Loose Horizon Research Articles

Petrographic and geochemical characterization of weathered materials developed on BIF from the Mamelles iron ore deposit in the Nyong unit, South-West Cameroon

Iron ore deposits hosted by Precambrian banded iron formation (BIF) are the most important source of mineable iron. In Cameroon, they are located in the southern part of the country. This study reports the petrological and geochemical data of iron ores collected from a weathering profile in the Mamelles BIF deposit, SW Cameroon. The profile is composed of three levels which are from the bottom to the top: the saprock, the ferruginous horizon, and the loose horizon. Eight representative iron ore samples (rock fragments and loose clayey material) were collected along the profile and were subjected to petrographic and geochemical analyses. Their mineralogy consists of martite, goethite, quartz, and lesser amounts of hematite, magnetite, kaolinite, and halloysite. The presence of minerals such as kaolinite and goethite in the Mamelles iron ores suggests their supergene origin. Geochemically, the saprock is characterized by high iron content (70.25 wt% Fe2O3t), and low silica (26.38 wt% SiO2) and alumina (1.14 wt% Al2O3). The rock fragments collected from the ferruginous horizon display higher Fe2O3t (72–76.40 wt%), Al2O3 (2.80–5.43 wt%), and lower SiO2 (16.70–18.35 wt%) contents, suggesting the leaching of silica during the enrichment process. The loose clayey samples collected from both the ferruginous horizon and the upper loose horizon show lower iron and higher silica contents. When normalized to the underlying BIF saprock, both rock fragments and loose clayey ores display LREE enrichment, suggesting that they formed through supergene processes. Economically, most of the Mamelles iron ores are classified as medium-grade ores and a few display acceptable contents in contaminants. Overall, this petrological and geochemical study of the Mamelles iron ores revealed encouraging results. Given its strategic location near the deep seaport, the deposit should be investigated in more detail for its mining potential.

The formation of the soil section by morphological characteristics in technosol of Semenivsko-Holovkivske lignite open-cut

Analysis of morphological descriptions of technosol sections indicates the following features of soil formation in technosol under common pine plantations. The particle-size composition in all variants depends above all on the composition of rocks, which form technosol. There is a color change in the upper layer of lithosol section, where the initial stage of humus formation and humus accumulation processes take place. The humus layer, which indicates the speed of these processes, is present only in the upper and lower third of the slope in 20-year-old common pine plantations and ranges from 7 cm in the upper to 10 cm in the lower third of the slope, which may be the result of flat erosion on the slope. There is no humus horizon in valley bottom, as spring water stagnation occurs there, which caused the death of most of the plantings, which were the main source of organic matter entering the soil. The inhibition in the processes of humus formation under the plantations that develop on the upland occurs due to the low content of the dusty fraction. The formation of the soil structure was not observed in technosols formed from rocks with light particle-size composition (sandy loam and sand); on loamy variants, it passes intensively in the upper 10–15 cm layer, which contains most plant roots, worms and insects. The presence of a plate structure has a industry-related origin and is formed when heavy machinery passes through rocks with a high moisture content. An important indicator of the initial processes of soil formation is the decompression of technosols and the speed of this process. This process is the most quick under 23-year-old plantations on a upland, cultivated on rocks with light particle-size composition (the loose horizon is 80 cm), on a slope in a 20-year-old plantation on heavier rocks, this process is much slower (the loose horizon is about 7–10 cm). Brown coal, inclusions of carbonaceous clays, rippled and kaolin clays can be found among the inclusions. The number and composition of the inclusions depends on the nature of the rocks, which form tehnosol. The new growths of biological origin should be mentioned, among which there are biological capillaries, represented by root passages with a diameter of 4–7 mm, filled with soil material enriched with organic substances and ones with the diameter of 2–3 mm to 1 cm, filled with coprolites and soil mass. Thus, changes in technosols identified by morphological features depend on open-pit rocks which form the edaphotope, their composition and properties, on the quality of the technical stage of reclamation and on the terrain of the formed territory.

