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Philip S. Neuhoff - One of the best experts on this subject based on the ideXlab platform.
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Thermodynamics of dehydration in Analcime: Absorption calorimetry and equilibrium observations
American Mineralogist, 2008Co-Authors: Jie Wang, Philip S. NeuhoffAbstract:Analcime forms in a wide range of geologic environments from alkaline lakes to primary igneous occurrences in Si-undersaturated lavas and pegmatites and is present in many radioactive waste repository settings. To evaluate its hydration state in these environments, calorimetric-hydration heat measurements, equilibrium observations, and thermodynamic modeling were performed. The enthalpy of hydration of Analcime determined by isothermal adsorption calorimetry was found to be independent of degree of hydration and relatively insensitive to temperature. Equilibrium hydration states of Analcime as a function of temperature and vapor pressure were assessed by isothermal thermogravimetry. In light of the lack of excess enthalpy of mixing in this solution, an ideal solution model was applied to the equilibrium observations along with previously determined heat capacities hydration to retrieve the standard Gibbs energy of hydration from water vapor (-47.29 ± 0.56 kJ/ mol H 2 O), standard enthalpy of hydration (-85.10 ± 1.03 kJ/mol H 2 O), and entropy of hydration (-126.81 ± 1.60 J/mol H 2 O·K) at 298.15 K, 1 bar. The standard enthalpy of hydration at 298.15 K, 1 bar regressed from the calorimetric data are consistent with these results (-86.4 ± 1.9 kJ/mol H 2 O). Calculations of the hydration state of Analcime as a function of temperature and pressure indicate that Analcime is essentially fully hydrated at temperature and pressure conditions attending its formation in geologic and experimental systems. Dehydration of Analcime under water-undersaturated conditions (as during heating in a radioactive waste repository) may contribute significantly to thermal budgets in systems where this phase is important.
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Thermodynamic properties of Analcime solid solutions
American Journal of Science, 2004Co-Authors: Philip S. Neuhoff, Guy L. Hovis, Giuseppina Balassone, Jonathan F. StebbinsAbstract:Analcime (NaxAlxSi3-xO6 · [(3-x)/2]H2O, where x varies from ∼0.78 to ∼1.06) is one of the most common rock-forming zeolites. It forms in a wide range of geologic environments that span a range of temperature and pressure from ambient to magmatic conditions. Cluster variation method analysis of 29Si magic angle spinning nuclear magnetic resonance spectra indicates 1) the presence of at least two distinct states of short range Si/Al disorder [low (less disordered) and high (more disordered) Analcime], and 2) that configurational entropy associated short-range Si-Al disorder within each of these states increases regularly with increasing Si content. Hydrofluoric acid (HF) solution calorimetry at 50°C was used to determine the enthalpy of formation of five pure Analcime samples of varying composition (range of x approximately 0.95 to 1.05) and Si-Al disorder. Enthalpies of formation from the elements at 25°C (ΔHf) for these samples fall on a linear trend, except for one sample of high Analcime for which ΔHf was about 6.1 ± 3.0 kJ/mol less stable than a low Analcime of the same composition. Comparison with the results of previous calorimetric studies indicates negligible excess enthalpies of mixing in both low and high Analcime solid solutions (that is, the solid solutions are athermal). The configurational entropies derived from cluster variation analysis were in turn used to derive activity-composition relationships for low Analcime solid solutions whose compositions are bounded by an aluminous endmember (Na1.05Al1.05Si1.95O6 · 0.975H2O) and a siliceous endmember (Na0.75Al0.75Si2.25O6 · 1.125H2O). These relationships were used to retrieve thermodynamic properties for the endmembers from experimental observations of equilibria between Analcime, albite, and aqueous solutions. Retrieved values of ΔHf are in excellent agreement with the calorimetric results of this study. Comparative analysis of equilibrium observations in the literature indicate that one sample of Analcime from the Mont St. Hilaire alkaline intrusive complex used for Analcime solubility measurements is high Analcime. The Gibbs energy of disordering at 298.15 K, 1 bar consistent with the retrieval calculations is ∼ 6 kJ/mol. The thermodynamic properties of disordering for Analcime indicate that hydrated low Analcime is stable with respect to hydrated high Analcime everywhere in Earth’s crust. Phase relations between low Analcime, quartz, albite, and aqueous solutions calculated from the retrieved thermodynamic data indicate that at quartz equilibrium, low Analcime should become more Si-rich with increasing temperature and pressure and that the composition of Analcime is a sensitive function of the chemical potential of SiO2. Stable equilibrium between Analcime, albite, quartz and H2O occurs at much lower temperatures than suggested by earlier phase equilibrium experiments. The breakdown of Analcime plus quartz to form albite in geologic systems probably reflects metastable equilibrium in which the composition of Analcime did not equilibrate with quartz.
