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Dennis K. Bird - One of the best experts on this subject based on the ideXlab platform.
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Experimental determination of thermodynamic properties of ion-exchange in Heulandite: Binary ion-exchange experiments at 55 and 85°C involving Ca2+, Sr2+, Na+, and K+
American Journal of Science, 2004Co-Authors: Thráinn Fridriksson, Philip S. Neuhoff, Brian E. Viani, Dennis K. BirdAbstract:Heulandite is a common rock-forming zeolite that exhibits wide solid solution of extraframework cations, presumably due to ready ion exchange with aqueous solutions. In order to provide a quantitative basis for interpreting and predicting the distribution of aqueous species between Heulandite and aqueous solutions, ion exchange equilibrium between Heulandite and aqueous solutions with respect to the binary cation pairs Ca2+ –K+, Ca2+ –Na+, K+ –Na+, K+ –Sr2+, Na+ –Sr2+, and Ca2+ –Sr2+ was investigated. Homoionic Ca-, K-, and Na-Heulandites prepared from natural Heulandite were equilibrated with 0.1 N Cl− solutions containing various proportions of the cations in a given binary pair at 55 and 85 °C to define isotherms describing partitioning of the cations over a wide range of Heulandite and solution composition with respect to the cations in each pair. In general, the experiments equilibrated rapidly, within 11 to 15 weeks at 55 °C and 3 to 4 weeks at 85 °C. The exception was the Ca2+ –Sr2+ binary exchange, which did not equilibrate even after 3 months at 55 °C and 4 weeks at 85 °C. Slow exchange of Sr2+ for Ca2+ also prohibited preparation of homoionic Sr-Heulandite from the natural (Ca-rich) Heulandite within 10 weeks in 2N SrCl2 solution at 90 °C, although near homoionic Sr-Heulandite was produced by exchange of K- and Na-Heulandite. Experimentally determined isotherms were used to derive equilibrium constants for the ion exchange reactions and asymmetric Margules models describing the extent of non-ideality in extraframework solid solutions in Heulandite. Regressed equilibrium constants for Ca2+-Na+, Ca2+-K+, and K+-Na+ binary cation pairs at 55 °C are internally consistent among each other (complying with the triangle rule), indicating good accuracy of these data. The maximum departure from internal consistency among the equilibrium constants for three binary pairs was 900 J per mole of charge equivalents (eq) for the 55 °C experiments and 2300 J eq−1 for the 85 °C experiments. The applicability of the present experimental results and thermodynamic models was assessed by calculating the composition of Heulandite in Icelandic geothermal systems from known compositions using the regressed thermodynamic properties of Ca2+-Na+ exchange at 85 °C. Calculations predict an average Ca mole fraction [defined as Ca/(Ca+Na)] in Heulandite of 0.74, in excellent agreement with observed compositions of Heulandite from geothermal and metamorphic systems in Iceland (0.75). Thermodynamic data for Heulandite ion-exchange derived in this study can be used to predict partitioning of Ca, K, Na, and Sr between Heulandite and aqueous solutions in geologic systems. Because Heulandite is the most effective sink for Sr in basaltic aquifers that have undergone zeolite facies metamorphism, the experimental results of this study will provide essential data for modeling Sr transport in aquifers in low-grade metabasalts.
