Ca-Mg Exchange Research Articles

Solidus and melting of carbonated phlogopite peridotite at 3–6.5 GPa: Implications for mantle metasomatism

It is well known that water significantly lowers mantle solidi. But it turns out this paradigm is not always true. Here, we studied the interaction of K-rich carbonate melts with the natural garnet lherzolite from the Udachnaya kimberlite (Russia) in the presence of water at 3.0–6.5 GPa, corresponding to depths of 100–200 km. We found that at ≤ 1100 °C, the metasomatic interaction consumes garnet, orthopyroxene, and melt to produce phlogopite ± K-richterite + magnesite ± dolomite. Besides, primary clinopyroxene is replaced by one with a lower amount of jadeite component. Thus, the addition of water to the K-rich carbonate melt, infiltrating the subcontinental lithospheric mantle, should yield its partial or complete disappearance accompanied by phlogopitization and carbonation.The studied systems have H2O/K2O = 2, like that in phlogopite, and therefore correspond to carbonated phlogopite peridotite under fluid-absent conditions. At 4.0–6.5 GPa, the solidus of carbonated phlogopite peridotite is controlled by the following reaction: phlogopite + clinopyroxene + magnesite = garnet + orthopyroxene + olivine + hydrous K-carbonatite melt, which is bracketed between 1100 and 1200 °C. At 3 GPa, the solidus temperature decreases to about 1050 °C presumably owing to the Ca-Mg exchange reaction, clinopyroxene + magnesite = orthopyroxene + dolomite, which stabilizes dolomite reacting with phlogopite at a lower temperature than magnesite.Our results suggest that the phlogopite- and carbonate-rich metasomatic vein networks, weakening rigid lithosphere and promoting continental rifting, could be formed as a result of infiltration of hydrous K-carbonatite melt at the base of subcontinental lithospheric mantle. Stretching and thinning of the cratonic lithosphere make geotherms warmer and shifts their intersections with the solidus of carbonated phlogopite peridotite to shallower depths. Consequently, the successive erosion of the continental lithosphere and ascent of the lithosphere-asthenosphere boundary during continental rifting should be accompanied by remelting of phlogopite-carbonate metasomes, upward percolation of K-rich melt, and its solidification at the front of the magmatic-metasomatic mantle system.

Metasomatic interaction of the eutectic Na- and K-bearing carbonate melts with natural garnet lherzolite at 6 GPa and 1100–1200°C: Toward carbonatite melt composition in SCLM

The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed.We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K‑carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2–0.3 wt%), and higher K2O (0.5–1.0 wt%), whereas the Na‑carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3∙60Ca0.5Mg0.4Fe0.1CO2 + 0.6–1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100∙Ca/(Ca + Mg + Fe). We have also found that alkali-poor (≤ 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures.Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100–1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3∙70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40–45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ≤ 30–40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM.Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30–40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40–60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.

Kinetics of Calcite Dissolution and Ca-Mg Ion Exchange on the Surfaces of North Sea Chalk Powders.

Calcite dissolution and Ca–Mg ion exchange on carbonate rock surfaces have been proposed as potential mechanisms occurring during smart waterflooding in carbonate reservoirs. However, there is still a lack of fundamental understanding of these reactions to quantitatively evaluate their effects in the reservoir flooding process. Especially, the data on precipitation and dissolution kinetics are insufficient. In this work, the equilibration kinetics of calcite dissolution and Ca–Mg exchange was experimentally studied. The behavior of three powders was compared: pure calcium carbonate, Stevns Klint outcrop chalk, and North Sea reservoir chalk. It was found that the equilibration time for calcite dissolution was of the order of seconds for a given surface-area-to-liquid-volume ratio. The existing theory of calcite dissolution could well reproduce our observations. The Ca–Mg exchange showed two-step kinetics: the first step was fast, and it dominated the process within the first hour of reaction; the second step was slow, and it continued longer than the time of observation (2 weeks). Characteristic times for the two steps were extracted by fitting the experimental curves. A two-layer adsorption model was proposed to characterize the kinetic process and successfully matched with experimental data. The findings were further extended to flow-through scenarios. By comparing with literature data and surface complexation models, it was concluded that calcite dissolution alone was unlikely to be able to explain the additional recovery reported in the literature. The Ca–Mg exchange process could dominate the fluid–rock interactions at a high temperature in pure calcium carbonate rocks, while competitive adsorption of cations appeared to control the process at a lower temperature. Different carbonate rocks possess different properties with regard to the ion-exchange process.

