintering, which promotes densification by reducing the porosity, and the conversion of metakaolinite into primary mullite upon temperature increase