Explore the vast range of titles available.

Although terra rossa has long been thought to form by residual dissolution of limestone and/or by accumulation of detrital mud, ash, or dust on preexisting karst limestones, we present conclusive new field and petrographic evidence that terra rossa forms by replacement of limestone by authigenic clay at a moving metasomatic front several centimeters wide. The red clay's major chemical elements, Al, Fe, and Si, probably come from dissolved eolian dust. The replacement of calcite by clay exhibits a serrated, microstylolitic texture that helps prove that replacement happens not by dissolution-precipitation, as conventional wisdom has it, but by pressure solution of calcite driven by the crystallization stress generated by the growth of clay crystals. The acid produced by the isovolumetric replacement of limestone by clay quickly dissolves out additional porosity/permeability in an adjacent slice of limestone within the front, triggering a reactive-infiltration instability that should, theoretically, convert the moving reaction front into a set of wormholes, then funnels, then sinks-the very karst morphology that in nature does contain the terra rossa itself. This beautifully explains why terra rossa and karst are associated.

It has been previously established that Western Europe underwent an important episode of lateritic weathering during early Cretaceous times. In this context, the red clays and ferrugineous pisoids deposits occurring in the graben of Saint-Maixent-l'Ecole (Poitou saddle) may represent remnants of lateritic formations associated with this paleoweathering episode. However, their exact age is unknown, and since they occur on top of the Oligocene limestones filling the graben, it cannot be ruled out that these lateritic soils formed more recently. In order to better constrain the age of these lateritic formations, goethite pisoids were sampled in the Fouilloux forest and investigated for Rb-Sr, U-Th and δO-δD isotopic systems. Rb and Sr have been measured on bulk pisoids, and also on leachates and residues from leaching experiments carried out in the attempt to isolate a radiogenic component. No reliable Rb-Sr isochron age could be derived, although the isotopic data would favor an early Cretaceous age for these lateritic formations. δ18O-δD measurements have been done on bulk goethite pisoids, and also on sub-fragments. The results indicate that the pisoids likely formed under a warm and/or arid climate. No argument for a late of post-Oligocene age can be found from the oxygen and hydrogen isotope data. Finally, U-Th analyses have been carried on bulk samples of goethites, and on sub-fragments as well. 234U/238U activity ratios are typically slightly higher than the equilibrium value, which indicates that goethites have recently evolved as open systems (if not still open). Hence, pedogenesis processes are still active from a geochemical point of view in these lateritic soils. This study has not allowed to derive a reliable age for these lateritic soils. However, when the results from the different isotopic systems investigated here are considered together, the best interpretation is that these lateritic soils developed most probably over the Kimmeridgian limestones along the edge of the Saint-Maixent-l'Ecole graben, and were reworked and redeposited at the bottom of the graben later on.