Thermal history of the Donjek harzburgite massif in ophiolite from Yukon, Canada with implications for the cooling of oceanic mantle lithosphere

We examine the partial melting and the cooling history of a ~5 km section of mantle lithosphere preserved in the Donjek massif, part of a Permian ophiolite in the northern Cordillera of Yukon, Canada. The mantle rocks are depleted spinel harzburgite containing <3% clinopyroxene displaying steep rare-earth element (REE) chondrite-normalized profiles and low (Gd/Yb)n (0.02 to 0.07) compared to most other ophiolites. The REE patterns of clinopyroxene can be modeled as 16–20% partial melts of typical depleted mid-ocean ridge (MOR) mantle. The REE exchange between coexisting ortho- and clinopyroxene preserves temperatures (TREE) of 1150–1360 °C, some of the highest values recorded in ophiolites and abyssal peridotites, and show a positive correlation with CaMg exchange (solvus) temperatures (TBKN) of 900–970 °C. The harzburgite represents lithosphere formed at an initial melting temperature of ~ 1350 °C that cooled at rate of 10−1 to 10−4 °C/year as deduced by TREE values with cation diffusion and grain size data. The TREE temperatures and cooling rates for the Donjek massif show a regular systematic variation with depth from the crust-mantle transition along a trend similar to the Samail ophiolite of Oman, consistent with conductive heat transfer beneath a cool lower crust. High near-solidus temperatures and the cooling rates in the massif were a consequence of rapid obduction against oceanic crust along either a transform or low angle detachment soon after melt extraction. Final emplacement of the ophiolite as klippen on underlying continental crust occurred ~ 40 m.y. later.

Contrasting thermal and melting histories for segments of mantle lithosphere in the Nahlin ophiolite, British Columbia, Canada

The Permo-Triassic Nahlin ophiolite is the largest and best-preserved ophiolite in the Canadian Cordillera of British Columbia and Yukon, Canada. The ophiolite is well-exposed along its ~ 150 km length with mantle segments divisible into the Hardluck and Menatatuline massifs. Both massifs comprise mostly depleted spinel harzburgite (< 2 wt% Al2O3 and ~ 45 wt% MgO). Chondrite normalized REE abundances in clinopyroxene vary in (Gd/Yb)N from 0.2 to 1.1. Inversion modelling of clinopyroxene REE abundances requires 10–16% and 16–20% partial melting in the Hardluck and Menatatuline massifs, respectively. The two-pyroxene and Fe–Mg exchange temperatures in the mantle of the ophiolite also change systematically along strike with the degree of partial melt depletion. The temperatures recorded by REE and Ca-Mg exchange between coexisting pyroxenes require markedly higher peak temperatures and cooling rates for the Menatatuline massif (1250 °C, 0.1–0.01 °C/year) compared to the Hardluck massif (< 1100 °C, ~ 10− 4 °C/year). The differences between these two contiguous massifs can be reconciled by their evolution as two separate segments along a ridge system having varying melt depletion, with contrasting cooling rates controlled by presence or absence of a crustal section above the mantle lithosphere, or by rapid exhumation along a detachment.

Merwinite in diamond from Sao Luiz, Brazil: A new mineral of the Ca-rich mantle environment