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Si-Al disorder and solid solutions in Analcime, chabazite, and wairakite
American Mineralogist, 2003Co-Authors: Philip S. Neuhoff, Jonathan F. Stebbins, Dennis K. BirdAbstract:Quantitative determination of the abundance of Si( n Al) tetrahedral structural units (where n = 0, 1, 2, 3, or 4) through analysis of 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra was used to assess the state of Si-Al disorder in the zeolites Analcime [(NaAl) x Si48− x O96·16H2O], chabazite [(Ca0.5,Na,K) x Al x Si12− x O24·12H2O], and wairakite [CaAl2Si4O12·2H2O]. Short-range Si-Al ordering in chabazite and Analcime is a regular function of Al mol fraction and is fully consistent with Al avoidance, as has generally been reported for zeolites not subjected to heat treatment. The results of this study and previously reported 29Si MAS NMR spectra suggest that natural Analcime samples are more Si-Al ordered than either their synthetic counterparts or chabazite. Cluster variation method (CVM) calculations were used to calculate the configurational entropy ( S CON) due to SiAl disorder in chabazite and Analcime. The calculations predict that long-range Si-Al ordering develops when Al occupies 5 out of 12 tetrahedral sites in chabazite and synthetic Analcime and 17 out of every 48 tetrahedral sites in natural Analcime. The difference between the calculated entropies and ideal entropies of mixing was used to derive activity-composition relationships for Si-Al substitution in these frameworks. Comparison between calculated values of S CON and the results of calorimetric and phase equilibrium studies on Analcime indicate that the CVM accurately assesses S CON. The 29Si MAS NMR spectrum obtained for natural wairakite indicates that this mineral is largely SiAl ordered, but comparison with a previously published spectrum indicates that natural and synthetic wairakites can exhibit significant variation in Si-Al disorder.
Atsushi Kyono - One of the best experts on this subject based on the ideXlab platform.
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A reply to comment on “An experimental study of symmetry lowering of Analcime”
Physics and Chemistry of Minerals, 2018Co-Authors: Atsushi KyonoAbstract:The fracture surfaces of Analcimes reported by Sugano and Kyono (Phys Chem Miner, 2018) were re-investigated using field-emission scanning electron microscopy. The results of scanning electron microscopic observation show the fractures can be characterized by smoothly curved surfaces, called sub-conchoidal fracture, and neither the lamellar twin nor the domain walls of twin were found on the surfaces of the Analcimes. In addition, the lamellar twin is generally formed by transformation from the high-temperature phase or from pseudomorphic replacement under strong alkaline conditions. Actually, the symmetry lowering of Analcime reported by Sugano and Kyono (2018) occurs at 200 °C. The temperature is much lower than the transformation from the high-temperature phase. In the hydrothermal experiment (Sugano and Kyono 2018), moreover, the Analcimes were synthesized under acidic condition and reheated in pure water. No twin domain is likely to be formed under the hydrothermal condition. These facts strongly deny the possibility that the twin domains cause the pseudo-symmetry of tetragonal Analcime. Consequently, it can be concluded that the observed forbidden reflections for the cubic Ia3d symmetry are not due to the presence of twin domains, but due to the symmetry lowering of Analcime from cubic Ia3d to orthorhombic Ibca.