Le début du Crétacé a été marqué en Europe occidentale par le développement de paléoaltérations de type latéritique. Les argiles rouges à cuirasse ferrugineuse pisolitique du graben de Saint-Maixent-l’École (seuil du Poitou) peuvent constituer un exemple-témoin de ce type d’altération. Toutefois, leur âge reste inconnu et leur position stratigraphique, au-dessus des calcaires oligocènes du fond du graben, ne permet pas d’exclure un âge de formation beaucoup plus récent. Des pisolites de goethite provenant de ces sols latéritiques ont été échantillonnés dans la forêt de Fouilloux et ont fait l’objet d’analyses isotopiques Rb-Sr, U-Th et δ 18 O-δD, afin de tenter de lever l’indétermination sur l’âge de leur formation. Pour Rb-Sr, outre les analyses sur pisolites totaux, des expériences de lessivages ménagés ont été effectuées afin d’essayer d’isoler un composant radiogénique. Les résultats obtenus n’ont pas permis de donner un âge fiable par la méthode des isochrones, même si les données tendraient plutôt à faire remonter l’épisode d’altération au début du Crétacé. Les analyses δ 18 O-δD ont été effectuées sur des pisolites de goethite totaux, ainsi que sur des fragments sous-échantillonnés. Les résultats plaident en faveur de conditions de formation en zone climatique chaude et/ou aride. Aucun argument en faveur d’un âge fini ou post-oligocène ne peut être tiré de l’étude des isotopes de l’oxygène et de l’hydrogène. Enfin, les analyses U-Th ont été effectuées sur des goethites totales ainsi que sur des fragments sous-échantillonnés. Les rapports d’activités 234 U/ 238 U sont tous légèrement supérieurs à l’équilibre séculaire, indiquant que les goethites évoluent actuellement et/ou ont évolué récemment en système ouvert. Ainsi, les sols latéritiques apparaissent toujours comme géochimiquement actifs. Si cette étude n’a pas permis d’apporter une réponse claire sur l’âge des sols latéritiques, l’hypothèse la plus probable au vu des données isotopiques est que ces sols latéritiques se sont développés au-dessus des calcaires kimméridgiens sur les bordures du graben de Saint-Maixent-l’École, puis remaniés et finalement resédimentés au fond du graben. It has been previously established that Western Europe underwent an important episode of lateritic weathering during early cretaceous times. In this context, the red clays and ferrugineous pisoids deposits occurring in the graben of Saint-Maixent-l’École (Poitou saddle) may represent remnants of lateritic formations associated with this paleoweathering episode. However, their exact age is unknown, and since they occur on top of the Oligocene limestones filling the graben, it cannot be ruled out that these lateritic soils formed more recently. In order to better constrain the age of these lateritic formations, goethite pisoids were sampled in the Fouilloux forest and investigated for Rb-Sr, U-Th and δO-δD isotopic systems. Rb and Sr have been measured on bulk pisoids, and also on leachates and residues from leaching experiments carried out in the attempt to isolate a radiogenic component. No reliable Rb-Sr isochron age could be derived, although the isotopic data would favor an early Cretaceous age for these lateritic formations. δ 18 O-δD measurements have been done on bulk goethite pisoids, and also on sub-fragments. The results indicate that the pisoids likely formed under a warm and/or arid climate. No argument for a late of post – oligocene age can be found from the oxygen and hydrogen isotope data. Finally, U-Th analyses have been carried on bulk samples of goethites, and on sub-fragments as well. 234 U/ 238 U activity ratios are typically slightly higher than the equilibrium value, which indicates that goethites have recently evolved as open systems (if not still open). Hence, pedogenesis processes are still active from a geochemical point of view in these lateritic soils. This study has not allowed to derive a reliable age for these lateritic soils. However, when the results from the different isotopic systems investigated here are considered together, the best interpretation is that these lateritic soils developed most probably over the kimmeridgian limestones along the edge of the Saint-Maixent-l’École graben, and were reworked and redeposited at the bottom of the graben later on.

The bauxites deposits of Kachchh area in Gujarat are investigated to characterize them based on mineralogical and petrographic studies. The major bauxitic mineral in these occurrences is gibbsite, with minor concentration of boehmite and diaspore. Apart from the bauxitic minerals, the other associate minerals are kaolin, calcite, alunite and the iron ore minerals such as hematite and goethite and titanium rich anatase. The iron ore minerals (hematite and goethite) are 10-50 microns in size and are disseminated throughout the oolitic and pisolitic bauxitic minerals. At places the goethite exhibits colloform texture. The preservation of basaltic texture in some of the samples indicate that the insitu nature of these bauxites, which are formed by the alteration of calcic plagioclase from the parent basalt. Although, the basalt occurs as the main parent rock for these bauxites, the presence of calcite in some of the samples represent the possibility of having a limestone parent rock at least in some of the bauxite occurrences. Copyright 2012 Geological Society of India

Directions of remanent magnetism in an ∼2-m interval of terra rossa claystone at Bloomington, Indiana, cluster around the current Earth magnetic field direction, a fact consistent with authigenic formation of the terra rossa over the last 0.78 m.yr., the date of the latest earth's magnetic field reversal, and suggesting a minimum terra rossa formation rate of ∼2.5 m/m.yr. Paleomagnetic directions in limestone \"floaters\" within the terra rossa are indistinguishable from directions of the bedrock Mississippian Harrodsburg Limestone, indicating that the floaters are in situ and perfectly preserve their original stratigraphic position. Thus, the red claystone containing the floaters is interpreted as an authigenic, isovolumetric replacement of the Harrodsburg--the replacement of which the floaters escaped--and precludes the possibility that the floaters and the surrounding claystone are a mudflow that had carried limestone blocks in it. The implication that the claystone is authigenic independently confirms the petrographic observations reported previously that the red clay grew in situ, replacing the underlying limestone at a moving metasomatic front. Rock magnetic studies of the terra rossa clays show evidence of both goethite and maghemite as main magnetic carriers of this remanence. Both minerals are independently known to occur in pisolites of the terra rossa. The ferric-iron-bearing red clays also are weakly magnetic.

