Ion exchange between tectosilicates with the nepheline-kalsilite framework and molten MNO3 or MO (M = Li,Na, K,Ag)

Natural nepheline, a synthetic Na-rich nepheline, and synthetic kalsilite were ion exchanged in molten MNO₃ or MCl (M = Li, Na, K, Ag) at 220–800° C. Crystalline products were characterized by wet chemical and electron microprobe analysis, single crystal and powder X-ray diffraction, and transmission electron microscopy and diffraction. Two new compounds were obtained: Li-exchanged nepheline with a formula near (Li,K_0.3,□)Li₃[Al₃(Al,Si)Si₄O₁₆] and a monoclinic unit cell with a = 951.0(6) b = 976.1(6) c = 822.9(5)pm γ = 119.15°, and Ag-exchanged nepheline with a formula near (K,Na,□)Ag₃[Al₃(Al,Si)Si₄O₁₆] and a hexagonal unit cell with a = 1007.4(8) c = 838.2(1.0) pm. Both compounds apparently retain the framework topology of the starting material. Ion exchange isotherms and structural data show that immiscibility between the end members is a general feature in the systems Na-Li, Na-Ag, and Na-K. For the system Na-K, a stepwise exchange is observed with (K,D)Na₃[Al₃(Al,Si)Si₄O₁₆] as an intermediate composition which has the nepheline structure and is miscible with the sodian end member (Na,□)Na₃[Al₃(Al,Si)Si₄O₁₆], but not with the potassian end member (K,□)₄[Al₃(Al,Si)Si₄O₁₆] which shows the kalsilite structure; there was no indication for the formation of trior tetrakalsilite (K/(K + Na)≈0.7) at the temperatures studied (350 and 800° C). The exact amount of vacancies □ on the alkali site depends upon the starting material and was found to be conserved during exchange, with ca 0–0.2 and 0.3–0.4 vacancies per 16 oxygen atoms for the synthetic and natural precursors, respectively. Thermodynamic interpretation of the Na-K exchange isotherms shows, as one important result, that the sodian end member is unstable with respect to the intermediate at K/(K+Na)≈0.25 by an amount of ca 45 kJ/mol Na in the large cavity at 800° C (52 kJ/mol at 350° C).     

Experimental determination of the speciation,partitioning, and release of perrhenate as a chemical surrogate for pertechnetate from a sodalite-bearing multiphase ceramic waste form

A key component to closing the nuclear fuel cycle is the storage and disposition of nuclear waste in geologic systems. Multiphase ceramic waste forms have been studied extensively as a potential host matrix for nuclear waste. Understanding the speciation, partitioning, and release behavior of radionuclides immobilized in multiphase ceramic waste forms is a critical aspect of developing the scientific and technical basis for nuclear waste management. In this study, we evaluated a sodalite-bearing multiphase ceramic waste form (i.e., fluidized-bed steam reform sodium aluminosilicate [FBSR NAS] product) as a potential host matrix for long-lived radionuclides, such as technetium (⁹⁹Tc). The FBSR NAS material consists primarily of nepheline (ideally NaAlSiO₄), anion-bearing sodalites (ideally M₈[Al₆Si₆O₂₄]X₂, where M refers to alkali and alkaline earth cations and X refers to monovalent anions), and nosean (ideally Na₈[AlSiO₄]₆SO₄). Bulk X-ray absorption fine structure analysis of the multiphase ceramic waste form, suggest rhenium (Re) is in the Re(VII) oxidation state and has partitioned to a Re-bearing sodalite phase (most likely a perrhenate sodalite Na₈[Al₆Si₆O₂₄](ReO₄)₂). Rhenium was added as a chemical surrogate for ⁹⁹Tc during the FBSR NAS synthesis process. The weathering behavior of the FBSR NAS material was evaluated under hydraulically unsaturated conditions with deionized water at 90 °C. The steady-state Al, Na, and Si concentrations suggests the weathering mechanisms are consistent with what has been observed for other aluminosilicate minerals and include a combination of ion exchange, network hydrolysis, and the formation of an enriched-silica surface layer or phase. The steady-state S and Re concentrations are within an order of magnitude of the nosean and perrhenate sodalite solubility, respectively. The order of magnitude difference between the observed and predicted concentration for Re and S may be associated with the fact that the anion-bearing sodalites contained in the multiphase ceramic matrix are present as mixed-anion sodalite phases. These results suggest the multiphase FBSR NAS material may be a viable host matrix for long-lived, highly mobilie radionuclides which is a critical aspect in the management of nuclear waste.     

Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study

Subsolidus phase relations for a K-doped lherzolite are investigated in the model system K₂O–Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O at 1.5–6.0 GPa and 680–1,000°C. Phlogopite is ubiquitous and coexists with Ca-amphibole up to 3.2 GPa and 900°C. High-pressure phlogopites show a peculiar mineral chemistry dependent on pressure: e.g., at 5.5 GPa and 680°C, excess of Si (up to 3.4 apfu) coupled with deficiency in Al (as low as 0.58 apfu) and K + Na (as low as 0.97 apfu), suggest a significant amount of a talc/10 Å phase component ([v]^XIISi₁K_?1Al _?1 ^IV , where [v]^XII is interlayer vacancy). Mixed layering or solid solution relations between high-pressure phlogopites and the 10 Å phase, Mg₃Si₄O₁₀(OH)₂ nH₂O, are envisaged. Phlogopite modal abundance, derived by weighted least squares, is maximum at high-pressure and relative low-temperature conditions and therefore along the slab–mantle interface (10.3 ± 0.7 wt.%, at 4.8 GPa, 680°C). In phlogopite-bearing systems, Ca-amphibole breaks down between 2.5 and 3.0 GPa, and 1,000°C, through the water conservative reaction 5(pa + 0.2 KNa_?1) + 17en + 15phl = (10di + 4jd) + 5py + 12fo + 20(phl + 0.2 talc), governed by bulk composition and pressure-dependent variations of K/OH in K-bearing phases and as a result, it does not necessarily imply a release of fluid.     

The compositional variation of synthetic sodic amphiboles at high and ultra-high pressures

Sodic amphiboles in high pressure and ultra-high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experiments at 20–30?kbar and 700–800?°C to investigate the stability and compositional variations of sodic amphiboles, based on the reaction glaucophane=2jadeite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite–talc?±?quartz does not fix its composition at glaucophane. This is because other amphibole species such as cummingtonite (Cm), nyböite (Nyb), Al–Na-cummingtonite (Al–Na-Cm) and sodium anthophyllite (Na-Anth) are also buffered via the model reactions: 3cummingtonite?+?4quartz?+?4H₂O=7talc, nyböite?+?3quartz=3jadeite?+?talc, 3Al–Na-cummingtonite + 11quartz + 2H₂O=6jadeite + 5talc, and 3 sodium anthophyllite?+?13quartz?+?4H₂O=3 jadeite + 7talc. We observed that at all pressures and temperatures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al^IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: [Na^AAl^IV]<=>[□^ASi] (edenite) toward nyböite, and [Na^(M4)Al^VI]<=>[Mg^(M4)Mg^VI] toward cummingtonite. The extent of nyböite and cummingtonite substitution increases with temperature and decreases with pressure in the experiments. Similar compositional variations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.     

The effects of sulfur intercalation on the optical properties of artificial ‘hackmanite’, Na8[Al6Si6O24]Cl1.8S0.1; ‘sulfosodalite’, Na8[Al6Si6O24]S; and natural tugtupite, Na8[Be2Al2Si8O24](Cl,S)2?δ

The minerals ??hackmanite?? and tugtupite exhibit tenebrescence (reversible photochromism) and photoluminescence. These features are generally attributed to the presence of sulfide species within their structures. But how these optical properties might be affected by intercalating additional amounts of sulfur into their structures was until now unknown. Artificial ??hackmanite??, Na₈[Al₆Si₆O₂₄]Cl_1.8S_0.1, and ??sulfosodalite??, Na₈[Al₆Si₆O₂₄]S, were heated with sulfur in evacuated quartz-glass ampoules over the temperature range 450?C1,050°C. This work has shown that sulfur intercalation into Na₈[Al₆Si₆O₂₄]Cl_1.8S_0.1 destroys the tenebrescence and induces a permanently pale blue and, at higher temperature, a pale green coloration. The effect on Na₈[Al₆Si₆O₂₄]S induced similar colorations but of a deeper hue. Annealing tugtupite, Na₈[Be₂Al₂Si₈O₂₄](Cl,S)_2??? under a sulfur atmosphere over the range 600?C700°C, destroyed the tenebrescence and resulted in a colorless tugtupite; but did not effect the photoluminescence. This suggests that the chemical species responsible for the tenebrescence in tugtupite is unlikely to be the same as that for the luminescence.     