Tropical chemical weathering of a garnet rich micaschist in the rainforest zone of Cameroon

Soils developed on garnet-rich micaschist in the Southern Cameroon plateau were investigated. Morphologically, two weathering materials constituted the study profile, fine earth and nodules. Each material has a singular evolution from the coarse saprolite to the upper set of clayey and loose horizons. There is Al2O3 enrichment in the fine earth concomitantly to Fe2O3, Na2O and K2O enrichment in nodules in major element–Al2O3 variation diagrams. A–CN–K and A–CNK–FM diagrams show a progressive depletion of Al in nodules concomitantly to an enrichment of this element in the fine earth. Three pedogenetic processes emerge from the evolution of weathering products. There is a ferruginization process whose importance increases from the coarse saprolite to the upper set of clayey and loose horizons, giving rise to the formation of dense nodules composed of hematite, goethite and gibbsite. This ferruginization occurs concomitantly with a desaluminization process, which causes Al2O3 depletion in nodules. These nodules are formed by the process of induration where the hydrated amorphous hydroxide and oxides are dehydrated to their crystalline oxide form, due to condensation. In addition, there is aluminization, which occurs in the fine earth, leading to the development of a red clayey material composed of kaolinite and gibbsite.

Iron bands, fragipans and duripans in the northeastern plateaus of Brazil — properties and genesis

Iron bands, fragipans and duripans are common in yellow low-activity clay soils developed from the Barreiras Group in coastal plateaus of northeastern Brazil. Such indurated horizons are found in depressions of the plateaus where sugar cane growth is greatly reduced. Little research has being done on the nature of their bonding agents and their genesis. Research carried out in arid zones has frequently attributed duripan consistency to amorphous silica; however, duripans in northeast Brazil occur under higher rainfall (1500–2000 mm). The objective of this work was to study the nature and distribution of soil features in a plateau to better understand the processes associated with soil hardening. Two transects, across a small and a large depression, were investigated using field soil characterisation, micromorphological techniques, and mineralogical and geochemical analyses.The formation of the indurated horizons studied is due to two sequential processes: development of aquic conditions and incipient podzolization. Such processes have affected the upper part of a thin (<0.6 m in small depressions) or thick (>0.6 m in large depressions) compact clay horizon, showing horizontal plans likely inherited from the sediment. Onset of aquatic conditions formed the first fragipans overlying iron bands. Later, aquic conditions are combined with incipient podzolization to produce bleached loose horizons overlying duripans and iron bands. As the bonding agents in the duripans are organo-metallic complexes, these duripans appear to be very different from those described in arid environments. Key words: Low activity clay Ultisols, Barreiras Group, iron bands, fragipans and duripans, organo-metallic complexes

A hard sandy‐loam soil from semi‐arid Northern Cameroon: I. Fabric of the groundmass

SummaryIn semi‐arid tropics soil hardening may reduce both water infiltrability and biological activity, thus inducing the development of large almost bare areas. A sandy‐loam soil with contrasting loose and underlying hard horizons was studied in the southern plain of the Chad basin. The fabrics of these horizons were studied using combined sieving and sedimentation techniques, mercury porosimetry and scanning electron microscopy. The horizons had similar particle size distributions of the skeleton grains. The hard horizon differs by a small increase in its fine clay (< 0·2 μm) content. The hardness is closely related to a fabric with clay coatings on the skeleton grains and clay wall‐shaped bridges linking the latter. This induces a strong continuity of the solid phase. This fabric requires a minimum of clay content (6%) to make the coatings and the wail‐shaped bridges, and it can be 30% less porous than the loose horizon, without any change in the packing of the skeleton grains. These characteristics of the fabric of the hard horizon are like those of fragipans elsewhere. The continuity of the solid phase, from the microscopic to the macroscopic scale, as well as the absence of a network of cracks explains the considerable strength of the hard horizon, and consequently the difficulties for water infiltration, root penetration and tillage.