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experimental determination of thermodynamic properties of ion exchange in Heulandite binary ion exchange experiments at 55 and 85 c involving ca2 sr2 na and k
American Journal of Science, 2004Co-Authors: Thráinn Fridriksson, Philip S. Neuhoff, Brian E. Viani, Dennis K. BirdAbstract:Heulandite is a common rock-forming zeolite that exhibits wide solid solution of extraframework cations, presumably due to ready ion exchange with aqueous solutions. In order to provide a quantitative basis for interpreting and predicting the distribution of aqueous species between Heulandite and aqueous solutions, ion exchange equilibrium between Heulandite and aqueous solutions with respect to the binary cation pairs Ca2+ –K+, Ca2+ –Na+, K+ –Na+, K+ –Sr2+, Na+ –Sr2+, and Ca2+ –Sr2+ was investigated. Homoionic Ca-, K-, and Na-Heulandites prepared from natural Heulandite were equilibrated with 0.1 N Cl− solutions containing various proportions of the cations in a given binary pair at 55 and 85 °C to define isotherms describing partitioning of the cations over a wide range of Heulandite and solution composition with respect to the cations in each pair. In general, the experiments equilibrated rapidly, within 11 to 15 weeks at 55 °C and 3 to 4 weeks at 85 °C. The exception was the Ca2+ –Sr2+ binary exchange, which did not equilibrate even after 3 months at 55 °C and 4 weeks at 85 °C. Slow exchange of Sr2+ for Ca2+ also prohibited preparation of homoionic Sr-Heulandite from the natural (Ca-rich) Heulandite within 10 weeks in 2N SrCl2 solution at 90 °C, although near homoionic Sr-Heulandite was produced by exchange of K- and Na-Heulandite. Experimentally determined isotherms were used to derive equilibrium constants for the ion exchange reactions and asymmetric Margules models describing the extent of non-ideality in extraframework solid solutions in Heulandite. Regressed equilibrium constants for Ca2+-Na+, Ca2+-K+, and K+-Na+ binary cation pairs at 55 °C are internally consistent among each other (complying with the triangle rule), indicating good accuracy of these data. The maximum departure from internal consistency among the equilibrium constants for three binary pairs was 900 J per mole of charge equivalents (eq) for the 55 °C experiments and 2300 J eq−1 for the 85 °C experiments. The applicability of the present experimental results and thermodynamic models was assessed by calculating the composition of Heulandite in Icelandic geothermal systems from known compositions using the regressed thermodynamic properties of Ca2+-Na+ exchange at 85 °C. Calculations predict an average Ca mole fraction [defined as Ca/(Ca+Na)] in Heulandite of 0.74, in excellent agreement with observed compositions of Heulandite from geothermal and metamorphic systems in Iceland (0.75). Thermodynamic data for Heulandite ion-exchange derived in this study can be used to predict partitioning of Ca, K, Na, and Sr between Heulandite and aqueous solutions in geologic systems. Because Heulandite is the most effective sink for Sr in basaltic aquifers that have undergone zeolite facies metamorphism, the experimental results of this study will provide essential data for modeling Sr transport in aquifers in low-grade metabasalts.
Mark E. Davis - One of the best experts on this subject based on the ideXlab platform.
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Alteration of perlite to calcium zeolites
Microporous Materials, 1997Co-Authors: Shervin Khodabandeh, Mark E. DavisAbstract:Hydrothermal synthesis of calcium zeolites by alteration of perlite (a rhyolitic glass) is presented. Synthetic versions of the zeolites gismondine, epistilbite and Heulandite are synthesized from perlite using calcium-containing solutions, and the effects of the solution phase, pH and temperature on the distribution of the zeolitic products obtained are discussed. Transient formation of a gismondine-type zeolite during transformation of perlite to Heulandite suggests a novel synthesis route for the crystallization of Heulandite-type zeolites from synthetic zeolite P1.
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Synthesis of a Heulandite-type zeolite by hydrothermal conversion of zeolite P1
Chemical Communications, 1996Co-Authors: Shervin Khodabandeh, Mark E. DavisAbstract:A synthetic analogue of the zeolite Heulandite is synthesized by hydrothermal treatment of zeolite P1 with solutions containing CaCl_2 and NaOH.
Boris A. Fursenko - One of the best experts on this subject based on the ideXlab platform.