Diamonds from Juina province, Brazil, and some others localities reveal the existence of a deep, Ca-rich carbonate-silicate source different from ultramafic and eclogite compositions. In this study, we describe the first observation of merwinite (Ca 2.85 Mg 0.96 Fe 0.11 Si 2.04 O 8 ) in a diamond; it occurs as an inclusion in the central growth domain of a diamond from the Sao Luiz river alluvial deposits (Juina, Brazil). In addition, the diamond contains inclusions of walstromite-structured CaSiO 3 in the core and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine in the rim. According to available experimental data, under mantle conditions, merwinite can only be formed in a specific Ca-rich and Mg- and Si-depleted enviroment that differs from any known mantle lithology (peridotitic or eclogitic). We suggest that such chemical conditions can occur during the interaction of subduction-derived calcium carbonatite melt with peridotitic mantle. The partial reduction of the melt could cause the simultaneous crystallization of Ca-rich silicates (CaSiO 3 and merwinite) and diamond at an early stage, and (Mg 0.86 Fe 0.14 ) 2 SiO 4 olivine and diamond at a later stage, after the Ca-Mg exchange between carbonatite melt and peridotite has ceased. This scenario is supported by the presence of calcite microinclusions within merwinite.

The formation of Ca-Cl-rich groundwaters in the Dry Valleys of Antarctica: Field measurements and modeling of reactive transport

Ca-Cl-rich brines have been found in shallow subsurface flows, groundwater systems, lakes, and ponds throughout the Dry Valleys of Antarctica. The apparent abundance of Ca-Cl-rich waters near the surface is unusual compared to global surface water compositions and a number of theories have been proposed to explain the genesis of these brines. We show that an ice-cemented soil developing on fluvial sediment in Taylor Valley also contains Ca-Cl-rich brine. The distribution of soluble ions, exchangeable cations, and stable isotopes down to 2.1m depth in the soil suggests that CaCl2 was formed by cation exchange reactions during downward reactive transport of Na-Cl-rich brine from the soil surface. To explore the implications of exchange reactions for the formation of Ca-Cl-rich brine, Ca-Na and Ca-Mg exchange properties were measured in 1mM, 0.1M, and 4.75M solutions. Low-temperature reactions and brine transport were modeled in PHREEQC by incorporating FREZCHEM Pitzer parameters and solubility products into PHREEQC. Modeling shows that by freezing soils in equilibrium with Dry Valley surface waters, a strong Ca-Mg enrichment of the soil solution is caused by the exchange of aqueous Na+ with exchangeable Ca2+ and Mg2+. Ca-Mg enrichment also occurs as Na-Cl-rich brine from the soil surface advects into ice-cemented soil. By modeling this process in the borehole soil, trends in ion distributions with depth can be predicted. Brine compositions from cation exchange reactions are consistent with Ca-Cl-rich brine compositions in the Dry Valleys, although additional water–rock interaction is proposed to account for the low Mg2+ concentrations in Don Juan Pond. Furthermore, the amount of CaCl2 that can be produced by exchange reactions is consistent with estimated amounts of CaCl2 in groundwaters beneath Don Juan Pond. This suggests that cation exchange reactions can explain the Ca-Cl-rich composition of the enigmatic Don Juan Pond and other brines in the Dry Valleys.

Hydrochemical-isotopic tendencies to define hydraulic mobility of formation water at the Samaria-Sitio Grande oil field, Mexico

The chemical (major elements) and isotopic ( 2H, 3H, 13C, 18O) composition of formation water is presented for the Sitio Grande oilfield, SE-Mexico, extracted from 28 production wells from the carbonate reservoir at a depth between 3585 and 4545 m.b.s.l. The linear δ 18O/δD-trend explains the formation of reservoir water as part of three subsequent stages: a) the evaporation of marine water at the surface, causing enrichment of both, δ 18O and δD-values, b) a reversal trend with decreasing δ 18O-and δD-ratios by the extreme evaporation of brines and, c) the subsequent mixing with isotopically depleted meteoric water. A SW-NE directed flow direction of deep groundwater systems is indicated by parallel-oriented isoline trends of stable isotope ratios and conservative elements, supported by the dominance of parallel directed microfractures and extensional faults, and by tracer test results. The arrival of artificial tritium, three years after tracer injection in the well SG-85, reflects a) the importance of long-term monitoring of tracer tests, and b) an estimated flow velocity of 2.2×10 −5 m/s. As the arrival of the tracer was detected exclusively in three production wells towards the SW of the injection site, lateral migration of groundwater occurs mainly along defined, channel-like conducts with a narrow dispersion angle of 39 °. Surface water, injected as part of an enhanced oil recovery program, was not detected in the production zone by chemical and isotopic methods. This fact is not a proof for lacking hydraulic conductivity, as the large reservoir size and the termination of the injection program in 1996 could have caused a complete dilution of the injected fluids. Restricted vertical flow movement between reservoir layers and insignificant temperature-dependent reaction processes is indicated by the heterogeneous distribution of δ 18O-and Cl-values throughout the groundwater column of most reservoir zones. Little influence of water-rock interaction, such as Ca-Mg exchange processes between formation water and host rock is reflected by the heterogeneous abundance of calcium in the liquid phase in both, dolomite-free and dolomitized carbonate host rock layers.