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synthesis of thallium leucite tlalsi 2 o 6 pseudomorph after Analcime
Mineralogical Magazine, 1999Co-Authors: Atsushi Kyono, Mitsuyoshi Kimata, Norimasa Nishida, Masahiro Shimizu, S. Saito, T. HattaAbstract:Thallium leucite, TlAlSi 2 O 6 , has been synthesized at 450 degrees C for 7 days, under ambient conditions, by the transformation of dehydrated Analcime NaAlSi 2 O 6 in the presence of excess T1C1. This substitution of T1 for Na leads to confirmation of a thallium-leucite pseudomorph after Analcime. Their optical properties, X-ray powder diffraction patterns, electron microprobe analysis, infrared spectra, and X-ray photoelectron spectroscopy have characterized the synthetic T1-leucites. The IR spectra show that the mid-IR modes T-O stretching and T-O-T bending vibrations for TlAlSi 2 O 6 are more resemblant of those for Analcime than for leucite, KAlSi 2 O 6 . This resemblance implies that T1 cation enters the W-site rather than the S-site in the Analcime structure: Na (S)+H 2 O (W) []+K (leucite) +[]Tl (Tl-leucite), where [] represents an S-site vacancy. The mechanism of this substitution is supported by the crystal chemical constraints: inasmuch as the S-site is smaller than the W-site, Tl (super +) cations being larger than Na (super +) plainly prefer the latter site to the former. One inference from the binding energy for Tl (super +) by XPS is that Tl (super +) occupies the extra-framework site in synthetic leucite pseudomorph, rather than the smaller tetrahedral site. The difference in A1/Si disordering between Analcime and leucite and the nonstoichiometry due to the solid solution of the []Si 3 O 6 component into the leucite structure may provide a fundamental insight into understanding why TlAlSi 2 O 6 deviates from the trend defined by K-, Rb- and CsAlSi 2 O 6 leucite series on the a-c parameter diagram, inasmuch as these three cations in the leucite structure occupy the W-sites. Finally, synthesis of TlAlSi 2 O 6 leucite has an implication for the existence of other polymorphs due to different degrees of Al/Si disordering, except for high- and low-temperature leucites already known: natural leucites crystallized directly through igneous processes are different from those formed by substitution of K for Na in Analcimes.
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synthesis of thallium leucite tlalsi2o6 pseudomorph after Analcime
Mineralogical Magazine, 1999Co-Authors: Atsushi KyonoAbstract:Thallium leucite, T1A1Si206, has been synthesized at 450~ for 7 days, under ambient conditions, by the transformation of dehydrated Analcime NaA1Si206 in the presence of excess T1C1. This substitution of TI for Na leads to confirmation of a thallium-leucite pseudomorph after Analcime, Their optical properties, X-ray powder diffraction patterns, electron microprobe analysis, infrared spectra, and X-ray photoelectron spectroscopy have characterized the synthetic Tl-leucites. The IR spectra show that the mid-lR modes T-O stretching and T-O-T bending vibrations for TIA1Si206 are more resemblant of those for Analcime than for leucite, KA1Si206. This resemblance implies that TI cation enters the W-site rather than the S-site in the Analcime structure: Na (S) + H20 (W) ~[] + K (leucite) ~ [] + T1 (T1leucite), where [] represents an S-site vacancy. The mechanism of this substitution is supported by the crystal chemical constraints: inasmuch as the S-site is smaller than the W-site, T1 + cations being larger than Na + plainly prefer the latter site to the former. One inference from the binding energy for T1 + by XPS is that T1 + occupies the extra-framework site in synthetic leucite pseudomorph, rather than the smaller tetrahedral site. The difference in A1/Si disordering between Analcime and leucite and the nonstoichiometry due to the solid solution of the []Si306 component into the leucite structure may provide a fundamental insight into understanding why T1A1Si206 deviates from the trend defined by K-, Rband CsA1Si206 leucite series on the a-c parameter diagram, inasmuch as these three cations in the leucite structure occupy the W-sites. Finally, synthesis of T1A1Si206 leucite has an implication for the existence of other polymorphs due to different degrees of A1/Si disordering, except for highand low-temperature leucites already known: natural leucites crystallized directly through igneous processes are different from those formed by substitution of K for Na in Analcimes.