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Thermochemical study of calcium zeolites—Heulandite and stilbite
American Mineralogist, 2001Co-Authors: Irina A. Kiseleva, Alexandra Navrotsky, I. Belitsky, Boris A. FursenkoAbstract:Calorimetric measurements were made on natural samples of Heulandite having the composition Ca0.86Na0.37K0.06Al2.14Si6.86O18·6.1H2O and stilbite Ca1.01Na0.12Al2.12Si6.88O18·7.27H2O both from the same locality (Nidym River, E. Siberia, Russia). Enthalpies of formation and hydration were studied by calorimetry in lead borate solvent at 975 K. The enthalpies of formation from oxides and elements at 298 K of Heulandite are: −238.7 ± 4.9 kJ/mol, and −10656.3 ± 8.6 kJ/mol, and of stilbite are −232.0 ± 8.3 kJ/mol and −11017.9 ± 10.9 kJ/mol respectively. The integral hydration enthalpies including the enthalpies of phase transitions during Heulandite and stilbite dehydration are −209.4 ± 5.9 kJ/mol and −229.2 ± 7.4 kJ/mol at 298 K, respectively. The molar Gibbs free energies of formation for idealized calcium-sodium and pure calcium Heulandites and stilbites were calculated by combining these new calorimetric data with thermodynamic quantities from the literature. Equilibrium temperatures for the reaction: Ca-stilbite = Ca-Heulandite + H2O, calculated on the basis of these thermodynamic data, agree with experimental phase equilibria.
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thermochemical study of calcium zeolites Heulandite and stilbite
American Mineralogist, 2001Co-Authors: Irina A. Kiseleva, Alexandra Navrotsky, I. Belitsky, Boris A. FursenkoAbstract:Calorimetric measurements were made on natural samples of Heulandite having the composition Ca0.86Na0.37K0.06Al2.14Si6.86O18·6.1H2O and stilbite Ca1.01Na0.12Al2.12Si6.88O18·7.27H2O both from the same locality (Nidym River, E. Siberia, Russia). Enthalpies of formation and hydration were studied by calorimetry in lead borate solvent at 975 K. The enthalpies of formation from oxides and elements at 298 K of Heulandite are: −238.7 ± 4.9 kJ/mol, and −10656.3 ± 8.6 kJ/mol, and of stilbite are −232.0 ± 8.3 kJ/mol and −11017.9 ± 10.9 kJ/mol respectively. The integral hydration enthalpies including the enthalpies of phase transitions during Heulandite and stilbite dehydration are −209.4 ± 5.9 kJ/mol and −229.2 ± 7.4 kJ/mol at 298 K, respectively. The molar Gibbs free energies of formation for idealized calcium-sodium and pure calcium Heulandites and stilbites were calculated by combining these new calorimetric data with thermodynamic quantities from the literature. Equilibrium temperatures for the reaction: Ca-stilbite = Ca-Heulandite + H2O, calculated on the basis of these thermodynamic data, agree with experimental phase equilibria.
Thomas Armbruster - One of the best experts on this subject based on the ideXlab platform.