Free bonding energy between cations of Entisols, Inceptisols and Alfisols of India

One hundred twenty surface soil samples have been collected from two locations, viz. districts of Surguja and Raigarh, representing soils of order, Entisols, Inceptisols and Alfisols, to evaluate the free bonding energy (ΔG r ) between cations of the corresponding soils of North-Eastern part of Central India. The cation exchange capacity is higher in the Alfisols of both the sites, followed by the Inceptisols and the Entisols studied. The free bonding energy between K and Ca in Entisols and Inceptisols is found positive in both the areas showing ready availability of K over Ca and Na. In general, the ΔG r values for Na-Ca exchange in Entisols of both the study areas are found to be positive, which indicates that these soils also release more Na + than Ca 2 + . Inceptisols and Alfisols of both the areas are characterized by the negative values of free bonding energy for Na-Ca and Ca-Mg exchange, which indicates that these soils release more Ca 2 + compared to Na + , while the negative ΔG r values for Ca-Mg exchange suggest that these soils prefer Ca 2 + , thereby releasing Mg 2 + ion under favorable conditions. Due to the more Μg 2 + in the soil solution of Alfisols of the study area, these soils would pose more water-logging problem, if irrigated unjudiciously, since Mg is known to have similar effect like Na to create water-logging problem in the soils.

Zaoyang chondrite cooling history from Fe2+-Mg intracrystalline ordering in pyroxenes

AbstractThe crystal chemistry of clinopyroxene, orthopyroxene and olivine from a crushed fragment of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. The meteoritic pyroxenes have cell and polyhedral volumes which compare well with such data from terrestrial pyroxenes that typically crystallize at low-pressure. Fe2+ and Mg are rather disordered in M1 and M2 sites of clino- and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512°C respectively, which is consistent with a reasonably fast cooling rate, estimated to be of the order of 1°C/day.The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino- and orthopyroxenes is 900°C as calculated from clino- and orthopyroxene intergrowth.The cooling rates obtained from Fe2+-Mg intracrystalline partitioning suggest a cooling of the order of degrees per day at temperatures of 600–500°C due to a strong loss of heat by irradiation.

Garnet-pyroxene equilibria in the system CaO-MgO-Al2O3-SiO2 (CMAS): prospects for simplified (?T-independent?) lherzolite barometry and an eclogite-barometer

New experimental data on compositions of garnets in two-pyroxene — garnet assemblages in the system CaO −MgO −Al2O3 −SiO2 (CMAS) are presented for conditions between 1,100 and 1,570° C and 30 to 50 kb. Garnets in these assemblages become less calcic with increasing pressure. Garnet-orthopyroxene barometry (Al-solubility-barometry) pertinent to geobarometry for garnet lherzolites has been evaluated with a set of experimental data covering the range 900 to 1,570° C and 15 to 100 kb. Various formulations of this barometer work well to 75 kb. Phase equilibria are not sufficient to positively verify the thermodynamic validity of any of such models. Empirical garnet-orthopyroxene barometry at least in the system CMAS can be formulated to obtain a pressure estimate without previous temperature estimation (P(kb)=34.4-19.175 1n X Al M1 +17.702 1n X Ca M2 ). The potential application of an analogous garnetclinopyroxene equilibrium is limited because the amount of Ca-Tschermaks in natural clinopyroxenes is usually quite small in garnet lherzolites and many eclogites. The Ca-Mg exchange between garnet and clinopyroxene appears however sufficiently sensitive to pressure to allow calibration of a CMAS barometer. The reaction 3CaMgSi2O6+Mg3Al2Si3O12=3Mg2Si2O6+Ca3Al2Si3O12 has a ΔV o of 3.5 cm3. The total pressure dependency of this reaction is however closer to a theoretical ΔV o of about 5 cm3 when excess volume properties of the phases involved are taken into account. We have calibrated such a barometer (mean error of estimate 2.8 kb) for assemblages with pyrope-rich (py>80) garnets and orthopyroxenes. This may provide the basis for a geobarometer for eclogites from kimberlites.

