Leaching Of Mg Research Articles

Mass fluxes and clay mineral formation in soils developed on slope deposits of the Kowarski Grzbiet (Karkonosze Mountains, Czech Republic/Poland)

Weathering, mineral formation, and transformation processes along slopes are complex. In cool mountainous regions, undisturbed soil development with a strong vertical leaching element may abruptly change as a result of erosion, accumulation, lateral water fluxes and aeolian input. We investigated soils in the eastern Karkonosze Mountains that have developed on silicatic slope deposits. To date, illite, vermiculite and chlorite are the minerals that have been detected in the clay fraction. Although the climate and parent material should be favourable for the formation of smectites, expandable phases were not verified so far. We investigated if expandable phases could be detected and how they related to elemental fluxes along a short slope sequence (1142–1268ma.s.l. on the border between the Czech Republic and Poland). Mass balance calculations indicated intensive mineral weathering together with a significant leaching of Mg, Al, Ca and Mn on the shoulder and foot slope positions. In the middle zone, which has a concave or undulating surface shape, however, the mass balances of several elements (Na, K, Al, P) revealed a less pronounced leaching (corresponding to a lower degree of podsolization) and in some cases even accumulation. At all sites, mass balance calculations and detected soil minerals (e.g. the increase in illite towards the surface together with an increase in Al and K) indicate some aeolian input. Kaolinite was detected in all soil horizons. Its concentration slightly increased towards the soil surface. Together with the pronounced leaching of Ca, part of kaolinite originates from plagioclase weathering. Besides being a weathering product of primary minerals, part of the kaolinite is inherited from the parent material and probably is also due to aeolian input. In all soils, illite was being transformed into vermiculite and smectites (through regularly-interstratified illite-smectite phases). In addition, the content of chloritic components which increases with depth indicated their concurrent weathering and transformation into smectites. Amphibole also may have acted as a source of smectites. Not all smectite is being actively formed in the soil. Most likely due to slope processes (cover beds) that affected even the subsoil, some smectite has been transferred along the slope. Part of the smectite also seems to derive from the parent material. Active formation of expandable clay minerals is related to convex and planar landscape forms. This relationship suggests intense element leaching, inheritance from the parent material and cover bed mixing processes have contributed to the presence of smectite. Along the slope, zones with predominant vertical transport (shoulder, foot slopes) may repeatedly be interchanged with zones dominated by lateral transport (undulating slope, concave forms).

Dolomitization in the Carbonate Rocks of the Upper Turonian Wata Formation, West Sinai, NE Egypt: Petrographic and Geochemical Constraints

This study discusses the mechanism of dolomitization of carbonate rocks of the Upper Turonian Wata Formation in west Sinai, Egypt. It has been done in terms of study of textural, mineralogical, and geochemical characteristics of the dolostones and dolomitic limestones of the Wata Formation. The Wata Formation is composed mainly of dolostone and dolomitic limestone intercalated with few shale and sandstone beds. Beside the studied dolostone facies, five associated limestone microfacies have been identified. The limestone microfacies also provide an evidence of sparse dolomitization.Dolomites occur as replacive dolomites with minor dolomite cements. Four textural types of dolomite are distinguished: (1) fine-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite; (2) medium-to coarse-crystalline, planar-e (euhedral, idiotopic) replacive dolomite; (3) medium-crystalline, planar-s (subhedral, hypidiotopic) replacive dolomite, and (4) fine-crystalline, planar-e (euhedral, idiotopic) void-filling dolomite cements. The recorded dolomite is nearly non-stoichiometric (CaCO3 ranges from 51 to 56 mol % with an average of 53.5 mol %) and has similar geochemical features. It has δ18OVPDB values range from −7.16‰ to +0.26‰ and δ13C VPDB values vary between −0.52‰ and +5.2‰. Its strontium content lies between 11 and 53 ppm.Petrographic investigations and geochemical data indicated that the dolomitization of the studied carbonates probably took place in a meteoric water-sea water mixing zone at shallow burial depth during the early stage of diagenesis. In this situation, the dolomitization was developed through an increase of Mg/Ca ratio of the pore water, with no salinity increase. The increase of Mg/Ca ratio took place by two possible ways: (1) through leaching of Mg from high Mg-calcite grains (micrite) and aragonitic shells in the studied limestone (2) through leaching of Mg adsorbed on clays of the adjacent mudrock horizons. Such leaching brought about by freshwater-dominated fluid during meteoric water diagenesis.