Jonathan F. Stebbins - One of the best experts on this subject based on the ideXlab platform.
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Thermodynamic properties of Analcime solid solutions
American Journal of Science, 2004Co-Authors: Philip S. Neuhoff, Guy L. Hovis, Giuseppina Balassone, Jonathan F. StebbinsAbstract:Analcime (NaxAlxSi3-xO6 · [(3-x)/2]H2O, where x varies from ∼0.78 to ∼1.06) is one of the most common rock-forming zeolites. It forms in a wide range of geologic environments that span a range of temperature and pressure from ambient to magmatic conditions. Cluster variation method analysis of 29Si magic angle spinning nuclear magnetic resonance spectra indicates 1) the presence of at least two distinct states of short range Si/Al disorder [low (less disordered) and high (more disordered) Analcime], and 2) that configurational entropy associated short-range Si-Al disorder within each of these states increases regularly with increasing Si content. Hydrofluoric acid (HF) solution calorimetry at 50°C was used to determine the enthalpy of formation of five pure Analcime samples of varying composition (range of x approximately 0.95 to 1.05) and Si-Al disorder. Enthalpies of formation from the elements at 25°C (ΔHf) for these samples fall on a linear trend, except for one sample of high Analcime for which ΔHf was about 6.1 ± 3.0 kJ/mol less stable than a low Analcime of the same composition. Comparison with the results of previous calorimetric studies indicates negligible excess enthalpies of mixing in both low and high Analcime solid solutions (that is, the solid solutions are athermal). The configurational entropies derived from cluster variation analysis were in turn used to derive activity-composition relationships for low Analcime solid solutions whose compositions are bounded by an aluminous endmember (Na1.05Al1.05Si1.95O6 · 0.975H2O) and a siliceous endmember (Na0.75Al0.75Si2.25O6 · 1.125H2O). These relationships were used to retrieve thermodynamic properties for the endmembers from experimental observations of equilibria between Analcime, albite, and aqueous solutions. Retrieved values of ΔHf are in excellent agreement with the calorimetric results of this study. Comparative analysis of equilibrium observations in the literature indicate that one sample of Analcime from the Mont St. Hilaire alkaline intrusive complex used for Analcime solubility measurements is high Analcime. The Gibbs energy of disordering at 298.15 K, 1 bar consistent with the retrieval calculations is ∼ 6 kJ/mol. The thermodynamic properties of disordering for Analcime indicate that hydrated low Analcime is stable with respect to hydrated high Analcime everywhere in Earth’s crust. Phase relations between low Analcime, quartz, albite, and aqueous solutions calculated from the retrieved thermodynamic data indicate that at quartz equilibrium, low Analcime should become more Si-rich with increasing temperature and pressure and that the composition of Analcime is a sensitive function of the chemical potential of SiO2. Stable equilibrium between Analcime, albite, quartz and H2O occurs at much lower temperatures than suggested by earlier phase equilibrium experiments. The breakdown of Analcime plus quartz to form albite in geologic systems probably reflects metastable equilibrium in which the composition of Analcime did not equilibrate with quartz.