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Heulandite-Ba, a new zeolite species from Norway
European Journal of Mineralogy, 2005Co-Authors: Alf Olav Larsen, Thomas Armbruster, Fred Steinar Nordrum, Nicola Döbelin, Ole V. Petersen, Muriel ErambertAbstract:Heulandite-Ba, ideally (Ba,Ca,Sr,K,Na) 5 Al 9 Si 27 O 72 ·22H 2 O, is a new zeolite species in the Heulandite series, occurring as an accessory mineral in hydrothermal veins of the Kongsberg silver deposit type at the Northern RavnAs prospect, southern Vinoren, 14 km NNW of Kongsberg town, Kongsberg ore district, Flesberg community, Buskerud county, Norway. The mineral has also been found at the Bratteskjerpet mine, Saggrenda near Kongsberg, and in hydrothermal veins in quartzite at Sjoa in Sel community, Oppland county. Heulandite-Ba occurs as well developed, thick tabular, trapezoidal crystals up to 4 mm across, showing the forms {100}, {010}, {001}, 111} and {201}. The mineral is colourless to white, rarely very pale yellowish white or pale beige, with a white streak; transparent to translucent, with a vitreous lustre, pearly on {010}. The mineral has a perfect {010} cleavage; subconchoidal to uneven fracture. It is non-fluorescent in long- or short-wave ultraviolet light. The Mohs9 hardness is 3½; D meas = 2.35(1) and D calc = 2.350 g/cm 3 . Heulandite-Ba is biaxial positive with n α = 1.5056(5), n β = 1.5064(5) and n γ = 1.5150(5); Δ = 0.0094, n (mean) = 1.5090. 2V γ (calc) = 34.1°, 2V γ (meas) = 38(1)°; distinct dispersion, r > v; α Λ c varying from ≅ 39° to ≅ 51° in obtuse angle β, γ = b. An average of 14 electron microprobe analyses on Heulandite-Ba from the Northern RavnAs prospect, Kongsberg, gave SiO 2 54.26, Al 2 O 3 15.27, MgO 2 O 0.34, K 2 O 0.58, H 2 O 13.1 (from TGA), total 99.99, corresponding to (Ba 2.49 Ca 1.41 Sr 0.30 K 0.37 Na 0.33 ) Σ4.90 Al 8.96 Si 27.00 O 72.00 ·21.75H 2 Oon the basis of 72 framework oxygen atoms. Chemical zoning is frequent, with transitions to Heulandite-Ca and Heulandite-Sr. Heulandite-Ba is monoclinic, C 2/ m , with a = 17.738(3), b = 17.856(2), c = 7.419(1) A, β = 116.55(2)°, V = 2102.0(7) A 3 , Z = 1. The strongest five X-ray diffraction lines of the powder pattern [ d in A( I )( hkl )] are: 2.973(100)(151), 3.978(97)(131), 7.941(66)(200), 4.650(66)(-131), 2.807(65)(-621). The crystal structure refinements (R = 3.5%) of Heulandite-Ba were done in space groups C 2/ m, Cm, C 2, and C 1, but refinements in space groups with lower symmetry than C 2/ m did not improve the structural model.
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Stepwise dehydration and change of framework topology in Cd-exchanged Heulandite
Microporous and Mesoporous Materials, 2003Co-Authors: Nicola Doebelin, Thomas ArmbrusterAbstract:Abstract Fully Cd-exchanged Heulandite of Cd 4.00 Na 0.01 K Ca 0.09 [Al 8.70 Si 27.30 O 71.75 ] · 29H 2 O composition was used for stepwise dehydration experiments. The crystal was heated for approximately 12 h in a stream of hot air from 50 to 250 °C in steps of 50 °C. For X-ray single-crystal data collection the heating device was switched off and the crystal was immediately cooled to −173 °C. The crystal structures of the 50–200 °C data sets were refined in space group Cm , the 250 °C data were refined in C 2/ m . The original crystal had 29 H 2 O molecules per formula unit (pfu). Heating at 50, 100, 150, and 200 °C reduced the H 2 O content to 27(2), 25(1), 26(1), and 13(1) molecules pfu. At 250 °C Heulandite-Cd became essentially anhydrous. With loss of H 2 O the cell parameters changed: a decreased slightly, b decreased strongly, c remained more or less invariant, and β increased leading to a decrease of the cell volume with dehydration. Due to the loss of the surrounding H 2 O molecules the Cd ions moved from the center of the eight- and ten-membered rings towards the framework walls causing distortion of the Heulandite channels at 250 °C. Parts of the structure transformed to a heat collapsed B-phase. This new topology differs from the original A-phase by broken T–O–T bonds and the appearance of new T–O–T connections reducing the free aperture of the 10-membered ring channels. This new structure forms domains (≈28%) coherently intergrown with the heat collapsed (A type) Heulandite framework. It is suggested that in absence of H 2 O and within strongly Cd populated domains, the bonding requirements of Cd 2+ extra-framework cations give rise to the altered T–O–T connections.