Ordered mesoporous hematite promoted by magnesium selective leaching as a highly efficient heterogeneous Fenton-like catalyst

Selective leaching of Mg in Mg modified mesoporous hematite promotes highly efficient wet peroxide oxidation of methylene blue.

Pyrolysis of Spent Biomass from Mallee Leaf Steam Distillation: Biochar Properties and Recycling of Inherent Inorganic Nutrients

A set of 24 biochars were produced from the slow and fast pyrolysis (at 400–700 °C) of a mallee leaf and two spent leaves collected after distilling the leaf in steam for 30 and 60 min. Under the same experimental conditions, the pyrolysis of the raw and spent leaves produced biochars of similar proximate and elemental analyses at similar char yields. However, the biochars from the raw leaf generally have higher contents of Na, K, Mg, Ca, and Cl than those from the spent leaves. The biochars were then leached via water to evaluate the recycling of inherent inorganic nutrients (i.e., Na, K, Mg, Ca, and Cl) and the leaching of organic carbon (C) in these biochars. On the basis of those in the mallee leaf, the quantities of Na, K, and Cl leached out by water from the biochars of the mallee leaf are considerably higher than those from the biochars of the spent leaves. However, the leaching of Mg in the slow-pyrolysis biochars follows an opposite trend. At pyrolysis temperatures higher than 400 °C, there are c...

The study of soil acidification of paddy field influenced by acid mine drainage

The acidification of paddy fields was studied in Guizhou Province, China. Affected by acid mine drainage, the pH value of irrigation water was 2.9 with the concentrations of iron and aluminium above 40 mg/L. Based on the pH(H2O) of topsoil, the paddy fields studied were classified spatially into three zones, the natural zone (pH value from 6.2 to 5.5), the acidified zone (pH value from 5.5 to 4.5), and the seriously acidified zone (pH value from 4.5 to 3.2), respectively. Comparing to the natural zone, the buffering processes for acidification of paddy soil were discussed by considering the changes of calcium, magnesium, potassium and aluminium in soils. The Ca, Mg and K were leached from the soil by the decomposition of carbonate and kaolinite. The leaching of Mg became less with the enrichment of iron in topsoil layer. When the soil pH was below 5.0, aluminium was leached from soil because of the dissolution of alumino silicate minerals. In addition, the hydrolysis of iron and aluminium in soil provided more protons to promote the soil acidification. Furthermore, the buffer capacity of paddy soil was discussed by the results of buffer experiment, based on which the pH buffer curve was drawn and the empirical formula for calculating the acidification rate was developed. Because pH buffer capacity of soil is about 2.78 cmolc/kg pH for the pH(H2O) value above 5.0, it is estimated that only another 50 years are needed for the pH(H2O) of the paddy soil decrease to 3.5 in the acidified zone if the acid water is used for irrigation continuously.

