Weathering of gneiss in Calabria,Southern Italy

The effects of weathering in a Mediterranean climate on the mineralogy and microfabric of Paleozoic gneiss of the Sila Massif, Calabria, southern Italy, have been studied. Field observations show highly weathered rock forms a residual soil. Micromorphological and mineralogical properties of bedrock and saprolite show that the weathering process is characterized by at least two major stages, having two distinct rock microfabrics. In the first stage, the morphological features of the original rock are preserved and weathering is manifested mainly by microfracturing, and large portions of the rock remain unaltered. The second stage of weathering involves further development of microcracks and progressive chemical attack on the minerals. This latter stage occurs along both compositional and microstructural discontinuities, with etch pitting of feldspar, and neoformation of clay minerals and ferruginous products replacing feldspar, biotite, and iron-bearing garnet. The determination of quantitative petrographic indices provides a measure of the various stages of weathering.     

Characteristics of Regolith developed on basalt in Pahang,Malaysia

Abstract

The characteristics and degree of weathering in a deep regolith developed on basalt were investigated. A representative 15m deep regolith of soil‐saprolite‐rock sequence, located in Pahang state, Malaysia, was selected. The intensity of weathering in this deep regolith was assessed by various weathering indices as well as changes in the physicochemical properties and clay mineralogy of the regolith. The values of all weathering indices and other assessments gave strong evidences of intense weathering during saprolite formation. The low CËC and base cation values even in the saprolite layers were accounted By the extreme depletion of major elements such as potassium (K), sodium (Na), calcium (Ca), and magnesium (Mg), and significant enrichment of aluminum (Al), iron (Fe), titanium (Ti), copper (Cu), and niobium (Nb) occurred during saprolitization. The extreme weathering pattern of basalt differed from those described in granite and metamorphic rocks found in Peninsular Malaysia.     

Weathering geochemistry of a paleudult in tropical Surinam

Chemical analyses and mineral norm calculations of a Paleudult, situated on a dolerite dike, revealed that the soil was not derived from the dolerite but from schistose material that is found in adjacent areas and which has been deposited over the dolerite.Thin-section micromorphology of the argillic horizon indicates that clay illuviation is now not an active process. With respect to the clay mobility the soil is paleosolic.The weathering of the quartz-dolerite rocks under the deposit of schistose material was studied by analyzing the spheroidal weathering layers of the rock. It appeared that most minerals of the rock dissolve completely and that only little clay has been formed during the early stages of weathering.     

中国福建省土壤的主要粘土矿物

The clay minerals of more than 200 soil samples collected from various sites of Fujian Province were studied by the X-ray diffraction method and transmission electron microscopy to study their distribution and evolution.Montmorillonite was found in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit,and some lateritic red soil,red soil and yellow soil with a low weathering degree.Chlorite existed mainly in coastal solonchak and paddy soil developed from marine deposit.1.4nm intergradient mineral appeared frequently in yellow soil,red soil and lateritic red soil.The content of 1.4nm intergradient mineral increased with the decrease of weathering degree from lateritic red soil to red soil to yellow soil.Hydrous micas were more in coastal solonchak,paddy soils derived from marine deposit,lacustrine deposit and river deposit.and puple soil from purple shale than in other soils.Kaolinte was the most important clay mineral in the soils iun this province.The higher the soil weathering degree,the more the kaolinite existed.From yellow soil to red soil to lateritic red soil,kaolinite increased gradually,Kaolinite was the predominant clay mineral accompanied by few other minerals in typical lateritic red soil. Tubular halloysite was a widespread clay mineral in soils of Fujian Province with varying quantities.The soil derived from the paent rocks rich in feldspar contained more tubular halloysite.Spheroidal halloysite was found in a red soil and a paddy soil developed from olivine basalt gibbsite in the soils in this district was largely“primary gibbsite” which formed in the early weathering stage.Gibbsite decreased with the increase of weathering degree from yellow soil to red soil to lateritic red soil.Goethite also decreased in the same sequence while hematite increased.     