Structure and properties of fluorine-bearing aluminosilicate melts: the system Na2O-Al2O3-SiO2-F at 1 atm

The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO₂-SiO₂] has been studied with Raman spectroscopy. Fluorine is dissolved to form F^– ions that are electrically neutralized with Na⁺ or Al³⁺. There is no evidence for association of fluorine with either Si⁴⁺ or Al³⁺ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si⁴⁺ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na⁺) reacting to form fluoride complexes. In this process an equivalent amount of Na⁺ (orginally required for Al-³⁺charge-balance) or Al³⁺ (originally required Na⁺ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.     

Vladimirivanovite, Na6Ca2[Al6Si6O24](SO4,S3,S2,Cl)2 · H2O, a new mineral of sodalite group

The results of an examination of vladimirivanovite, a new mineral of the sodalite group, found at the Tultui deposit in the Baikal region are discussed. The mineral occurs in the form of outer rims (0.01–3 mm thick) of lazurite, elongated segregations without faced crystals (0.2 to 3–4 mm in size; less frequently, 4 × 12–15 × 20 mm), and rare veinlets (up to 5 mm) hosted in calciphyre and marble. Vladimirivanovite is irregular and patchy dark blue. The mineral is brittle; on average, the microhardness VHN is 522–604, 575 kg/mm²; and the Mohs hardness is 5.0–5.5. The measured and calculated densities are 2.48(3) and 2.436 g/cm³, respectively. Vladimirivanovite is optically biaxial; 2V _meas = 63(±1)°, 2V _calc = 66.2°; the refractive indices are α = 1.502–1.507 (±0.002), N _m = 1.509–1.514 (±0.002), and N _g = 1.512–1.517 (±0.002). The chemical composition is as follows, wt %: 32.59 SiO₂, 27.39 Al₂O₃, 7.66 CaO, 17.74 Na₂O, 11.37 SO₃, 1.94 S, 0.12 Cl, and 1.0 H₂O; total is 99.62. The empirical formula calculated based on (Si + Al) = 12 with sulfide sulfur determined from the charge balance is Na_6.36Ca_1.52(Si_6.03Al_5.97)_Σ12O_23.99(SO₄)_1.58(S₃)_0.17(S₂)_0.08 · Cl_0.04 · 0.62H₂O; the idealized formula is Na₆Ca₂[Al₆Si₆O₂₄](SO₄,S₃,S₂,Cl)₂ · H₂O. The new mineral is orthorhombic, space group Pnaa; the unit-cell dimensions are a = 9.066, b = 12.851, c = 38.558 Å, V = 4492 ų, and Z = 6. The strongest reflections in the X-ray powder diffraction pattern (dÅ—I[hkl]) are: 6.61–5[015], 6.43–11[020, 006], 3.71–100[119, 133], 2.623–30[20.12, 240], 2.273–6[04.12], 2.141–14[159, 13.15], 1.783–9[06.12, 04.18], and 1.606–6[080, 00.24]. The crystal structure has been solved with a single crystal. The mineral was named in memoriam of Vladimir Georgievich Ivanov (1947–2002), Russian mineralogist and geochemist. The type material of the mineral is deposited at the Mineralogical Museum of St. Petersburg State University, St. Petersburg, Russia.     