Evidence for weathering on early Mars from a comparison with terrestrial weathering profiles

Phyllosilicates on Mars are widespread in the ancient crust suggesting the presence of liquid water at the martian surface and therefore warmer conditions during its early history. However, the role of the ancient climate in the alteration process, which produced these phyllosilicates, remains under debate, because similar mineral assemblages can be produced by hydrothermal alteration at depth. This paper focuses on the origin of coincident outcrops of Fe/Mg bearing phyllosilicates and Al-bearing phyllosilicates, which are observed in several regions of Mars. We performed a detailed mineralogical comparison between a section in Nili Fossae, Mars, and a weathering profile located at Murrin Murrin, Western Australia. The Murrin Murrin profile is developed in Archaean serpentinized peridotite massifs over a ∼40 m thick sequence. It has three alteration zones: the serpentine mineral saprolite is found at the bottom, immediately overlain by Fe/Mg-bearing smectites and then Al-bearing phyllosilicates (kaolinite) mixed with iron hydroxides. This example illustrates how Al-dominated minerals can derive from the alteration of initially Al-poor ultramafic rocks by the intense leaching of Mg 2+. This mineralogical sequence is very similar to that detected locally in Nili Fossae by orbital spectroscopy. By analogy, we propose that the mineral assemblage detected on Mars is the result of long-term weathering, and thus could be the best evidence of past weathering as a direct result of a climate significantly warmer than at present.

Water pollution in relation to mineral exploration: a case study from Alayi-Ovim area of Southeastern Nigeria

Water samples from rivers, streams, springs, and shallow wells in Alayi-Ovim area of southeast Nigeria have been analyzed for Pb, Ca, Mg, Fe, Mg, PO(4), NO(3), CO(3), SO(4), Cl, and pH. The analyses were carried out using atomic absorption spectrometer and Hach Direct Reading Equipment. Results of the analyses from the area conform to the WHO (1995) standards for drinking water. However, the results show relative enrichment of Ca, pH, Mg, CO(3), and Cl. Low values were obtained for Fe, SO(4), and NO(3). While the Cl and Pb enrichment in the area north of Alayi-Ovim axis is attributed to proximity to the lead-zinc and chloride-rich formations of the Turonian Eze-Aku and the Albian Asu River; the Ca, Mg, SO(4), and CO(3) enrichment in Southern part of Alayi-Ovim is due to the limestone-bearing Late Maastrichtian Nsukka Formation. Furthermore, the very low values of less than 5ppm for these characters in water in the central region correlate well with the relatively clean Maastrichtian quartz arenite Ajali Sandstone Formation. The Pb-Zn and Cl incursions into the water system from the Older Albian Asu River/Turonian Eze-Aku Formations in the northern part of Alayi-Ovim area and the leaching of Mg, and Ca into the water system in the Maastrichtian limestone area in the south thus constitute geochemical indices for chemical pollution and mineral exploration for brine and dolomitic limestone in the area.

Surface application of gypsum in low acidic Oxisol under no-till cropping system

The conditions in which a favorable response to a gypsum application can be expected on crop yields are not clear. A 3-year field trial was carried out to evaluate the effects of gypsum application on soil chemical attributes and nutrition and yield of corn (Zea mays L.) and soybean (Glycine max L. Merrill) on a clayey Typic Hapludox of high fertility and low acidity under no-till in Guarapuava, Parana State, Brazil. Treatments were arranged in a randomized complete block design with four replications, and consisted of gypsum application on the soil surface at 4, 8, and 12 Mg ha-1. Gypsum application increased the P content in the soil most superficial layer (0.0 - 0.1 m) and also the exchangeable Ca and S-SO4(2-) contents and the Ca/Mg ratio in the soil profile (0.0 - 0.6 m). Gypsum also caused leaching of Mg and K exchangeable in the soil. An increase in Ca concentrations in the corn leaves, and in P and S concentrations in the corn and soybean leaves occurred following the gypsum application. A yield response of corn to initial application of gypsum was found, but subsequent soybean crops did not respond. Gypsum application proved to be an effective practice to maximize no-till corn grain yield.