Structural changes induced by calcium and magnesium in a high affinity calcium-binding protein from crayfish sarcoplasm.

The sarcoplasmic calcium-binding protein (SCP) from crayfish has in its dimer form two calcium-specific and four Ca-Mg sites. The conformations of this protein in the calcium- (SCP2 *Cas), magnesium- (SCP2 -Mg4), and metal-free forms as well as in intermediary states have been studied by circular dichroism (CD) in the far and near-ultraviolet regions, tryptophan fluorescence, sulfhydryl reactivity, and trypsin susceptibility. SCP appears to contain eight a helical segments (per subunit), each consisting of about 10 amino acid residues. Three of these helices require eaz+ to be formed and are stable even in the presence of 8 M urea provided SCP is saturated by calcium. This is not the case for the remaining five helical segments. One of the three calcium-sensitive helices is unaffected by Mg+, and is formed upon ea2+ binding to the Ca-specific site. In the calcium-saturated SCP, 8 to 9 Phe residues out of 13 (per monomer) are immobilized in a helical segments. Two Phe side chains are present in the Ca-sensitive helix adjacent to the Ca-specific site. Some of the Tyr chromophores are located in the metal-sensitive helices flanking both the calcium-specific and the Ca-Mg sites, contrary to the Trp residues, which are not included in these a helical segments. Two thiols out of three (per monomer) display a marked decrease in reactivity when SCP binds Ca2+ or M8+, and are less accessible to 5,5’-dithiobis(2-nitrobenzoic acid) in the calciumthan in the magnesium-form of the protein. In the presence of physiological levels of Mg+, Ca2+ induces major structural changes (SCPz-Mg, -+ SCPZ*C~~) which involve the calcium-specific sites rather than the Ca-Mg sites. This contrasts sharply with the situation prevailing in vertebrate SCP (parvalbumin) as the latter has no calcium-specific sites and displays very limited conformational changes upon Ca-Mg exchange.

Calcium, magnesium and the conformation of parvalbumin during muscular activity

The conformation of perch parvalbumin in the Ca-, Mg- and metal-free state was studied by intrinsic fluorescence, trypsin susceptibility, thiol titration and circular dichroism. The data reveal that Ca-parvalbumin has a more compact structure than the metal-free protein, with a high α-helical content and a buried thiol. No difference in conformation could be detected between Mg- and Ca-parvalbumin, indicating that the Ca-Mg exchange that may take place during muscular activity is accompanied by little or no structural changes. Furthermore, recently published kinetic parameters can now be interpreted as meaning that, during the contraction-relaxation cycle, parvalbumin often stays in the Mg-form instead of switching to the Ca-form which is predominant in vitro.

The thermodynamics of pyroxene geotherms

Correctly formulated thermodynamics provides the only reliable means of applying the results of mineral equilibria experiments to the calculation of the temperature and pressure of formation of mineral assemblages. The problems of formulating geothermometers and geobarometers are considered with particular reference to calculating the conditions of formation of garnet lherzolites in kimberlites. Incorrect expressions for the thermodynamics of minerals will provide unreliable methods for calculating conditions of formation. The reactions on which these methods are based must be chosen so that composition parameters which are uncertain in a mineral analysis (for example, the amounts of ferrous iron on each octahedral site in a pyroxene) do not have a central position in a method. For this reason it is suggested that the use of alumina solubility in enstatite in equilibrium with garnet is not suitable as a geobarometer for garnet lherzolites. The orthopyroxene-Ca-rich clinopyroxene miscibility gap geothermometer is shown to be very sensitive to the mineral compositions when more correctly formulated, treating the pyroxenes as non-ideal solutions. Two new methods are presented which are based on Ca-Mg exchange reactions involving orthopyroxene-Ca-rich clinopyroxene and orthopyroxene-garnet. These methods allow the calculation of both temperature and pressure for garnet lherzolites. It is observed that the sheared lherzolites and the granular lherzolites formed at lower pressures than has been suggested previously, implying a steeper geotherm at the time the minerals attained their present compositions.