Electron density distributions and bonded interactions for the fibrous zeolites natrolite, mesolite and scolecite and related materials

 For the fibrous zeolites natrolite, Na₂[Al₂Si₃O₁₀]·2H₂O, mesolite, Na₂Ca₂[Al₂Si₃O₁₀]₃·8H₂O, and scolecite, Ca[Al₂Si₃O₁₀]·3H₂O, with topologically identical aluminosilicate framework structures, accurate single-crystal X-ray diffraction data have been analyzed by least-squares refinements using generalized scattering factor (GSF) models. The final agreement indices were R(F ) = 0.0061, 0.0165, and 0.0073, respectively. Ensuing calculations of static deformation [Δρ(r)], and total, [ρ(r)], model electron density distributions served to study chemical bonding, in particular by topological electron density analyses yielding bond critical point (bcp) properties and in situ cation electronegativities. The results for 32 SiO, 24 AlO, 14 CaO, and 12 NaO unique bonds are compiled and analyzed in terms of both mean values and correlations between bond lengths, bonded oxygen radii, bcp densities, curvatures at the bcps, and electronegativities. Comparison with recent literature data obtained from both experimental electron density studies on minerals and model calculations for geometry-optimized molecules shows that the majority of the present findings conforms well with chemical expectation and with the trends observed from molecular modeling. For the SiO bond, the shared interaction is indicated to increase with decreasing bond length, whereas the AlO bond is of distinctly more polar nature, as is the NaO bond compared to CaO. Also, the observed ranges of the Si and Al in situ electronegativities and their mean electronegativities agree well with both Pauling's values and model calculation results, and statistically significant correlations are obtained which are consistent with trends described for oxide and nitride molecules. Received: 10 May 1999 / Revised, accepted: 14 September 1999     

Synthetic dumortierite: its PTX-dependent compositional variations in the system Al2O3-B2O3-SiO2-H2O

Dumortierite, generally simplified as Al₇BSi₃O₁₈, was synthesized in the pure system Al₂O₃–B₂O₃–SiO₂–H₂O (ABSH) using gels with variable Al/Si ratios mixed with H₃BO₃ and H₂O in known proportions as starting materials. Synthesis conditions ranged from 3 to 5 and 15 to 20 kbar fluid pressure at 650° to 880°C. On the basis of analyses, synthetic dumortierite shows relatively narrow homogeneity ranges with regard to Al/Si which, however, vary as a function of pressure: at low pressures (3–5 kbar) Al/Si is 2.77–2.94 versus 2.33–2.55 at high pressures (15–20 kbar). Outside of these homogeneity limits, dumortierite was found to coexist with quartz or corundum, depending on the starting composition. Whereas synthetic dumortierite invaribly contains 1.0 boron atom per formula unit (p.f.u.) based on 18 oxygens, the water contents vary drastically as a function of pressure and temperature (1.32–2.30 wt.% H₂O or 0.85–1.47 H p.f.u.). H₂O is an essential component in dumortierite. Structural formulae based on complete chemical analyses of the dumortierites synthesized reveal that there is invariably an Si-deficiency against the ideal number of 3.0 p.f.u. In the calculation procedure used here, this deficiency is balanced by assuming tetrahedral Al. The remaining Al, taken to occupy the octahedral sites, is always below the ideal number of 7.0 p.f.u. Charge-balancing the structure with the hydrogen found analytically leads to two different mechanisms of H incorporation: (1) 3H⁺ + octahedral vacancy for Al^[6]; (2) H⁺ + tetrahedral Al for Si^[4]. Dumortierite synthesized at high fluid pressure contains little Al^[4] and, thus, little H⁺ of type 2; its hydrogen is predominantly present as type 1. Conversely, dumortierite formed at low fluid pressures is high in Al^[4] and hydrogen type 2. The amounts of hydrogen type 1 in low-pressure dumortierites decrease with rising temperatures of synthesis. Typical structural formulae are: (Al_6.670.33)[Al_0.49Si_2.51–O_13.53(OH)_1.47](BO₃) for a low-pressure product, and (Al_6.680.32)[Al_0.09Si_2.91O_13.94(OH)_1.06](BO₃) for a high-pressure product. Independently of the synthesis conditions, dumortierite was found always to be orthorhombic, with b₀/a₀ deviating slightly, but significantly from the valid for hexagonal lattice geometry. As a function of increasing Al/Si in the synthetic crystals, their a₀, c₀, and V₀ rise, whereas b₀ decreases. Thus b₀/a₀ decreases most sensitively with rising Al/Si and also with growing Al^[4]. More experimentation is required before the compositional variations of dumortierite found here can be applied successfully to geothermobarometry of natural rocks.     