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Si-Al disorder and solid solutions in Analcime, chabazite, and wairakite
American Mineralogist, 2003Co-Authors: Philip S. Neuhoff, Jonathan F. Stebbins, Dennis K. BirdAbstract:Quantitative determination of the abundance of Si( n Al) tetrahedral structural units (where n = 0, 1, 2, 3, or 4) through analysis of 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra was used to assess the state of Si-Al disorder in the zeolites Analcime [(NaAl) x Si48− x O96·16H2O], chabazite [(Ca0.5,Na,K) x Al x Si12− x O24·12H2O], and wairakite [CaAl2Si4O12·2H2O]. Short-range Si-Al ordering in chabazite and Analcime is a regular function of Al mol fraction and is fully consistent with Al avoidance, as has generally been reported for zeolites not subjected to heat treatment. The results of this study and previously reported 29Si MAS NMR spectra suggest that natural Analcime samples are more Si-Al ordered than either their synthetic counterparts or chabazite. Cluster variation method (CVM) calculations were used to calculate the configurational entropy ( S CON) due to SiAl disorder in chabazite and Analcime. The calculations predict that long-range Si-Al ordering develops when Al occupies 5 out of 12 tetrahedral sites in chabazite and synthetic Analcime and 17 out of every 48 tetrahedral sites in natural Analcime. The difference between the calculated entropies and ideal entropies of mixing was used to derive activity-composition relationships for Si-Al substitution in these frameworks. Comparison between calculated values of S CON and the results of calorimetric and phase equilibrium studies on Analcime indicate that the CVM accurately assesses S CON. The 29Si MAS NMR spectrum obtained for natural wairakite indicates that this mineral is largely SiAl ordered, but comparison with a previously published spectrum indicates that natural and synthetic wairakites can exhibit significant variation in Si-Al disorder.
J. F. G. Wilkinson - One of the best experts on this subject based on the ideXlab platform.
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SOME Analcime-BEARING PYROCLASTIC AND SEDIMENTARY ROCKS FROM NEW SOUTH WALES ~
2016Co-Authors: J. F. G. Wilkinson, J. Whetten T. ZAbstract:Analcime in acid-intermediate tufts and volcanic sedimentary ocks at Currabubula, New South Wales, has probably formed due to mild P/T (zeolite mineral facies) reconstitution ofglass shards and fine volcanic ash matrix. The Analcime co-exists with quartz-alkali feldspar-heulandite-chlorite-montmorillonite-(calcite-albite). Analyses of two Analcimes from rocks of differing bulk composition i dicate the mineral to be moder.ately si!ica-rich. Optical properties and cell dimensions of these Analcimes accord reasonably well with synthetic analclmes of similar composition. Aspects relating to the stability field of aaalcime and its derivation from potentially anal-cime-bearing volcanic glasses are also discussed. Soda metasomatism is not considered an important factor in Analcime genesis n the rocks under discussion
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Nephelines and Analcimes in some alkaline igneous rocks
Contributions to Mineralogy and Petrology, 1994Co-Authors: J. F. G. Wilkinson, H. D. HenselAbstract:The paper presents electron microprobe analyses of nephelines and Analcimes in alkaline igneous rocks ranging from theralite and basanite to mugearite and tinguaite. With few exceptions, the nephelines are Si-rich types whose Qz (quartz) components exceed those defining the limits of excess SiO_2 in solid solution in the Ne-Ks-Qz-H_2O system at 700°C and 1 kbar P _H _2 _O. Unlike the nephelines in the basanites which show only limited grain-to-grain compositional variation, those in theralites and tinguaites from the differentiated Square Top intrusion, New South Wales, and in a New Zealand tinguaite vary significantly in Ne (nepheline), Ks (kalsilite) and Qz, even within individual samples, and they also may be strongly zoned. The rims of zoned nephelines are enriched in Si and Fe^3+, relative to core compositions. These zoning trends contrast with the composional trend of successive “bulk” nepheline fractions in the Square Top sequence theralite → tinguaite whereby Qz decreases. The nephelines coexist with high-temperature alkali feldspars. In the Ne-Ks-Qz system they plot on the Ne side of the Barth compositional join defined by the omission solid solution series with end-members K_2Na_6Al_8Si_8O_32 (the Buerger ideal composition; Ne_75Ks_25 mol%) and □_2Na_6Al_2Si_10O_32 (□=cavity cation vacancy). The compositions of most natural nephelines are restricted to the field defined by Ne and the Barth join, compositions more K-rich than the