Weathering Of Granitic Rocks Research Articles

Garnet as a carrier of HREEs in highly fractionated peraluminous granite: Implications for the formation of ion-absorption HREE deposits

Ion-absorption rare earth element (REE) deposits in South China are the world’s most important source of heavy REEs (HREEs). These deposits were formed by the weathering of granitic rocks whose formation involved primary HREE enrichment. Previous studies have identified the key role of late-stage magmatic evolution, especially the magmatic–hydrothermal transition stage played in HREE enrichment, but the detailed processes need further investigation. Garnet is a common HREE carrier in parent rocks and also a main contributor of these elements in formation of ion-absorption HREE deposits. Here, we investigate textural and compositional variations in garnets from parent rock (muscovite granite) of the Dabu ion-absorption HREE deposit to constrain the primary HREE enrichment of the parent rock during late-stage magmatic evolution. Mass-balance calculations reveal that garnet accounts for ∼67 % of the Y and 64 % of the REEs in the Dabu muscovite granite. The garnets can be classified into three types: i) magmatic garnets (Grt-1A) are intergrown with plagioclase, K-feldspar, and quartz, host both melt and mineral inclusions, and have high REE + Y contents (6488–19,215 ppm); ii) magmatic–hydrothermal garnets (Grt-1B) occur as overgrowths on Grt-1A, host both melt and fluid inclusions, and have intermediate REE + Y contents (2681–8683 ppm); and iii) hydrothermal garnets (Grt-2) are intergranular with quartz and altered biotite, host primary fluid inclusions, and have the lowest REE + Y contents (476–1247 ppm). The texture and composition of the three types of garnet indicate that the magma have undergone a transition from a volatile-undersaturated to a volatile-oversaturated aqueous system. The fluid, from which some REE minerals precipitated, present in the magma system was derived from the magma itself rather than from an external source, as evidenced by the similarity in Nd isotopic composition between the REE minerals and the whole-rock samples. During this transition, the presence of high-HREE garnet prevents the HREE partitioning into refractory minerals (e.g., zircon, REE-bearing phosphate) or extracting from the magma system by the fluid. Our findings show that granites containing high-HREE garnet have high potential for forming ion-absorption HREE deposits and that garnet can reliably record their magmatic evolution.

Fluoride abundance and their release mechanisms in groundwater along with associated human health risks in a geologically heterogeneous semi-arid region of east India

The present study investigated the source, release mechanisms, mobility and controlling factors of fluoride in groundwater. It has been conducted in the semi-arid rural tract of Birbhum district, located in the eastern part of peninsular India which comprises of the granite genesis aquifers, soft to medium hard rocks with laterite and lateritic soils and alluvial aquifers. The groundwater fluoride level of the study area varied from 0.3 to 9.36 mg/L with an average value of 1.7 mg/L. In the granite region, weathering of granitic rocks, dissolution and iron-exchange processes are responsible for fluoride occurrence in groundwater and its concentration is controlled by the concentration of carbonate and bicarbonate ions. In clay, laterite soil and sedimentary aquifer regions, pH of the circulating water and clay/soil, surface charge characteristics of clay/soils, soil permeability and residence time control the groundwater fluoride through adsorption/desorption of fluoride by the clay and laterite soil. High evaporation rate of the study area also plays an important role to increase the groundwater fluoride concentration. The study also evaluated the health risks to the residents based on the dose-response characterization and USEPA human health risk assessment model through ingestion and dermal contact exposure pathways. The population of infants and children within the study area is more susceptible to the non-carcinogenic health hazards of fluoride than adults. This takes place mainly through direct ingestion route where the dominance order of the population is infants > children > men > women.