南极Fildes半岛成土过程中不同时期的岩石风化趋势

From the view of energy state of material,this paper introduces a concept a concept of weathering potential in carrying out quantitative calculation of the relevant products at different stages of rock-weathering and primary soil-forming processes,elaborates respectively on weathering degree in the bio-weathering layer of rocks and during the formation of soil material and clay,and evaluats the further tendency of weathering in the above-mentioned stages.The authors have discovered that the scales of weathering potential of the materials increase successively in the three stages,which indicates that the products in the above-mentioned three stages must have undergone stronger and stronger weathering in the primitive forming process of soil in Fildes Peninsula,Antarctic.But,Because of relatively weak chemical weathering,it is reasonable that there are much more skeleton grains and little clay in priamry soils in this region.Meanwhile the authors have also verified that the weathering potential of crde rock determines to some extent decrease in the products‘ weathering potential in the different stages in primary soil-forming,thereby plays an important role in the genesis and development of the primary soil in the studied area.     

Alteration profonde preglaciaire d'un gneiss a Charlesbourg,Quebec (Canada)

A saprolite derived from a gneiss has been sampled in a fault zone at Charlesbourg, near Quebec city. The weathering largely exceeding 6 m in depth and a till overburden suggest preglacial alteration under warmer climatic conditions. This is further supported by the presence of high (up to 15% ) percentages of crystalline iron oxides (goethite and lepidocrocite). Four different facies were defined in the saprolite. Mineralogical analyses were performed on the clay fractions and the following associations were found: swelling minerals + kaolinite, kaolinite, kaolinite + gibbsite, swelling minerals + kaolinite.The degree of weathering of 2:1 minerals into kaolinite seemed to be related to internal drainage conditions. Cation exchange capacity of the saprolite was adequately explained by the clay and silt fraction contributions.     

Mineralogy of two red soil (Alfisol) profiles of mysore state,India

Quantitative mineralogical composition of the clay fractions of two red soil (Alfisol) profiles developed on gneissic rocks in Mysore, India, is discussed in relation to the genesis of these soils. Data on the mineralogy of their silt and sand fractions are also presented. Both the silt and clay fractions are kaolinitic and contain considerable amorphous material. A possible weathering sequence of transformation of the minerals, with probable mechanisms involved, is suggested.     

Morphogenesis and erodibility of soil-saprolite complexes from magmatic rocks in Swaziland (Southern Africa)

The Kingdom of Swaziland in Southern Africa is affected by diverse forms of severe soil erosion as a consequence of population growth, deforestation, and overgrazing of the grass savanna. The physical, chemical and mineralogical properties of soil-saprolite complexes show that the presence of relict saprolite underlying the soils is an essential precondition for the development and rapid expansion of deep incising erosion gullies in areas with magmatic rocks. The polygenetic isovolumetric weathering of saprolite led to mass losses of more than 50 vol.-% due to the formation of solution cavities by leaching in warm and humid climatic phases. Therefore, the saprolite is highly susceptible to erosion. Low structure stability of the saprolite, indicated by shear strength values < 5 kPa, results from a silty texture, absence of organic matter, and low contents of Fe- and Al-oxides and exchangeable Al-ions. On the contrary, the clay-rich ferrallitic soils developed from saprolite are comparatively stable against erosion. The relatively high structure stability of the soil, indicated by shear strength values ranging from 7 to 12 kPa, results from the organic matter content of about 3 wt.-% C_org in the topsoil, up to 7 wt.-% pedogenic Fe- and Al-oxides, pH(KCI)-values around 4, and Al-saturations above 50 %. Concerning the genesis and the susceptibility to erosion, the results show a distinct bipartition of the weathering mantle. Therefore, it is necessary to investigate the site properties and spatial distribution of complete soil-saprolite complexes from magmatic rocks as a basis of soil erosion control, land evaluation, and land use planning.     