“Pseudosinhalite”, a new hydrous MgAl-borate: synthesis, phase characterization, crystal structure, and PT-stability

The new synthetic phase Mg₂Al₃O[BO₄]₂(OH) provisionally named “pseudosinhalite” is optically, chemically, and structurally similar to the mineral sinhalite, MgAl[BO₄], isostructural with forsterite. It grows hydrothermally from appropriate bulk compositions in the range 4–40?kbar at temperatures that increase with pressure (～650?→?900?°C), and it breaks down at higher temperatures to sinhalite?+?corundum?+?H₂O. At P?≥?20?kbar single-phase products of euhedral twinned crystals could often be obtained. Pseudosinhalite is monoclinic with a?=?7.455 (1) Å, b?=?4.330 (1) Å, c?=?9.825 (2) Å, β?=?110.68 (1)°, and space group P2₁/c. Crystal structure analysis reveals that pseudosinhalite is also based on hexagonal close packing (hcp) of oxygen atoms with Mg and Al in octahedral and B in tetrahedral coordination. In pseudosinhalite the winged octahedral chains in the plane of hcp are not straight as in sinhalite but have a zigzag, 3-repeat period (Dreierkette), and only 1/10 instead of 1/8 of all tetrahedral sites are filled by boron. Hydrogen is located at a split position between two oxygen atoms O5—O5, which are only 2.550 Å apart and thus generate strong hydrogen bonding. This may be responsible for the absence of an hydroxyl absorption band between 2800?cm^?1 and 3500?cm^?1 in the powder IR spectrum. The equilibrium breakdown curve of pseudosinhalite to form sinhalite, corundum, and water was determined by bracketing experiments to pass through 10?kbar, 745?°C and 35?kbar, 950?°C, giving a slope of about 8?°C/kbar, similar to dehydration curves of some silicates at high pressure. In nature pseudosinhalite could have been misidentified as sinhalite. A possible appearance, like sinhalite in boron-rich skarns, would require more aluminous bulk compositions than for sinhalite at relatively low temperatures. However, pseudosinhalite might also form as a hydrous alteration product of sinhalite at low temperatures, perhaps in association with szaibelyite, MgBO₂(OH).     

A high pressure-high temperature study of TiO2 solubility in Mg-rich phlogopite: implications to phlogopite chemistry

The solubility of Tio₂ in phlogopites has been experimentally determined in the system K₂Mg₆Al₂Si₆O₂₀(OH)₄-K₂Mg₄TiAl₂Si₆O₂₀(OH)₄-K₂Mg₅TiAl₄Si₄O₂₀(OH)₄ between 825–1300°C and 10–30 kbar under vapour absent conditions. Starting compositions lie along the join K₂Mg₆Al₂Si₆O₂₀(OH)₄-K₂Mg_4.5TiAl₃Si₅O₂₀(OH)₄ which represents a combination of the Mg^[VI]2Si^[IV] = Ti^[VI]2Al^[VI] and $\text{2Mg}{}^{\mathrm{[VI]}}\text{=}\text{Ti}{}^{\mathrm{[VI]}}\text{□}{}^{\mathrm{[VI]}}$ substitution mechanisms for Ti in phlogopites. The results of the experiments indicate a systematic increase in solubility of Ti with increasing temperature and decreasing pressure for given bulk Tio₂ content. Under isobaric conditions high temperature Ti-saturated phlogopite breaks down to Ti-deficient phlogopite + rutile + vapour. Mass balance calculations suggest that the vapour phase may contain K₂O dissolved in H₂O and that the reaction is controlled by the vapour phase. Analyses of phlogopites coexisting with rutile and vapour can be represented in terms of the end-member components phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄], eastonite [K₂Mg₅Al₄Si₅O₂₀(OH)₄], an octahedral site deficient Ti-phlogopite (Ti-OSD) of composition K₂(Mg₄Ti□)Al₂Si₆)O₂₀(OH)₄, and Ti-eastonite [K₂Mg₅TiAl₄Si₄O₂₀(OH)₄]. With decreasing amounts of Ti in these phlogopites there is a decrease in the Ti-eastonite component and increase in the eastonite component.The general equation for the breakdown of Ti-phlogopite solid solution to Ti-free phlogopite + rutile + vapour is: 14 Ti-eastonite + 7 Ti-OSD ? 16 eastonite + 3 phlogopite + 21 rutile + 4 H₂O + 2 K₂O. Lack of knowledge of H₂O and K₂O activities in the vapour phase does not permit evaluation of thermodynamic constants for this reaction. The Ti solubility in phlogopites and hence its potential as a geothermobarometer under lower crustal to upper mantle conditions is likely controlled by common mantle minerals such as forsterite.     