Assessment of Groundwater Quality, with special reference to Fluoride Contamination in Bhopalpatnam Block, District Bijapur,Chhattisgarh, India

<div><p><em>Groundwater samples were collected for physico-chemical analysis during pre-monsoon period (May, 2015) from fluoride affected Bhopalpatnam block, District-Bijapur. The major objective of this study was to locate the vulnerable areas of fluoride contamination. The results reveal that the fluoride concentration in the existing groundwater in Bijapur-District is ranging from 0.1 to 3.58 mg/l. It is found that the study area is affected mostly by high Fluoride concentration and is evident from the 59% of total samples collected showing >1.5 mg/l of Fluoride concentration, which is a maximum permissible limit suggested by Bureau of Indian Standard [9].The evaluation of physico-chemical parameters reflects that weathering of granitic rocks are responsible for major ion chemistry of groundwater in Bhopalpatnam area. This study will be helpful in solving problem related to drinking water in the study area.</em></p></div>

Crystallization modifiers applied in granite desalination: The role of the stone pore structure

Soluble salts of marine origin are one of the main agents in the weathering of granitic rocks used in buildings and monuments; they cause a loss of cohesion between the mineral grains, which results in granular disintegration. This deterioration is produced by crystallization–dissolution processes of salts, mainly sodium chloride, in the interior of the rock. Desalination procedures (mainly immersion and application of adsorbent poultices) are the more usual intervention in this circumstance, but complete extraction of the salts is generally difficult to achieve. In this paper we present the results of a study of the effectiveness of two NaCl crystallization modifiers, potassium ferrocyanide and sodium ferrocyanide, in desalinating two types of granitic rock with significantly different pore structure. Evaluations have been made of how these modifiers influence, first, the transport of NaCl solution inside the rocks; second, the susceptibility of the granite to salt crystallization cycles; and third, the effectiveness of desalination by immersion in distilled water. The results obtained are that the effectiveness of the two modifiers is completely different in each of the two granites studied. In one type of rock studied, both ferrocyanides increase the evaporation rate of the saline solution inside the structure of the rock by permitting NaCl to crystallize on surfaces as efflorescences; consequently, first, the intensity of sand disaggregation during the salt crystallization test is reduced, and second, when either of the modifiers is added to distilled water, desalination is more effective than extraction with distilled water only. For the second type of granite evaluated, neither of the modifiers was found to be effective. Therefore, we conclude that the network of fissures in the rock has a significant effect on the behaviour of the solutions with modifiers, since the pore structure of the stone determines the dissolution/evaporation kinetics of the solutions of salts. Thus, generalizations about the effectiveness of specific modifiers in granites are not possible: their effectiveness must be studied for each specific stone.

Weathering of granitic rocks by chlorides: effect of the nature of the solution on weathering morphology

AbstractFive granites from NW Spain were subjected to two salt‐spray weathering tests under controlled atmospheric conditions. Granite samples were exposed to a sodium chloride solution in the first test and to a complex solution (sea water) in the second. Subsequent examination of the rocks by light and petrographic microscopy clearly demonstrated the development of different weathering morphologies in each test. The distribution of dissolved ions in samples taken at different depths from the weathered surfaces at the end of the experiments, and the changes in weight of the samples during the tests, also differed. In our opinion, these results were determined by the nature of the solution used; although sodium chloride is the predominant salt in sea water, the presence of other ions modifies its solubility, thereby varying the dynamics of mobility and precipitation and thus the weathering morphology that develops. Copyright © 2003 John Wiley & Sons, Ltd.

Un karst sous la glace de l'Antarctide ?

Are there karstic landforms under Antarctic icecap ? A new bedrock map of the Dome C area based on all radar data collected during Italian Antarctic Expeditions in 1995, 1997, 1999 and 2001 is presented. The map can clearly distinguish the Dome C plateau, along with some valleys and ridges develop. The plateau develops at three different altimetric levels and its morphology is characterised by hills and closed depressions. There are no visible features which can be ascribed to glacial erosion or deposition. The major valley is 15 km wide and 500 m deep ; its axis is parallel to that of other valleys and ridges in the plateau. The valley bottom is not flat, but contains a saddle in its centre. The morphology of the major valley could be considered as a relict one which was not modified by the overlying ice cap. Two big ridges, characterised by hills, saddles and depressions, lie near the boundaries of the area. The hill and depression landscape may be the result of two different processes : the weathering of granitic rocks, with the development of a "demi-oranges" and inselberg landscape, or the karstification of limestones, and development of a cone karst. The karstic hypothesis should be the more suitable, but it is impossible to exclude the granitic rock wheathering. Both proposed genetic hypotheses call for a warm, humid climate and a long period of stability in a continental environment. Consequently, the ice cap did not largely modify the landscape.