Two mathematical models of Mg and Sr partitioning into solution during incongruent calcite dissolution: Implications for dripwater and speleothem studies

Positive correlation between Sr/Ca and Mg/Ca in cave dripwaters and speleothem is often attributed to ‘prior calcite precipitation’ (PCP), where exclusion of Mg and Sr from calcite precipitated upstream of the dripwater/speleothem site results in enrichment in the Mg/Ca and Sr/Ca ratios of residual effluent. Modelling predicts that PCP will produce a straight line correlation in graphs of ln(Sr/Ca) vs. ln(Mg/Ca) with a theoretical slope that is given by ( Kd Sr − 1) / ( Kd Mg − 1) (calculated to be ∼ 0.88 ± 0.13) regardless of rock or absolute dripwater composition ( Sinclair et al., submitted for publication). While this makes the correlation slope potentially diagnostic for PCP, Mg and Sr are also preferentially released from calcite during incongruent calcite dissolution (ICD), and it is possible that ICD produces a similar correlation between ln(Sr/Ca) and ln(Mg/Ca). To examine this possibility, mathematical formulations for two different mechanisms of ICD are presented. Model 1 simulates net dissolution as a dynamic kinetic process where forward and reverse reactions compete: dissolution is congruent but ions are fractionated by a simultaneous reverse reaction where new calcite forms with Sr/Ca and Mg/Ca ratios that are determined by the solution composition and Kd Sr and Kd Mg. Model 2 simulates leaching of Mg and Sr from a fresh calcite surface by tracking the dissolution of a thin layer where high ion mobility allows the calcite to maintain a homogenous composition controlled by Kd Sr and Kd Mg. In both models, generalised kinetic equations are used in the derivations meaning that the results are not specific to a particular kinetic model. In both models of ICD, the mathematics predict that under conditions that may prevail during dissolution of limestone, the correlation between ln(Sr/Ca) and ln(Mg/Ca) is approximately linear with a slope given by ( Kd Sr − 1) / ( Kd Mg − 1). Thus, this property is not specific to just PCP. However, for both PCP and ICD, this slope is universal—expected to occur regardless of kinetics and the absolute composition of solution or host rock. A correlation slope of ∼ 0.88 in either karst waters or speleothem is therefore diagnostic for calcite–water interaction.

Effect of mineralogy and pedoclimatic variations on Ni and Cr distribution in serpentine soils under temperate climate

Part of the soils occurring in the serpentinite Szklary Massif (SW Poland) have developed in Quaternary under temperate climate on the parent rock not affected by chemical weathering. In studied parent ultrabasite, Ni occurs in Cr-magnetite (up to 0.96 wt.% of NiO), forsterite (up to 0.55 wt.% of NiO), iddingsite (up to 0.99 wt.% of NiO), and serpentine (up to 0.55 wt.% of NiO), while Cr is bounded with Cr-magnetites (up to 21.77 wt.% of Cr 2O 3) and clinochlores (up to 1.68 wt.% of Cr 2O 3). Cr-magnetites are highly resistant to weathering while other primary minerals, more susceptible to weathering release parts of Ni and Cr which are trapped in authigenic phases. The latter, however, contain less Ni and Cr than corresponding primary minerals: clayous matrix 0.37 wt.% of NiO and 0.08 wt.% of Cr 2O 3, smectites 0.30 wt.% of NiO and 0.07 wt.% of Cr 2O 3 and mixture of oxy-hydroxides and clay minerals 0.36 wt.% of NiO and 0.14 wt.% of Cr 2O 3. At the mineral scale, smectite plays an important role in the trapping of Ni. At the profile scale, Ni and Cr are moderately diluted upwards the soil profile and Cr is generally less mobile than Ni. The soil formed in Szklary is chemically different than the analogous serpentine soils occurring more to the south in Europe (French Massif Central, Vosges Mountains and Ligurian Apennines, NW Italy). The difference is due to participation of fluvioglacial material produced by Quaternary continental glaciacion in Poland. However, similar vertical relative variations of Ni and Cr were found in all compared soils in spite of differences in parent rock composition, allogenic contaminations and degree of soil development. Comparison with other well-drained pedons located in similar uphill positions shows a climatic control of the Cr and Ni vertical distribution in serpentine soils. Humid mountainous variant of temperate climate induces leaching of Mg and subsequent lack in smectite neoformation which lead to the release of Ni. Ni is less mobile in the colder and dryer climate of Poland. Moreover, some primary minerals which are generally considered as not stable under soil conditions (olivine, serpentine and chlorite) are still present even in the A horizon of the studied Polish pedon. Their resistance is due to slower rates of weathering related to the climate but also due to former hydrothermal processes affecting the parent rock fabric.