Weathering and morphogenesis in a tropical plateau

Weathering phenomena on the Campos do Jorda?o Plateau have been analysed in order to show their relationship to geomorphic compartition and morphogenetic dynamics.Correlations between weathering characteristics of surficial materials and geomorphic compartments are more conspicuous in saprolite than in soils.On the plateau highest areas gibbsite is the most important secondary mineral; at lower altitudes kaolinite prevails. Thus the spatial distribution of secondary minerals seems to out-line two weathering processes—alitization and monosialitization—defined and interpreted as two different stages within a general alitic weathering trend.On hillslopes, bedrock weathering degree reflects the intensity of morphogeneticactions. Hillslope deposit profiles frequently show degrees and sequences of weathering related to downslope mouvement processes rather than to incipient post-depositional pedogenetic activity.Red ferralitic materials overlaid by superficial podzolized ones point to the super-position of two pedogenetic trends linked to variations in environmental conditions.Ferralitic materials would be remnants of weathering processes active during the Tertiary at lower altitudes, prior to the plateau uplift.     

Impact of climate and related weathering processes on the authigenesis of clay minerals: Examples from circum-Baikal region,Siberia

The widely outcropping igneous and metamorphic rocks of the circum-Baikal region all contain illite and chlorite resulting from early alteration of primary minerals. These rocks were deeply weathered and leached, mainly to kaolinite, under the warm and mostly wet Upper-Cretaceous and Palaeogene climates. During the following severely dry and cold Pleistocene episodes, the glacier cover was restricted, allowing the formation of permafrost, which is still several hundred metres thick in the northern Baikal region, while the lowlands of the central and southern regions belong to the so-called “sporadic permafrost area”. The lack of glacial abrasion during the cryo-arid climate preserved somehow the weathering products of the previous warm periods that presently crop out next to the relics of the periglacial Pleistocene weathering. These relict deposits derived from the same substrate rocks as those weathered during the warm and wet climate, but they differ in the clay composition with the occurrence of a montmorillonite-type smectite that is dominant in some sites.     

Mineralogical study of weathering products of granodiorite at Shinshiro City (I)

Rock weathering has long been a subject of study for geologists, mineralogists, chemists and soil scientists since the dawn of this century. In methods for investigating rock weathering, three aspects seem to be present. The one is a chemical aspect in which weathering process is considered by comparing chemical composition of fresh parent rock with that of the weathered rock, the difference being attributed to gains or losses of chemica,l elements with respect to a supposed immobile element, usually aluminum. This aspect can elucidate the chemical behaviour of rock, that is, of material as an assemblage of constituent minerals in the environment of weathering. Among many such studies mentioned. PoLYNov's “Cycle of Weathering” (6) is one of the most comprehensive and fruitful acheivements. The second aspect is a mineralogical one, in which interests are directed toward skeletal minerals surviving against severe attacks of weathering. It is commonly observed that some of the original constituent minerals still remain in weathered material after others have been extinguished. The former minerals are more stable than the latter. On the basis of these observations, the sequence of resistantiability or stability to weathering can be determined for many rock forming minerals. GOLDICH's study of rock-weathering (3) is a representative one in this aspect. The third aspect is concerned with clay mineralogy. Primary rock-forming minerals are weathered into very finegrained materials most of which had been believed amorphous until techniques now used in clay mineralogical reserch proved their crystalline state. Besides primary skeletal minerals, weathered materials are now known to consist mainly of both amorphous and crystalline secondary minerals, mostly appearing in minus two micron fraction and being objects of interest in clay mineralogy. Any study of rock weathering hitherto performed stood more or less on the three aspects above mentioned but, all the abovementioned seem to fall into the common tendency of dealing with materials as bulk mass. That is, they were concerned much more with fresh rock versus its weathered end products as a whole rather than with the process or mechanism by which fresh rock changed into weathered material. Thus, in the temperate to subtropical humid region, it is known that, for example, kaolinite minerals, gibbsite, and some of 2: 1 type clay minerals are found in weathered materials of rocks and further that Na, K. Mg. Ca, and Si are leached away, while H, Al, and Fe are concentrated in the weathered products, but it is scarcely understood from what constituent minerals of the parent fresh rock any of the clay minerals now present in the weathered material were derived. Though, a mineralogical or chemical tracing of the courses of decomposing minerals from their initial phases to subsequent modified phases was already pioneered by STEPHEN (8). such a trend is believed, by the author, to be a fourth aspect necessary for further thorough understanding of rock weathering. This way of study may also serve in bridging between experimental data on chemical reactions of specific minerals with reagent solutions on the one hand and observations of mineralogical interrelation of parent minerals to resultant weathering products on the other hand. Granitic rock offers a suitable situation for this fourth aspect because of the ease in picking up mineral grains at various weathering stages due to the coarseness of its constituent minerals and also to its unique mode of physical disintegration.     