Rock erosion is attracting increasing attention from scientists worldwide. The area encompassing the Saint John Baptist Church, Saint John Village, XVII century ruins in Rio Grande do Sul at the UNESCO World Heritage Site is considered a Brazilian treasure. However, the risk of damage to this site from rock erosion has recently increased tremendously. Generally, the rocky construction such as fence, wall and tomb stone, seems strong but is actually extremely sensitive to erosion caused by lichens, fungi, molds and bacteria. Because of biological erosion and massive exposure, the fresh rock is dominated by clays and microorganisms. Water-adsorbing clays and microorganisms influence the mechanisms of the rock erosion. In this study, the formation of bio-clay-minerals in porous structure of pisolite was demonstrated using electron microscopy. Bacterial clay mineralization can deform the rock structure and even produce organic materials. Biological activity could easily corrode rocky constructions around the Saint John Baptist Church site. The rocks are pisolitic laterites possibly formed in Tertiary over the Kretaceous Parana flood Basalts. Samples inhabited by lichens and fungi were collected from a collapsed wall in the ancient church. The zonal reddish-brown pisolites are 4 mm in diameter in a matrix of clays associated with porous and empty spaces. Elemental distribution maps from X-ray fluorescence microscopy show iron-rich spherules of pisolite, whereas the matrix is composed of Al, Si, Mn, and Sr; thus producing goethite and kaolinite. Transmission electron microscopic observation showed that various types of bacteria inhabit the spherule and are associated with clay minerals and graphite. STEM elemental analysis confirmed the bio-clay-mineralization with Al, Si, S, and Fe, around bacterial cells. The results presented here will improve our understanding of nm-scale bio-mineralization and bio-erosion in lateritic rocks.

The procedures used for bauxite resource estimation are summarised using examples from Australia, Africa, South America and Asia. In the all studied cases the bauxite resources were estimated by drilling and sampling drill holes at 0.25–0.5 m intervals. Short sampling intervals are necessary for accurate estimation of the mineralisation contacts. Samples of the non-consolidated bauxites usually beneficiated by sieving and removing the barren fine grained material prior to chemical assays. Consolidated bauxites are not beneficiated and processed in a conventional manner. Bauxites density is preferably measured using the sand replacement method which is a formally certified technique for measuring bauxite density at the Australian deposits. Bauxites grade is estimated using conventional geostatistical techniques, which are commonly applied after geometry of the bauxite seam is flattened using ‘equal thickness unfolding’ method, or, in some cases, using ‘top flattening’ approach. Conversion resources to the ore reserves requires the following mining and processing conditions to be taken into considerations:Haulage distance;A vertical mining selectivity of 0.25–0.5 m;Pre-production infill drilling is usually used for grade control purposes;Grindability of the bauxites and quantification of the Bond Work Index;Optimal mesh size for bauxite beneficiation;Detailed chemical and mineralogical characterisation of the crude bauxite, including deleterious components, in particular iron and silica;Characterisation of the pre-desilication behaviour of the bauxite;Characterisation of the alumina extraction;The refinery parameters, including recovered alumina, refinery caustic consumption and red mud loading; Presence of the organic carbon, oxalate formation rate and carbonate formation rate;The mud settling rate, mud compaction and overflow clarity.

Ferruginous crusts and pisolites have chemical and mineralogical properties that differ from the surrounding soil due to Fe and Al enrichments which cause cementations that can harden irreversibly. In addition to, and possibly as a result of the Fe and Al accumulation, other ions, particularly phosphate are often also enriched by a factor of 2-20 relative to the surrounding soil. The P accumulated in ferruginous materials is normally bound to the Fe or Al in amorphous forms of low solubility. Distinct minerals have rarely been identified. We examined a section through a Venezuelan ferruginous crust, which contained portions with P contents >100 g kg-1, chemically, mineralogically and micromorphologically with the aim to show some of the mechanisms that cause such extreme P accumulation and segregation in a landscape that is otherwise very nutrient poor. Except for the cementation, manifested as an in-filling of pores by Fe, the morphology of the ferruginous crusts reflected that of the original soil. At approx. 30 cm below the crust's surface, goethite, strengite and leucophosphite (KFe2(PO4)2OH· 2H2O) were identified along a downward sequence of pores nearer the surface to pores at greater depth to the matrix of the lower crust. While the lower crust contained highly soluble P, Fe oxides from outer pore spaces showed high P sorption. The element and mineral distribution across thin sections suggested that incoming Fe had interacted with a soil matrix that was exceptionally rich in K and P to form highly soluble leucophosphite, followed by less soluble strengite and finally Fe oxides that essentially occluded the more soluble minerals found in the lower crust. Associated organic C dated at 18,700 y b.p., suggesting that the occlusion process occurred around the last glacial maximum, when the region became more arid. Although extreme in its extent, the process of separation and occlusion of minerals demonstrated here, may be useful for interpreting similar processes in soils and soil cementations that affect the biogeochemical turnover of elements.