Pseudomorphic transformation of plagioclases during the weathering of granitic rocks in Galicia (NW Spain)

Microscopic study of twelve weathering profiles developed on granitic rocks in Galicia (NW Spain) showed that, in most cases, the transformation of the parent rock into saprolite occurs with pseudomorphic alteration of the plagioclases. It was seen that the transformation and the retention of plagioclases in the alteration profile are directly related to the conservation of the general structure of the rock and this, in its turn, depends on the mineralogical composition and texture. The most unstable constituents, in the Galician environment, are biotites and plagioclases, there being a clear difference in the rate of alteration between the plagioclases and the K-feldspars. This implies that a large part of the fabric of the rock remains unaltered when the plagioclases are completely weathered, so that the pores caused by the weathering of the plagioclases are filled by their own alteration products. With regard to the influence of the texture, it was observed that fine grained rocks are altered more easily than coarse grained ones, especially if they are rich in biotites. In this study we show various examples in which, even with high levels of alteration, the morphological features of the original plagioclase crystals are preserved: twins, original zoning, myrmekitic textures and perthites. The pseudomorphic transformation of plagioclase is independent of the products to which it gives rise, and here we show examples where gibbsite, 1:1 dioctahedral phyllosilicate, or a mixture of both, appear as secondary minerals. The mineralogical nature of a newly formed product depends mainly on the intensity of the leaching to which the profile is subjected, which is influenced by the rainfall in the area where they are found, as well as the size of existing voids in the material.

Behavior of REE and other trace and major elements during weathering of granitic rocks, Évora, Portugal

The distribution of elements within a weathering profile depends on several factors, including the relative stability of the mineral phases of the parent rock, redox and leaching conditions. Three main processes are involved in weathering: ( 1 ) the breakdown of the parent minerals; (2) the removal in solution of some of the released constituents; and (3) the formation of secondary phases with components from the atmosphere (water, oxygen and carbon dioxide), from the parent rock and from shallower levels, and transported by the percolating solutions (Fritz and Ragland, 1980; Nesbitt, 1980; Chesworth et al., 1981 ). The purpose of this work is to investigate how major and trace elements behave during the weathering of granitic rocks from l~vora, Portugal. The prevailing climatic characteristics of this region are: 700 mm annual rainfall; dry summers; and a mean annual temperature of 16°C.

Chemical processes affecting the mobility of major, minor and trace elements during weathering of granitic rocks

The behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some Portuguese granitic rocks. Element mobilities are calculated from geochemical data normalized with respect to Ti in the fresh parent rock. Chemical elements are divided into two groups, immobile and mobile, on the basis of their geochemical distribution during weathering. Elements that are immobile during weathering are Zr, Hf, Fe, Al, Th, Nb, Sc and the REE. Very mobile are Ca, Na, P, K, Sr, Ba, Rb, Mg and Si. Mobile elements are derived mainly from leachable minerals such as feldspars, micas and apatites, whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals. The geochemical behaviour of Mn, Cr, V, Fe and Ce is very dependent on redox conditions. Redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite. The REE are mobilized or fractionated during late stages of weathering, but not during moderate stages of weathering. This fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing REE.

The effect of oxalate on the dissolution rates of oligoclase and tremolite

The effect of oxalate, a strong chelator for Al and other cations, on the dissolution rates of oligoclase feldspar and tremolite amphibole was investigated in a flow-through reactor at 22°C. Oxalate at concentrations of 0.5 and 1 mM has essentially no effect on the dissolution rate of tremolite, nor on the steady-state rate of release of Si from oligoclase. The fact that oxalate has no effect on dissolution rate suggests that detachment of Si rather than Al or Mg is the rate-limiting step. At pH 4 and 9, oxalate has no effect on the steady-state rate of release of Al, and dissolution is congruent. At pH 5 and 7, oligoclase dissolution is congruent in the presence of oxalate, but in the absence of oxalate Al is preferentially retained in the solid relative to Si. Large transient “spikes” of Al or Si are observed when oxalate is added to or removed from the system. The cause of the spikes is unknown; we suggest adsorption on feldspar surfaces away from sites of active dissolution as a possibility. Solutions in the reactors are undersaturated with respect to both gibbsite and kaolinite, so neither the spikes nor the incongruent dissolution can be explained by formation of a secondary precipitate. The rate of dissolution of tremolite is independent of pH over the pH range 2–5, and decreases at higher pH. The rate of dissolution of oligoclase in our experiments was independent of pH over the pH range 4–9. Since the dissolution rate of these minerals is independent of pH and organic ligand concentration, the effect of acid deposition from the atmosphere on the rate of supply of cations from weathering of granitic rocks should be minor.