ideal composition being relatively rare. The compositions of nephelines on the Ne side of the join are controlled by a number of factors which include the physical conditions attending nepheline crystallization and the compositions of the alkaline hosts. Interstitial Analcimes from the Square Top intrusion display extensive NaAl⇌Si substitution and their compositions extend from Analcime of natrolite composition to compositions slightly more Si-rich than “ideal” NaAlSi_2O_6. Groundmass Analcimes in the basanites, mugearite and New Zealand tinguaite have relatively constant compositions which approach stoichiometric NaAlSi_2O_6. An unusually Si-rich deuteric Analcime (60.2% SiO_2) is also present in vugs in the New Zealand tinguaite. Experimental and other evidence, including P/T data defining the coexistence of Analcime and silica-undersaturated silicate melt in the NaAlSiO_4-KAlSiO_4-SiO_2-H_2O system, and inferred solidus temperatures of the various hosts (they would have exceeded the stability range of Analcime) preclude a primary magmatic origin for the interstitial and groundmass Analcimes. These are interpreted as subsolidus phases produced by nepheline interaction with deuteric and/or hydrothermal fluids. Analyses of nephelines and their derivative Analcimes indicate that the latter may form from both Si-rich and more Si-poor nephelines.
Guy L. Hovis - One of the best experts on this subject based on the ideXlab platform.
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Thermodynamic properties of Analcime solid solutions
American Journal of Science, 2004Co-Authors: Philip S. Neuhoff, Guy L. Hovis, Giuseppina Balassone, Jonathan F. StebbinsAbstract:Analcime (NaxAlxSi3-xO6 · [(3-x)/2]H2O, where x varies from ∼0.78 to ∼1.06) is one of the most common rock-forming zeolites. It forms in a wide range of geologic environments that span a range of temperature and pressure from ambient to magmatic conditions. Cluster variation method analysis of 29Si magic angle spinning nuclear magnetic resonance spectra indicates 1) the presence of at least two distinct states of short range Si/Al disorder [low (less disordered) and high (more disordered) Analcime], and 2) that configurational entropy associated short-range Si-Al disorder within each of these states increases regularly with increasing Si content. Hydrofluoric acid (HF) solution calorimetry at 50°C was used to determine the enthalpy of formation of five pure Analcime samples of varying composition (range of x approximately 0.95 to 1.05) and Si-Al disorder. Enthalpies of formation from the elements at 25°C (ΔHf) for these samples fall on a linear trend, except for one sample of high Analcime for which ΔHf was about 6.1 ± 3.0 kJ/mol less stable than a low Analcime of the same composition. Comparison with the results of previous calorimetric studies indicates negligible excess enthalpies of mixing in both low and high Analcime solid solutions (that is, the solid solutions are athermal). The configurational entropies derived from cluster variation analysis were in turn used to derive activity-composition relationships for low Analcime solid solutions whose compositions are bounded by an aluminous endmember (Na1.05Al1.05Si1.95O6 · 0.975H2O) and a siliceous endmember (Na0.75Al0.75Si2.25O6 · 1.125H2O). These relationships were used to retrieve thermodynamic properties for the endmembers from experimental observations of equilibria between Analcime, albite, and aqueous solutions. Retrieved values of ΔHf are in excellent agreement with the calorimetric results of this study. Comparative analysis of equilibrium observations in the literature indicate that one sample of Analcime from the Mont St. Hilaire alkaline intrusive complex used for Analcime solubility measurements is high Analcime. The Gibbs energy of disordering at 298.15 K, 1 bar consistent with the retrieval calculations is ∼ 6 kJ/mol. The thermodynamic properties of disordering for Analcime indicate that hydrated low Analcime is stable with respect to hydrated high Analcime everywhere in Earth’s crust. Phase relations between low Analcime, quartz, albite, and aqueous solutions calculated from the retrieved thermodynamic data indicate that at quartz equilibrium, low Analcime should become more Si-rich with increasing temperature and pressure and that the composition of Analcime is a sensitive function of the chemical potential of SiO2. Stable equilibrium between Analcime, albite, quartz and H2O occurs at much lower temperatures than suggested by earlier phase equilibrium experiments. The breakdown of Analcime plus quartz to form albite in geologic systems probably reflects metastable equilibrium in which the composition of Analcime did not equilibrate with quartz.