Formation of loose and cemented B horizons in Podzolic soils: evaluation of biological actions from micromorphological features, C/N values and 14C datings

Micromorphological observations, C/N values and 14C datings of organic matter were used to evaluate impacts of biological activities on the development of loose and cemented podzolic horizons of a toposequence in the St-Lawrence Lowland. In well-drained soils, the B horizons containing roots have two distinct microstructure types: (1) a microstructure with organic aggregates in root zones (< 5% of the soil volume) resulting from microbial or animal actions, and (2) organo-mineral aggregates and grain coatings in inter-root zones which are due to aggregate transformation of root zones under biological and chemical processes. The organic matter of these loose horizons has C/N values of 23–28, and mean residence time of about 500 yr. The cemented Bfc horizon without roots is characterized almost exclusively by organo-mineral grain coatings, and has a higher C/N ratio (38) and a longer mean residence time (800 yr) indicating weaker biological activities than in loose B horizons. In the poorly drained soil, the cemented horizon has a microstructure with organic grain coatings only. The organic matter has a very high C/N value of 46 and a mean residence time of about 3500 yr. This horizon could represent an older "fossil" situation. Key words: Podzol, biological activity, micromorphology, 14C datings

Major mechanisms in formation of spodic horizons

Current explanations of the formation of spodic horizons do not accomodate all features of the horizons in their natural state. In this paper, a more complete explanation of major mechanisms is proposed, using two principles of colloid chemistry: (1) organic substances may form hydrophylic colloids with surface charges, and (2) the hydrophylic character and negative surface charges determine the dispersibility of the colloids. The hydrophylic character is due to the presence of hydrophylic radicals as parts of the organic compounds in soils. The surface charges are the result of dissociation of -COOH and possibly phenol-OH radicals. The neutralization of the surface charge can in principle occur: (1) through electrostatic or physical adsorption and (2) through chemisorption. The first case is typical for monovalent alkali cations. The adsorbed cations are distributed in a double layer, which favours dispersion. Chemisorption occurs mostly with polyvalent cations. This process corresponds in reality to the formation of organo-metallic compounds. It results in a relatively complete disappearance of the double layer and in the formation of large immobile “polymerized” organo-metallic compounds. Because these compounds contain much hydrophylic water, they form a gel. Transition into the solid state is accompanied by the loss of most of the hydration water. The dehydration may be induced by a decrease in thickness of the double layer. At a certain stage of the dehydration process, Van der Waals bonds and protonic bridges can form and bring about a certain degree of hydrophoby. In soils, mobile organic substances are formed during breakdown of plant remains. If at the top of the mineral soil enough polyvalent cations, especially Al and Fe, are available, the mobile organic substances formed are immobilized immediately and no migration occurs. In case insufficient amounts of Al and/or Fe are available to completely immobilize the mobile compounds, these cations are complexed by the mobile compounds and transported downward. Immobilization may occur at some depth through supplementary fixation of cations, through dessication or on arrival at a level with different ionic concentration. In nature, spodic horizons range from loose, with many roots, to very cemented with few roots. These differences can be related to changes in microstructure. Loose spodic horizons have a predominance of polymorphic pellets and aggregates, whereas organans or monomorphic coatings prevail in cemented horizons. The former horizons have many features suggesting major biological influences during their formation, viz., high numbers of roots, thorough mixing of the organic units with clay and silt, the presence of pedotubules and relatively young mean residence times. The latter horizons have features consistent with organo-metallic compounds immobilized in a gel-state, viz., the coatings are strongly cracked, indicating the transition of a gel into a solid; they contain much Al or Al plus Fe but very little or no Si, and the mean residence time is considerably higher than in loose horizons. The two processes seem to operate simultaneously during the formation of spodic horizons and their relative intensities determine the composition of each spodic horizon at any moment in its evolution. As long as the biological activity predominates, the horizon remains loose; if the accumulation of mobile organo-metallic compounds starts to prevail, the horizon is gradually cemented and fossilized.