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Cd-exchanged Heulandite: symmetry lowering and site preference
Microporous and Mesoporous Materials, 2000Co-Authors: Jano Stolz, Ping Yang, Thomas ArmbrusterAbstract:Abstract Fully Cd-exchanged Heulandite was obtained from a Na-exchanged sample by treatment with 1 M Cd acetate solution at 373 K. Subsequent electron microprobe analyses revealed 4.0 Cd 2+ per formula unit (pfu), that was in good agreement with 4.11 Cd pfu refined from single-crystal X-ray data collected at 293 K. Crystal structure refinements were performed in the space groups C 2/ m , C 2, Cm , C 1 , and C 1, of which only the Cm model yielded reasonable agreement with the observed diffraction data. Cadmium preferentially occupied the center of the A channel, where it formed a distorted octahedral Cd 2+ (H 2 O) 6 complex, and a central position in the B channel that was sevenfold coordinated by oxygen atoms of the framework and channel H 2 O molecules. Seven additional low-populated Cd sites were located in the channels. Cd-exchanged Heulandite was refined in Cm symmetry in contrast to most other natural and cation-exchanged Heulandites that commonly show C 2/ m symmetry. Symmetry lowering is attributed to Si,Al ordering in the tetrahedral framework as well as to the asymmetrical distribution of Cd 2+ ions due to preferred Cd bonding of oxygen atoms sharing Al-enriched tetrahedra and cation–cation repulsion.
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Partially dealuminated Heulandite produced by acidic REECl 3 solution; a chemical and single-crystal X-ray study
American Mineralogist, 1999Co-Authors: Tobias Wuest, Jano Stolz, Thomas ArmbrusterAbstract:Single crystals (0.1-0.5 mm) of natural Heulandite from Nasik (India) were treated in 4 M NaCl solution at 423 K for 12 weeks, yielding almost fully Na-exchanged Heulandite (composition: Na (sub 8.44) Ca (sub 0.09) K (sub 0.01) [Al (sub 8.63) Si (sub 27.37) O 72 ].nH 2 O). This precursor phase was subsequently treated in a Teflon autoclave with 0.5 M rare-earth element (REE) solution (0.25 M ErCl 3 .6H 2 O and 0.25 M LaCl 3 .7H 2 O; pH of 2.8) for 15 weeks also at 423 K. Er and La in the zeolite were subsequently measured by inductively coupled plasma (ICP) mass spectrometry yielding only 656 ppm Er and 195 ppm La, whereas electron-microprobe (EMP) analyses indicated that the Na concentration decreased from originally 8.44 Na pfu to 0.25 Na pfu. The low REE content may be explained by the relatively small free diameter of the channel windows and the large size of hydrated REE ions. The low Na concentration is caused by partial dealumination of the tetrahedral framework where Si replaced some Al of the framework and Al migrated into the structural channel. Dealumination or more generally dissolution phenomena on the crystal surface occurring due to the acidic milieu in the exchange solution were observed as etch pits on scanning-electron microscope (SEM) images. X-ray single-crystal data of REECl 3 -treated Heulandite were collected at 100 and 293 K and at 100 K after partial dehydration at 323 and 378 K. Structure refinement using all data sets suggested the presence of low concentrations of octahedrally coordinated Al (super 3+) that was dissolved from the framework and incorporated into the channels. T-O distances in the framework, corrected for rotational disorder, are significantly shortened compared with the Na-exchanged precursor Heulandite. This indicates that REECl 3 -treated Heulandite has a significantly lower Al concentration in the frame-work than the Na-exchanged precursor phase.