Correlation between electron paramagnetic resonance and thermoluminescence in natural sodalite

Samples of natural sodalite, Na₈Al₆Si₆O₂₄Cl₂, submitted to gamma irradiation and to thermal treatments, have been investigated using the thermoluminescence (TL) and electron paramagnetic resonance (EPR) techniques. Both, natural and heat-treated samples at 500°C in air for 30 min, present an EPR signal around g = 2.01132 attributed to oxygen hole centers. The EPR spectra of irradiated samples show an intense line at g = 2.0008 superimposed by a hyperfine multiplet of 11 lines due to an O⁻ ion in an intermediate position with respect to two adjacent Al nuclei. In the TL measurements, the samples were annealed at 500°C for 30 min and then irradiated with γ doses varying from 0.001 to 20 kGy. All the samples have shown TL peaks at 110, 230, 270, 365, and 445°C. A correlation between the EPR g = 2.01132 line and the 365°C TL peak was observed. A TL model is proposed in which a Na⁺ ion acts as a charge compensator when an Al³⁺ ion replaces a Si⁴⁺ lattice ion. The γ ray destruction of the Al–Na complex provides an electron trapped at the Na and a hole trapped at a non-bridging oxygen ion adjacent to the Al³⁺ ion.     

Molar volume and thermal expansion of glaucophane

The molar volume of glaucophane [Na₂Mg₃Al₂Si₈O₂₂(OH)₂] has been determined in this study by correcting synthetic glaucophane-rich amphiboles made in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O for very small deviations from ideal glaucophane composition using recent volume data on key amphibole components. The derived unit-cell volume for end-member glaucophane is 862.7±1.6 Å³, which gives a molar volume of 259.8±0.5 cm³/mol and a calculated density of 3.016±0.006 g/cm³. This value has been corroborated through an essentially independent method by correcting the volumes of natural sodic amphiboles reported in the literature for non-glaucophane components, particularly including calcium-rich components, to yield a value of 861.2±1.9 Å³. The unit-cell volume derived from the synthetic amphiboles, which is considered here to be more reliable, is somewhat smaller than that reported previously in the literature. A thermal expansion (α_V) at 298 K of 1.88±0.06×10^?5/K was derived from unit-cell volumes measured in the range of 25–500°C for a synthetic glaucophane sample, which is noticeably smaller than previously reported.     

Ferroelastic phase transition in cryolite,Na3AlF6, a mixed fluoride perovskite: High temperature single crystal X-ray diffraction study and symmetry analysis of the transition mechanism

Cryolite, Na₃AlF₆[ = 2Na⁺(Na_0.5 ⁺Al_0.5 ³⁺)F₃] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF₆] and [AlF₆] octahedra and the cavities are occupied by Na⁺ ions. At 295 K, it is monoclinic (α phase), space group P2 ₁/n with a = 5.4139 (7), b = 5.6012 (5) and c = 7.7769 (8) Å and β = 90.183 (3)°, Z = 2. A high temperature single crystal X-ray diffraction study in the range 295–900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at T ₀ ～ 885 K, in contrast to a previous report that it becomes cubic at ～823 K. The space group of the high temperature β phase is Immm with a = 5.632 (4), b = 5.627 (3) and c = 7.958 (4) Å, Z = 2 at 890 K. Above T ₀, the coordination number of the Na⁺ ion in the cavity increases from eight to twelve and the zigzag Na1 — Al octahedral chains parallel to c become straight with the Na1-F-Al angle = 180 °. The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [AlF₆] group and transforms according to the Γ ₄ ⁺ irreducible representation of Immm. Coupled to the [AlF₆] rotation is a second primary order parameter corresponding to the displacement of the Na2⁺ ion in the cavity from its equilibrium position. This order parameter transforms according to the X ₃ ⁺ irreducible representation of Immm. Following Immm → P2 ₁ /n phase transition, four equivalent domains of P2 ₁/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral Al-F and Na-F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above T ₀ indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The β phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the a phase. This view is consistent with the entropy of phase transition, ΔS_trans (11.43 JK^?1 mol^?1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK^?1 mol^?1), where 4 is the number of domains formed during the phase transition. The dynamic nature of the β phase is independently confirmed from a considerable narrowing of the ²⁷Al nuclear magnetic resonance (NMR) line-shape above T ₀ (Stebbins et al. 1992).     