Graulehmbildung durch hydrothermalen Gesteinszersatz

Formation of gray loams by low hydrothermal alteration Beside the formation of gray loams by strong protolytic weathering of silicates a possible alteration by aszending, CO₂-containing solutions is discussed. Questionable is the possibility to identify different formation from the elementary composition. Therefore the total contents of Si, Al, Ca, Mg, K and Na and the composition of the clay fraction were investigated in samples from sites with assumed low hydrothermal alteration. Arguments for alteration by CO₂-containing solutions are: No vertical but a distinct horizontal rock alteration in steep-dipping rocks. Bleached, soft and loamy material changes to unalterated fresh rock on a short distance. Softening is caused by dissolving the cementing SiO₂ in the rocks, bleaching is due to removing Fe and Mn. Akkumulation is appointed by redox-gradients. The contents of Al, Ca, Mg, K and Na are not clearly reduced in the bleached, loamy altered rocks. Composition of the clay fraction is equal in altered and fresh rocks. Illite is mostly dominant. Strong differences between weak altered material and unchanged parent material with distinct boundaries, dissolution of cementing SiO₂ and unchanged high base contents are contradictionary to an advanced stage of protolytic weathering.     

海拔与岩性变异对石灰岩发育土壤黏土矿物组成的影响

用X射线衍射法研究了中亚热带区海拔、岩性变异对石灰岩发育土壤黏土矿物组成的影响。结果表明:在海拔约2 000 m的重庆金佛山山顶区,灰黑色钙质页岩石灰岩发育的黑色石灰土,矿物组成复杂,除伊利石为主外,还有少量蛭石、绿泥石、高岭石和蒙脱石,有一定量的母质残留矿物(滑石和滑间皂石);由砂页岩和灰岩互层母质发育的山地黄棕壤,以蛭石为主,伊利石和高岭石其次,有三水铝石和弱结晶针铁矿。在金佛山中下部(600~1 200 m),石灰岩发育的黄色石灰土以蛭石或伊利石为主,有一定量的高岭石,普遍出现针铁矿。湖南怀化盆地海拔200多米,纯灰岩发育的红色石灰土为伊利石、蛭石和高岭石(含埃洛石)组合型,有结晶好的针铁矿和少量赤铁矿。海拔显著影响高岭石含量、针铁矿结晶度,海拔降低时高岭石含量增加,针铁矿结晶度提高。在金佛山山顶区,石灰岩母质岩性和微地形变异对土壤成土过程和黏土矿物组成影响显著。     

Intensive kaolinization during a lateritic weathering event in South-West Spain: Mineralogical and geochemical inferences from a relict paleosol

This study documents the mineralogical and geochemical record of a lateritic weathering event during the Pliocene in South-west Spain. The paleoweathering profile derived from arkosic sands and comprises a white sandy clayey saprolite, a red mottled clay zone overlain by a thick soft layer, and a ferruginous pisolitic hardcap partially dismantled by erosive processes. Kaolinite, quartz and degraded K-feldspars are the major minerals forming the saprolite, along with Al-goethite (mottled zone) as well as gibbsite and boehmite (soft layer), while the pisolitic duricrust is essentially composed of quartz grains embedded in a matrix of Fe oxy-hydroxides (hematite, goethite, and/or maghemite). Besides quartz, the phases most resistant to alteration were a variety of heavy minerals (mostly titanium oxides) that are present as minor impurities in the residual kaolin deposit.     