Preparation of novel porous solids from alumina-pillared fluorine micas by acid-treatment

New porous solids from alumina-pillared fluorine micas (APMs), which were obtained from synthetic Na-tetrasilicic fluorine mica [NaMg 2.5Si 4O 10F 2], were prepared by sulfuric acid-treatment under mild conditions at 25 °C. The products were investigated by XRD, ICP, SEM, TEM and N 2 adsorption–desorption isotherm at 77 K. XRD measurements indicated that the interlayer pillared structure having a large basal spacing collapsed during the early stages of the acid-treatment. ICP analyses indicated that Al 3+ and Mg 2+ ions were leached out from the pillared micas during the acid-treatment. The pore properties of the leached products were found to differ from those of the mother pillared micas: the acid leaching of the pillared micas leads to the formation of mesopores around 3.2 nm in diameter. The correlation between the change in pore properties and cation elution behavior suggests that the mesopore formation results from the leaching of Mg 2+ ions from the octahedral sheet of the pillared micas. The leached products thus obtained retained the flaky morphology of the mother pillared micas. These results show that the mild acid-treatment using APMs provides a novel route for obtaining unique mesopore solids having the large particle sizes of the mother micas.

Modelling water balance and nitrate leaching in temperate Norway spruce and beech forests located on the same soil type with the CoupModel

Two contrasting forest ecosystems located in close proximity to each other were selected for evaluating the importance of tree species and afforestation in relation to the water balance and the quality of the water leaving the forest root zone. Measurements included soil water content and the collection of precipitation, canopy throughfall, stem flow and soil solution on a weekly basis during 15 months (1999–2000). Soil solutions were extracted using suction probes installed at all major horizons within the upper 120 cm of a Norway spruce (N. spruce) stand ( Picea Abies [L.] Karst.) and a European beech stand ( Fagus Sylvatica L.) located on the same soil type. Soil solutions were analyzed for the content of all major ions, including nitrate. A water balance model (CoupModel) was used to estimate percolation rates beneath the root zone. Percolation at the beech stand was 292 mm and only 41 mm at the N. spruce stand mainly due to differences in the interception loss. The highest annual leaching of Mg, K, Na, Al, Cl, SO 4-S was noted in the N. spruce stand while leaching of NO 3-N was highest in the beech stand, corresponding to 39 kg ha −1 year −1. By contrast, the annual leaching of NO 3-N in the N. spruce stand was only 0.5 kg ha −1 year −1. The larger amount of NO 3-N was leaving the beech forest soil despite the fact that the N. spruce stand had the highest atmospheric N-deposition. Thus, differences in NO 3-N leaching between the stands must be related to differences in uptake and accumulation of N in the vegetation and within the upper 120 cm of the soil. Differences in the water balance and NO 3-N leaching between beech and N. spruce stands call for further attention to the selection of tree-species on a soil type basis when planning future afforestation projects, particularly when such projects aim to improve the quality of water infiltrating to the groundwater zone.