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Thermodynamic and structural behavior of Analcime–leucite analogue systems
American Mineralogist, 2002Co-Authors: Guy L. Hovis, Jacques Roux, Elizabeth RodriguesAbstract:Two synthetic solid-solution series, Analcime to Rb-leucite and Analcime to Cs-leucite (pollucite), have been investigated to understand more fully the thermodynamic and structural behavior of Analcime-leucite and similarmineral systems. Unit-cell dimensions and volumes in these series expand with the substitution of Analcime component in either Rb-leucite or pollucite, as H 2 O molecules structurally replace the smaller entities Rb + 1 or Cs + 1 , respectively. Unit-cell volumes vary linearly as functions of composition, but with changing slopes over several segments of compositional space, akin to thermal expansion in K-, Rb-, and Cs-end-member materials studied by previous workers. When symmetry changes displacively from tetragonal to isometric, as in the Rb-bearing series, the slope of volume expansion changes. Once structures have reached full expansion, volume slopes flatten and are little affected by additional Analcime component. Enthalpies of solution measured at 50 °C in 20.1 wt% hydrofluoric acid show single-slope linear relationships over the entire compositional ranges of both series. Thus, despite positive volumes of mixing, there are no enthalpies of mixing in either series, nor is there energetic evidence of displacive tetragonal/isometric inversion or the various stages of structural expansion. Overall, the data suggest that the Analcime-leucite system also can be modeled as close to thermodynamically ideal. The limited solid solution between natural Analcime and leucite must be attributed to energetically favored heterogeneous equilibria involving minerals such as feldspars and other feldspathoids, and not to immiscibility between the end-members.
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Thermodynamic and structural behavior of Analcime-leucite analogue systems.
American Mineralogist, 2002Co-Authors: Guy L. Hovis, Jacques Roux, Elisabeth RodriguesAbstract:Two synthetic solid-solution series, Analcime to Rb-leucite and Analcime to Cs-leucite (pollucite), have been investigated to understand more fully the thermodynamic and structural behavior of Analcime- leucite and similar mineral systems. Unit-cell dimensions and volumes in these series expand with the substitution of Analcime component in either Rb-leucite or pollucite, as H2O molecules structurally replace the smaller entities Rb+1 or Cs+1, respectively. Unit-cell volumes vary linearly as functions of composition, but with changing slopes over several segments of compositional space, akin to thermal expansion in K-, Rb-, and Cs-end-member materials studied by previous workers. When symmetry changes displacively from tetragonal to isometric, as in the Rb-bearing series, the slope of volume expansion changes. Once structures have reached full expansion, volume slopes flatten and are little affected by additional Analcime component. Enthalpies of solution measured at 50 ∞C in 20.1 wt% hydrofluoric acid show single-slope linear relationships over the entire compositional ranges of both series. Thus, despite positive volumes of mixing, there are no enthalpies of mixing in either series, nor is there energetic evidence of displacive tetragonal/isometric inversion or the various stages of structural expansion. Overall, the data suggest that the Analcime–leucite system also can be modeled as close to thermodynamically ideal. The limited solid solution between natural Analcime and leucite must be attributed to energetically favored heterogeneous equilibria involving minerals such as feldspars and other feldspathoids, and not to immiscibility between the end-members.