Podzolization as a deferralitization process: a study of an Acrisol–Podzol sequence derived from Palaeozoic sandstones in the northern upper Amazon Basin

Morphological, geochemical and mineralogical studies were carried out in a representative soil catena of the low‐elevation plateaux of the upper Amazon Basin to interpret the steps and mechanisms involved in the podzolization of low‐activity clay soils. The soils are derived from Palaeozoic sandstones. They consist of Hydromorphic Podzols under tree savannah in the depressions of the plateaux and predominantly of Acrisols covered by evergreen forest elsewhere. Incipient podzolization in the uppermost Acrisols is related to the formation of organic‐rich A and Bhs horizons slightly depleted in fine‐size particles by both mechanical particle transfer and weathering. Weathering of secondary minerals by organic acids and formation of organo‐metallic complexes act simultaneously over short distances. Their vertical transfer is limited. Selective dissolution of aluminous goethite, then gibbsite and finally kaolinite favour the preferential cheluviation of first Fe and secondly Al. The relatively small amount of organo‐metallic complexes produced is related to the quartzitic parent materials, and the predominance of Al over Fe in the spodic horizons is due to the importance of gibbsite in these low‐activity clay soils. Morphologically well‐expressed podzols occur in strongly iron‐depleted topsoils of the depression. Mechanical transfer and weathering of gibbsite and kaolinite by organic acids is enhanced and leads to residual accumulation of sands. Organo‐metallic complexes are translocated in strongly permeable sandy horizons and impregnate at depth the macro‐voids of embedded soil and saprolite materials to form the spodic Bs and 2BCs horizons. Mechanical transfer of black particulate organic compounds devoid of metals has occurred later within the sandy horizons of the podzols. Their vertical transfer has formed well‐differentiated A and Bh horizons. Their lateral removal by groundwater favours the development of an albic E horizon. In an open and waterlogged environment, the general trend is therefore towards the removal of all the metals that have initially accumulated as a response to the ferralitization process and have temporarily been sequestrated in organic complexes in previous stages of soil podzolization.     

Weathering patterns in representative soils of Guanxi Province, south-east China, as indicated by detailed clay mineralogy

The mineralogy of five soils situated on a south-west to north-east transect of Guanxi Province, south-east China was determined by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA) and chemical composition. One soil had formed on granite under tropical conditions, the others on sedimentary rocks under subtropical conditions. In all soils, kaolinite dominates the clay fractions, and is accompanied by vermiculite or chloritized vermiculite. In the saprolites of the granite-derived and sandstone-derived soils, the kaolinite has a morphology close to that of hydrated halloysite. The formamide and hydrazine/water tests suggested the presence of both poorly-crystalline (dehydrated halloysite) and well-ordered kaolinite. Gibbsite was identified in the saprolites of the granite-derived and sandstone-derived soils but not in the soils themselves. Gibbsite was also identified in the lowermost horizon of a soil derived from Pleistocene sediments. The upper horizon clay of a Rendzina soil formed on Permian limestone contains much more gibbsite and is much more weathered than the corresponding lower horizon clay. Only in the more humid, southernmost soil can the mineral composition be explained by present-day climatic conditions. In the soils from drier areas, the clay mineral composition reflects weathering that had taken place under moister, paleoclimatic conditions. Some of these paleoclimatic conditions had been conducive to lateritic weathering, as is indicated by the presence of goethite-containing pisolitic nodules in one of the soils and in its parent material. Two of the soils appear to be polymorphic, with one part of the soil having weathered more strongly than the other part.     

Potassium dynamics of a forest soil developed on a weathered schist regolith

Soils of the humid tropics are poor in available potassium due to intensive weathering and leaching of nutrients. A study was conducted to investigate the mineralogy and potassium supplying capacity of a forest soil developed on a weathered schist regolith. The quantity–intensity (Q/I) approach was used in thisstudy. The schist regolith showed deep weathering and intense leaching throughout the profile, resulting in low cation exchange capacity (CEC) and available K in soil and saprolite layers. The mineralogy of the regolith was dominanted by kaolinite, gibbsite and goethite. Feldspar, mica and mica–smectite minerals were observed in the lower saprolite layers. The Q/I parameters showed that the soils and saprolites were low in K supply power. This observation was attributed to weathering and intense leaching. The free energy values of K replacement (ΔG _r°) also suggest that soils and saprolites of the schist regolith were deficient in K. The Q/I parameters significantly correlated with organic carbon and clay content, CEC, pH and exchangeable K.     