Constraints from geochemistry and Sr–Nd isotopes for the origin of albitite deposits from Central Sardinia (Italy)

Major- and trace-element contents and Sr–Nd isotope ratios were determined in albitite, albitized and unaltered late-Variscan granitoid samples from the world-class Na-feldspar deposits of central Sardinia, Italy. The albite deposit of high economic grade has geological, textural, and chemical features typical of metasomatic alteration affecting the host granitoids. Albitization, locally accompanied by chloritization and epidotization, was characterized by strong leaching of Mg, Fe, K, and geochemically similar trace elements, and by a significant increase of Na. Ca, and P were moderately leached in the most metasomatized rocks. Other major (Si, Ti, Ca) and trace elements (U, Th, Y, and Zr), along with light (LREE) and middle (MREE) rare-earth elements, behaved essentially immobile at the deposit scale. The Nd-isotope ratios (0.512098 to 0.512248) do not provide information on the emplacement age of the unaltered late-Variscan granitoids. On the other hand, their Sr-isotope ratios fit an errorchron of 274±29 Ma (1σ error), in fair agreement with all published ages of Sardinian Variscan granitoids. The very low Rb content of albitized rocks precludes application of the Rb–Sr radiometric system to determine the age of albitization. The Sm–Nd system is not applicable either, because the 143Nd/144Nd ratios of albitized rocks and unaltered granitoids overlap. The overlap confirms that Sm and Nd were substantially immobile during albitization. On the other hand, the measured 87Sr/86Sr ratios of the albitized rocks are appreciably lower than those of the unaltered host granitoids, whereas, their initial Sr-isotope ratios are higher. This seems to suggest that a) albitization was induced by non-magmatic fluids rich in radiogenic Sr, and b) albitization occurred shortly after the granitoid emplacement. This conclusion is supported by Nd isotopes, because unaltered granitoids and albitites fit the same reference isochron at 274 Ma. The fluids acquired radiogenic Sr by circulation through the Lower Paleozoic metasedimentary basement. Specifically, it is estimated that Sr supplied by the non-carbonatic basement represents about 22 wt% of total Sr in albitite.

Downward Movement of Soil Cations in Highly Weathered Soils Caused by Addition of Gypsum

Besides supplying calcium (Ca) and sulfur (S) to plants, gypsum has recently been used in agriculture to ameliorate some soil physical and chemical properties, especially to alleviate aluminum phytotoxicity in subsoils. When applied in large quantities, however, gypsum may leach significant amounts of nutrients from the plow layer. This study was conducted to assess the effect of gypsum addition to the soil on the magnitude of cation leaching as well as the relationship of leaching with some soil properties in a group of seven Brazilian soils. Rates of gypsum equivalents to 0, 5.0, 10, and 20 t ha−1 (0, 2.5, 5.0, and 10 g kg−1) were mixed with triplicate soil samples consisting of 3.0 kg of dry base soil. After 60 days of incubation at room temperature (15–25°C), the experimental units were packed into polyvinyl chloride leaching columns (32‐cm‐high×10 -cm-diameter) at a density of 0.9 g cm−3. Thereafter, they were percolated once a week with a volume of distilled water equivalent to 1.5 times the total soil porosity over 11 weeks. Soil samples were collected before the first and after the last percolation, for chemical analysis. Averaged across soils, 11 percolation events leached about 26% of each Ca, magnesium (Mg), and potassium (K) from the treatment without gypsum. Averaged across soils and rates, addition of gypsum leached 41–94% of added Ca, 13–90% of exchangeable Mg, and 13–58% of exchangeable K, and the highest losses occurred on the sandiest soils. The relationship between soil parameters and Ca leaching varied with gypsum rate: in the treatments that received gypsum, leaching was negatively related to cation exchange capacity (CEC), clay, and organic matter, and positively correlated with sand; in the treatment with no gypsum, leaching correlated with the same parameters above, nevertheless, all coefficients presented opposite signs. Leaching of K caused by gypsum was negatively associated with clay and positively with sand, whereas leaching of Mg was poorly correlated with any soil parameter. Gypsum is a good source to promote high and fast downward movement of Ca in the soil profile, but rates must be cautiously chosen because of excessive leaching of Mg especially on soils with low CEC.