The role of magnesium in the crystal growth of calcite and aragonite from sea water

The seeded precipitation (crystal growth) of aragonite and calcite from sea water, magnesium-depleted sea water, and magnesium-free sea water has been studied by means of the steady-state disequilibrium initial rate method. Dissolved magnesium at sea water levels appears to have no effect on the rate of crystal growth of aragonite, but a strong retarding effect on that of calcite. By contrast, at levels less than about 5 per cent of the sea water level, Mg has little or no effect on calcite growth. Extended crystal growth on pure calcite seeds in sea water of normal Mg content resulted in the crystallization of magnesium calcite overgrowths, containing 7–10 mole % MgCO 3 in solid solution. This suggests that the rate inhibition by Mg is due to its incorporation within the calcite crystal structure during growth, which causes the resulting magnesian calcite to be considerably more soluble than pure calcite. The standard free energy of formation of 8.5 mole% Mg calcite calculated on this assumption is in good agreement with independent estimates of magnesian calcite stability. From the work of Katz ( Geochim. Cosmochim. Acta 37, 1563–1586, 1973), Plummer and Mackenzie ( Amer. J. Sci. 273, 515–522, 1974), and the present paper, it can be predicted that the most stable calcite in Ca-Mg exchange equilibrium with sea water contains between 2 and 7 mole%MgCO 3 in solid solution. Likewise, calcites containing more than 8.5 mole% MgCO 3 are less stable, and those containing less than 8.5 mole% MgCO 3 are more stable than aragonite plus Ca and Mg in sea water.

Selectivity coefficients of ca-mg exchange for three montmorillonitic soils

The selectivity coefficients of Ca—Mg exchange of three montmorillonitic soils are reported. The percent of montmorillonite in the clay fractions of the two grumusolic soils was about 50 and in that of the sandy clay loam hamra, 80. It was found that the selectivity coefficient for each of the soils, at a Mg saturation above 30%, was unity. Below 30% Mg saturation, the two grumusolic soils showed a preference for Mg, and the sandy clay loam hamra a preference for Ca, equal to that of “pure” montmorillonite. By comparing the selectivity coefficients for the three soils with that of “pure” montmorillonite, it was concluded that the exchange properties of the montmorillonite in soils were masked by cementing agents.

Calcium-Magnesium Exchange in Montmorillonite and Vermiculite

AbstractAn experimentally determined Ca-Mg exchange isotherm of montmorillonite is reported. The selectivity coefficient of this exchange over a wide range of Mg saturation was calculated and found constant.Standard free energies of exchange, thermodynamic equilibrium constants and activity coefficients of the exchangeable Ca and Mg ions in vermiculite and montmorillonite, were predicted from knowledge of the microstructure of these two clays, assuming that coulombic forces are the main ones playing a role in the interaction between the counterions and the charged clay surface. The standard free energies of exchange (ΔGCa°Mg = 238 cal/mole) predicted a preference for Ca in montmorillonite and a preference for Mg in vermiculite (ΔGCa°Mg = - 1665 cal/mole). The predicted thermodynamic equilibrium constants were compatible with the experimentally determined selectivity coefficients KsCaMg = 0·67 as compared with KsCaMg = 0·68 in montmorillonite, which remains constant over all the range of Mg saturation, and KCaMg = 16·7 as compared with KsCaMg = 13·9 in vermiculite at 95% Mg saturation. The activity coefficients of Ca and Mg counterions in montmorillonite were found to be and , respectively, and to remain constant. The activity coefficients of exchangeable Ca and Mg in vermiculite were found to be and , respectively, at an equivalent fraction of unity. The activity coefficient of exchangeable Mg increased as the saturation with Mg decreased, and was found to be 1·7 × 10-3 in the range of the low Mg saturation.The microstructure, the isomorphic substitution and the surface charge density provided an understanding of the changes taking place in the activity coefficients of the counterions.