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Na, K, Rb, and Cs exchange in Heulandite single-crystals : X-ray structure refinements at 100 K
Journal of Solid State Chemistry, 1996Co-Authors: Ping Yang, Thomas ArmbrusterAbstract:Abstract The crystal structures of Na-, K-, Rb-, and Cs-exchanged varieties of the zeolite Heulandite with the simplified composition M + 9 Al 9 Si 27 O 72 · n H 2 O were studied by single-crystal X-ray diffraction at 100 K. The structure refinements of Na-, K-, and Rb-exchanged Heulandite were performed in space group C 2/ m with resultant R values of 3.8, 3.0, and 4.9%, respectively. Cs-exchanged Heulandite was refined in space group C [formula], yielding an R value of 3.4%. X-ray single-crystal data of the Cs-exchanged variety indicated that many reflections of type h k l were not equivalent to h −k l as expected for monoclinic symmetry. With increasing radius of the incorporated channel cations, the b axis increases from 17.93 to 18.09 A leading to a slight widening of the channels. The number of H 2 O molecules also decreases with increasing cation radius due to space limitations. Three general cation positions (II-1, C 3, and B 4) were found in the four exchanged Heulandite samples. For Rb- and Cs-exchanged crystals, the additional cation site A 2 occurs. In Cs-exchanged Heulandite symmetry lowering is due to partial Si, Al ordering in the framework accompanied with a more asymmetric arrangement of channel Cs. Only if heavy elements in the channels are present the symmetry information of the framework is enforced, thus partial Si, Al ordering can be resolved.
Thráinn Fridriksson - One of the best experts on this subject based on the ideXlab platform.
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Experimental determination of thermodynamic properties of ion-exchange in Heulandite: Binary ion-exchange experiments at 55 and 85°C involving Ca2+, Sr2+, Na+, and K+
American Journal of Science, 2004Co-Authors: Thráinn Fridriksson, Philip S. Neuhoff, Brian E. Viani, Dennis K. BirdAbstract:Heulandite is a common rock-forming zeolite that exhibits wide solid solution of extraframework cations, presumably due to ready ion exchange with aqueous solutions. In order to provide a quantitative basis for interpreting and predicting the distribution of aqueous species between Heulandite and aqueous solutions, ion exchange equilibrium between Heulandite and aqueous solutions with respect to the binary cation pairs Ca2+ –K+, Ca2+ –Na+, K+ –Na+, K+ –Sr2+, Na+ –Sr2+, and Ca2+ –Sr2+ was investigated. Homoionic Ca-, K-, and Na-Heulandites prepared from natural Heulandite were equilibrated with 0.1 N Cl− solutions containing various proportions of the cations in a given binary pair at 55 and 85 °C to define isotherms describing partitioning of the cations over a wide range of Heulandite and solution composition with respect to the cations in each pair. In general, the experiments equilibrated rapidly, within 11 to 15 weeks at 55 °C and 3 to 4 weeks at 85 °C. The exception was the Ca2+ –Sr2+ binary exchange, which did not equilibrate even after 3 months at 55 °C and 4 weeks at 85 °C. Slow exchange of Sr2+ for Ca2+ also prohibited preparation of homoionic Sr-Heulandite from the natural (Ca-rich) Heulandite within 10 weeks in 2N SrCl2 solution at 90 °C, although near homoionic Sr-Heulandite was produced by exchange of K- and Na-Heulandite. Experimentally determined isotherms were used to derive equilibrium constants for the ion exchange reactions and asymmetric Margules models describing the extent of non-ideality in extraframework solid solutions in Heulandite. Regressed equilibrium constants for Ca2+-Na+, Ca2+-K+, and K+-Na+ binary cation pairs at 55 °C are internally consistent among each other (complying with the triangle rule), indicating good accuracy of these data. The maximum departure from internal consistency among the equilibrium constants for three binary pairs was 900 J per mole of charge equivalents (eq) for the 55 °C experiments and 2300 J eq−1 for the 85 °C experiments. The applicability of the present experimental results and thermodynamic models was assessed by calculating the composition of Heulandite in Icelandic geothermal systems from known compositions using the regressed thermodynamic properties of Ca2+-Na+ exchange at 85 °C. Calculations predict an average Ca mole fraction [defined as Ca/(Ca+Na)] in Heulandite of 0.74, in excellent agreement with observed compositions of Heulandite from geothermal and metamorphic systems in Iceland (0.75). Thermodynamic data for Heulandite ion-exchange derived in this study can be used to predict partitioning of Ca, K, Na, and Sr between Heulandite and aqueous solutions in geologic systems. Because Heulandite is the most effective sink for Sr in basaltic aquifers that have undergone zeolite facies metamorphism, the experimental results of this study will provide essential data for modeling Sr transport in aquifers in low-grade metabasalts.