Influence of carbonation and carbonation methods on leaching of metals from mortars

Mortars with varying water-to-cement (w/c) ratio were spiked with heavy metals. After hardening, some samples were carbonated in a CO 2 chamber, others were carbonated using supercritical CO 2 (SCC). Porosity and carbonation depth of treated and untreated samples were compared. Leaching was tested using the diffusion (NEN 7345) and extraction test (based on DIN 38414-S4). The results show that carbonation decreases the porosity of the samples. The decrease is more important with increasing w/c ratio. Carbonation runs deeper into the monolith with increasing w/c ratio. In the supercritical method, not all samples were carbonated to the same extent. The diffusion test shows that carbonation decreases leaching of Na, K, Ca, Ba, Cu, and Pb and increases leaching of Mg. Ni leaching depends on the pH induced by carbonation. Influence of pH and formation of metal carbonates is evaluated with the extraction test and enhances understanding of metal leaching in the diffusion test.

Canopy leaching of cations in Central European forest ecosystems – a regional assessment

AbstractThe leaching of Ca, Mg, and K from canopies is a major pathway of these cations into forest soils. Our aim was to quantify rates of canopy leaching and to identify driving factors at the regional scale using annual fluxes of bulk precipitation and throughfall from 37 coniferous and deciduous forests of North and Central Europe. Total deposition of Ca, Mg, K, and H+ was estimated with Na as an index cation. The median canopy leaching increased in the order: Mg (0.11 kmolc ha–1 a–1) < Ca (0.31 kmolc ha–1 a–1) < K (0.39 kmolc ha–1 a–1). Canopy leaching of Ca and K was positively correlated with the calculated total H+ deposition and H+ buffered in the canopy, whereas canopy leaching of Mg was not. With contrasting effects, fluxes of SO4‐S and NH4‐N in throughfall explained to 64 % (P<0.001) of the Ca canopy leaching. Fluxes of NH4‐N and Ca were negatively correlated, suggesting that buffering of H+ by NH3 deposition reduced canopy leaching of Ca. Amount of bulk precipitation and SO4‐S in throughfall were identified as much weaker driving factors for canopy leaching of K (r2=0.28, P<0.01). Our results show that Ca is the dominant cation in buffering the H+ input in the canopy. At the regional and annual scale, canopy leaching of Mg appears to be unaffected by H+ deposition and H+ buffering in the canopy.

Controls on trace element (Sr–Mg) compositions of carbonate cave waters: implications for speleothem climatic records

At two caves (Clamouse, S France and Ernesto, NE Italy), cave drip and pool waters were collected and sampled at intervals over a 2–3 year period. Mg/Ca and Sr/Ca concentration ratios, corrected for marine aerosols, are compared with those of bedrocks and, in some cases, aqueous leachates of soils and weathered bedrocks. Cave waters do not lie along mixing lines between calcite and dolomite of bedrock carbonate, but typically show enhanced and covarying Mg/Ca and Sr/Ca. Four factors are considered as controlling processes. (1) The much faster dissolution rate of calcite than dolomite allows for the possibility of increase of Mg/Ca if water–rock contact times are increased during drier conditions. A theoretical model is shown to be comparable to experimental leachates. (2) Prior calcite precipitation along a flow path is a powerful mechanism for generating enhanced and covarying Mg/Ca and Sr/Ca ratios. This mechanism requires the solution to lose CO 2 into pores or caverns. (3) Incongruent dolomite dissolution has only limited potential and is best regarded as two separate processes of dolomite dissolution and calcite precipitation. (4) selective leaching of Mg and Sr with respect to Ca is shown to be important in leachates from Ernesto where it appears to be a phenomenon of calcite dissolution. In general selective leaching can occur whenever Ca is sequestered into precipitates due to freezing or drying of soils, or if there is derivation of excess Sr and Mg from non-carbonate species. The Ernesto cave has abundant water supply which in the main chamber is derived from a reservoir with year-round constant P CO 2 of around 10 −2.4 and no evidence of calcite precipitation in the karst above the cave. Two distinct, but overlying trends of enhanced and covarying Mg/Ca and Sr/Ca away from the locus of bedrock compositions are due to calcite precipitation within the cave and, at a variable drip site, due to enhanced selective leaching at slow drip rates. Mg-enhancement in the first chamber is due to a more dolomitic bedrock and longer residence times. The Clamouse site has a less abundant water supply and presents geochemical evidence of prior calcite precipitation, both in the cave and in overlying porous dolomite/dedolomitized limestone bedrock. Initial P CO 2 values as high as 10 −1 are inferred. Experimental incubations of Clamouse soils which generated enhanced P CO 2 and precipitated CaCO 3 had compositions similar to the karst waters. Calcite precipitation is inferred to be enhanced in drier conditions. Hydrological controls on cave water chemistry imply that the trace element chemistry of speleothems may be interpretable in palaeohydrological terms. Drier conditions tends to promote not only longer mean residence times (enhancing dolomite dissolution and hence Mg/Ca), but also enhances degassing and calcite precipitation leading to increased Mg/Ca and Sr/Ca.