マサ地盤の圧縮性に関する研究 : 第 1 報小載荷板による載荷試験

The purpose of this paper is to investigate the characteristics of the compressibility of Masa-do derived from weathering of granitic rocks. The compressibility is closely related to weathering and indicated by the coefficient of subgrade reaction K, for instance the K-value of highly-weathered Masa-do is less than that of the less-weathered. This paper has dealt with the relation between the settlement and the load on the horizontal surface (5.48kgf/cm^2 maximum load by the loading plate of 4.82 cm in diameter). The results of the tests are summarized as follows : (1) After load increment, immediate settlement comes to an end within 15 seconds, and creep settlement can approximate a straight line after that for 15 minutes' period of logarithmic time in every load step. (2) Load-immediate settlement curve is concave and load-creep settlement curve convex; the total load-settlement curve can approximate a straight line, from which K-value is calculated. (3) The relationship between N_<sw>-value and K-value makes it possible to estimate roughly the settlement from N_<sw>-value. (4) The value of creep rate parameter α is approximately proportional to the load q. And the correlation between its proportional constant θ (=α/q) and K-value is indicated by the equation (4). (5) The value of viscosity parameter β can be estimated from equation (10). It indicates that the ratio of immediate settlement to creep settlement is related to the load and its increment.

Shear Strength of Undisturbed Sample of Decomposed Granite Soil

ABSTRACT The residual soils (called Masado in Japan) derived from weathering of granitic rocks are tested to make their shear strength clear by the improved direct shear tests on their undisturbed samples coated by hard gypsum plaster. The shear strength of Masado is closely related to two weathering indices; i. e., porosity and ignition loss. The apparent cohesions and the internal friction angles determined by the direct shear tests on small size samples under constant volum eand constant normal stress are well in accordance with those determined by the drained triaxial compression tests on large size samples (Shono, et ah, 1975). When concerned especially with clayey and silty Masados, the failure envelope derived from the direct shear tests under constant volume and constant normal stress is never straight. This is due to the reason that breaking of weathered particles occurs during the shear failure under relatively small normal stress less than 6kg/cm2.

Geochemical and mineralogical studies on the weathering of granitic rocks

Chemical, mineralogical, and autoradiographic techniques applied to the study of five weathering profiles developed on granitic rocks of Oklahoma and Georgia indicate that mineral stabilities generally follow the expected sequence: plagioclase, biotite, K-spar, quartz, from least to most stable respectively. Kaolinite is the dominant clay mineral in the Georgia profile; both illite and kaolinite are major constituents in the Oklahoma profiles. Stability diagrams indicate that the Georgia surface waters are in equilibrium with kaolinite and the Oklahoma surface waters are in equilibrium with kaolinite and montmorillonite, but not illite. The largest physical and chemical changes occur in the transition from the C- to the B-horizon. Ca and Na are mobilized in the early stages of weathering, whereas K and Rb are concentrated in the relatively stable K-spar

Weathering of granitic rocks of Georgia

Introduction† For the past three years the writer has been engaged in a careful detailed study of the granitic rocks of Georgia, giving special attention to the phenomena of their weathering. The natural outcrops have been carefully studied in the field, and specimens of the fresh and weathered rock, representing various stages in the decay, have been collected from the most typical and widely separated localities in the state and analyzed in the chemical laboratory of the State survey by the writer. The material is sufficiently representative and the work extensive and detailed enough to arrive at definite conclusions regarding the changes involved in the transition from fresh to decayed rock in the granitic group of rocks. Based primarily on texture and structure, three types of the granitic rocks are distinguished: (1) The massive even-granular granites; (2) the porphyritic granites, and (3) the banded or foliated granites—gneisses. Laboratory study . . .