Genesis and dynamics of an oxic dystrochrept and a typic haploperox from ultrabasic rock in the tropical rain forest climate of south-east Brazil

Oxisols are a product of long and intense weathering. Their actual characteristics can neither be traced back to a certain climatic period, nor do they reveal the present soil-forming factors. In this paper, therefore, a reconstruction of the soil genesis of a strongly weathered Oxisol is attempted by comparing the present mineralogical status and the recent dynamics of selected elements with a nearby Inceptisol from the same catena. The parent rock of both soils is serpentinized dunite with nepheline-syenitic veins; the soil moisture regime is udic. Chemical and mineralogical parameters, as well as the concentration of elements in the soil solution of a Typic Haploperox and an Oxic Dystrochrept from the same catena were analyzed for one year. The dynamics of Si and Mg strongly depended on the state of weathering. In the saprolite of the Dystrochrept, Si is partially removed and partially fixed by the formation of quartz and mixed layer minerals. Mg is first fixed in mixed layer minerals, but is during consecutive weathering leached from the transition zone from saprolite to the oxic horizon. Due to the high redox potential, Fe is not soluble, forming goethite in the upper part of the saprolite. Al is also not removed and is finally fixed in kaolinite. In the weathered upper part of both profiles the concentration of Al, Si, Mg and Fe in the soil solution is too low to perceive a further weathering. The present dynamics of Fe, Al, Mg and Si and the mineralogical status of the Dystrochrept indicate that these processes are similar to those that have led to the formation of the Haploperox. The soils, therefore, developed exclusively by the relative enrichment of Al and Fe, the partial loss of Si and the almost complete loss of Mg. Desilification and ‘laterisation’ are hence processes occurring also presently under a tropical rain forest climate.     

Formation of clay from granite and its distribution in relation to climate and topography

Chemical weathering can be approached from three interpretive points of view: distribution of the secondary minerals in landscapes, weathering of primary minerals and geochemistry of ions in solution. On granitic rocks it is posible to define both climatic and topographic sequences of distribution of clays in association with weathering products: gibbsite - kaolinite - hydrous iron oxides - hydrous manganese oxides - iron-rich montmorillonite - calcium carbonate - calcium sulfate - sodium carbonates and silicates. These stages appear as an absolute accumulation formed by filling of intergranular pores. Another sequence can be found not in intergranular pores but in the cracks and along the cleavages of primary minerals from which they are derived by relative accumulation. The order of appearance of these secondary minerals remains the same regardless of the nature of climates and the topographic position. The conditions of equilibrium are not identical in the interior of grains of different primary minerals nor in the pores outside of them and the secondary minerals found in the different situations are also not identical. This is well demonstrated by the chemistry of waters draining slopes. The equilibrium diagrams and balances between cations and silica sometimes result in disagreement, proving that equilibrium conditions are not satisfied between solutions circulating in the pores and clay minerals found inside the primary minerals.     

Differentiation of upland soils on the Palawan ophiolitic complex, Philippines

The morphological and chemical features of upland soils derived from ophiolitic parent materials in Palawan, Philippines, are summarised from soil survey data, and compared with those on geosynclinal sedimentary rocks. Although the ophiolitic crystalline rocks are lithologically heterogeneous, the soils have similar morphologies, with reddish colours, clay textures, and friable crumb structures. However, their chemical characteristics differ considerably, with a pH range of over one unit, and exchangeable base status ranging from 95% dominance by Mg in soils derived from ultramafic rocks to 75% dominance by extractable Al in soils from felsic rocks. There are also differences in the total contents of most heavy metals, but less for the extractable forms of Mn and Cu. The soils on nearby geosynclinal sedimentary parent materials are also finely textured but are morphologically distinct, with more yellowish subsoil colours, blockier structures, some clayskins and ‘corned beef’ saprolite. They are also more acid and base-deficient, with Al occupying up to 95% of their exchange complexes. In view of the morphological similarities, chemical criteria are needed to separate the edaphically different groups of ophiolitic soils. Exchangeable Mg/ECEC is the best simple criterion. Soil Taxonomy and the FAO World Reference Base fail to distinguish between the mafic and ultramafic ophiolitic soils at subgroup level. Interactions between droughtiness, cation imbalances, phosphate immobilisation, and heavy metal toxicities may determine the stunted vegetation characteristic of some of the ultramafic soils.