Characterization and catalytic properties of a saponite clay modified by acid activation

AbstractA natural saponite was acid activated at room temperature or 90°C with different acid/clay ratios and the products were characterized by powder X-ray diffraction, infrared spectroscopy and thermogravimetry. The leaching of Mg from the octahedral sheets is enhanced by an increase in the acid/clay ratio and by an increase in temperature of activation. Textural properties are reported, and it appears that they are strongly correlated to the presence of a noncrystalline silica phase which is formed during the acid activation process. The desorption of cyclohexylamine indicates that for samples activated at 90°C the number of acid sites in the acidactivated saponites decreases following severe acid treatment. Infrared spectroscopy of adsorbed pyridine on samples after calcination at 500°C suggests that acid activation at 90°C produces a single type of Bronsted site but two types of Lewis sites whereas activation at room temperature results in only one type of Lewis site in addition to a Brønsted site. The two Lewis sites are suggested to originate from residual Al in the clay structure and to AI exsolved from the layers during activation. The dehydration of pentan-1-ol has been used as a further probe to measure acidity by monitoring the degree of conversion and selectivity for the different samples.

Effects of mass transfer, compaction and secondary porosity on hydrothermal upgrading of Paleoproterozoic sedimentary manganese ore in the Kalahari manganese field, South Africa

In the northwestern part of the Kalahari manganese field low-grade carbonate-rich Mamatwan-type ore is altered to high-grade oxide-rich Wessels-type ore in association with normal faults. Mass balance calculations, based on the assumption that manganese was geochemically immobile, suggest that upgrading of the manganese ore can be attributed to leaching of Mg, Ca, CO2 and SiO2 from the sedimentary ore with residual enrichment of Mn. Hydrothermal alteration resulted in development of about 10 to 20% of secondary porosity in the ores and the orebed was compacted to two thirds of its original stratigraphic thickness.

蛇紋岩の風化鉱物について

Weathering products in a brown soil (sample S3) and in a partially weathered antigorite in yellowish brown color (sample A59b) were collected from the serpentinite zone in Sasaguri area, Fukuoka Prefecture, Japan. The products were studied by X-ray diffraction, transmission electron microscopy, analytical electron microscopy, infrared absorption spectrumand differential thermal analysis. Mineralogical composition of the brown soil are 14A intergrade, halloysite, goethite and ferrihydrite associated with Si, and hematite. Si-containing goethite, Si-containing ferrihydrite, hematite and hisingerite were found in the weathered antigorite. The results suggest that leaching of Mg and Si from the serpentine minerals occurred at initial weathering stages. Precipitation of hydrous iron oxides and hydrous iron silicates may occur after the leaching of Mg and Si. Poorly crystallized iron minerals showed aggregate of microcrystalline materials (<100A). The result suggests that the 14A intergrade and halloysite were produced from weathered chlorite and/or mica minerals in the crystalline schist, or weathered chlorite in the vein at the serpentinite zone, or 2 : 1 layer silicates in eolian dust.