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experimental determination of thermodynamic properties of ion exchange in Heulandite binary ion exchange experiments at 55 and 85 c involving ca2 sr2 na and k
American Journal of Science, 2004Co-Authors: Thráinn Fridriksson, Philip S. Neuhoff, Brian E. Viani, Dennis K. BirdAbstract:Heulandite is a common rock-forming zeolite that exhibits wide solid solution of extraframework cations, presumably due to ready ion exchange with aqueous solutions. In order to provide a quantitative basis for interpreting and predicting the distribution of aqueous species between Heulandite and aqueous solutions, ion exchange equilibrium between Heulandite and aqueous solutions with respect to the binary cation pairs Ca2+ –K+, Ca2+ –Na+, K+ –Na+, K+ –Sr2+, Na+ –Sr2+, and Ca2+ –Sr2+ was investigated. Homoionic Ca-, K-, and Na-Heulandites prepared from natural Heulandite were equilibrated with 0.1 N Cl− solutions containing various proportions of the cations in a given binary pair at 55 and 85 °C to define isotherms describing partitioning of the cations over a wide range of Heulandite and solution composition with respect to the cations in each pair. In general, the experiments equilibrated rapidly, within 11 to 15 weeks at 55 °C and 3 to 4 weeks at 85 °C. The exception was the Ca2+ –Sr2+ binary exchange, which did not equilibrate even after 3 months at 55 °C and 4 weeks at 85 °C. Slow exchange of Sr2+ for Ca2+ also prohibited preparation of homoionic Sr-Heulandite from the natural (Ca-rich) Heulandite within 10 weeks in 2N SrCl2 solution at 90 °C, although near homoionic Sr-Heulandite was produced by exchange of K- and Na-Heulandite. Experimentally determined isotherms were used to derive equilibrium constants for the ion exchange reactions and asymmetric Margules models describing the extent of non-ideality in extraframework solid solutions in Heulandite. Regressed equilibrium constants for Ca2+-Na+, Ca2+-K+, and K+-Na+ binary cation pairs at 55 °C are internally consistent among each other (complying with the triangle rule), indicating good accuracy of these data. The maximum departure from internal consistency among the equilibrium constants for three binary pairs was 900 J per mole of charge equivalents (eq) for the 55 °C experiments and 2300 J eq−1 for the 85 °C experiments. The applicability of the present experimental results and thermodynamic models was assessed by calculating the composition of Heulandite in Icelandic geothermal systems from known compositions using the regressed thermodynamic properties of Ca2+-Na+ exchange at 85 °C. Calculations predict an average Ca mole fraction [defined as Ca/(Ca+Na)] in Heulandite of 0.74, in excellent agreement with observed compositions of Heulandite from geothermal and metamorphic systems in Iceland (0.75). Thermodynamic data for Heulandite ion-exchange derived in this study can be used to predict partitioning of Ca, K, Na, and Sr between Heulandite and aqueous solutions in geologic systems. Because Heulandite is the most effective sink for Sr in basaltic aquifers that have undergone zeolite facies metamorphism, the experimental results of this study will provide essential data for modeling Sr transport in aquifers in low-grade metabasalts.