The crystal chemistry of mordenites

Mordenite is a zeolite whose approximate composition is (Na₂, K₂,Ca)₄[Al₈Si₄₀O₉₆] 28 H₂O. Unit cell dimensions, determined by X-ray powder diffractometry for 35 natural samples, fell within the following ranges: a=18.052–18.168, b=20.404–20.527, c=7.501–7.537 Å. The indexed powder pattern of a typical sample is reported. Complete wet chemical analyses of 12 samples, partial analyses of three others, and 6 analyses from the literature reveal that mordenites vary only slightly in chemical composition. Si occupies 80 to 85% of the tetrahedra, and the exchangeable cations are mainly Na and Ca, with minor K. The lattice constant b is negatively correlated to the ratio Si/(Si+Al+Fe?).     

The upper thermal stability of chlorite + quartz: an experimental study in the system MgO-Al2O3-SiO2-H2O

Experiments up to water pressures of 21 kbar have been undertaken to bracket the reactions chlorite + quartz = talc + kyanite + H₂O, chlorite + quartz = talc + cordierite + H₂O, and talc + kyanite + quartz = cordierite ± H₂O by reversed runs in the system MgO-Al₂O₃-SiO₂-H₂O (MASH). These reaction curves intersect at an invariant point (IP1) at PH₂O = 6.4 ± 0.2 kbar and a temperature of 624 ± 4°C. The curve of the chlorite + quartz breakdown to talc + kyanite + H₂O at water pressures above 6.4 kbar shows a negative dP/dT, with the slope decreasing with rising pressure, whereas the slope of the breakdown curve to talc + cordierite + H₂O at water pressures is clearly positive. The composition of the chlorite solid solution reacting with quartz has been estimated to be approximately Mg_4.85Al_1.15[Al_1.15Si_2.85O₁₀](OH)₈ over the entire pressure range investigated. The composition of the talc solid solution forming by the breakdown of chlorite + quartz appears to be Mg_2.94Al_0.06[Al_0.06Si_3.94O₁₀](OH)₂ at PH₂O = 2kbar. With increasing pressure, the Al content of talc decreases, reaching a value of about 0.06 atoms per formula unit at P,H₂O = 21 kbar. As a consequence of the new experimental data, the existing phase topologies of the MASH-system and K₂O-MASH-system have been revised. For example, the invariant point IP1 and the univariant reaction curve kyanite + talc + H₂O = chlorite + cordierite are stable. For this reason, the development of medium- to high-temperature metamorphic rocks compositionally approximating the MASH-system must be reconsidered. The whiteschists from Sar e Sang, Afghanistan, are treated as an example. The application of the present experimental data to metamorphic rocks of more normal composition requires the examination of the influence of further components. This leads to the conclusion that the introduction of Fe²⁺ into magnesian chlorite extends its stability field in the presence of quartz by 10°-15°C in comparison with pure Mg-chlorite.     

Thermobarometry of mafic igneous rocks based on clinopyroxene-liquid equilibria, 0–30 kbar

 Models for estimating the pressure and temperature of igneous rocks from co-existing clino-pyroxene and liquid compositions are calibrated from existing data and from new data obtained from experiments performed on several mafic bulk compositions (from 8–30 kbar and 1100–1475° C). The resulting geothermobarometers involve thermodynamic expressions that relate temperature and pressure to equilibrium constants. Specifically, the jadeite (Jd; NaAlSi₂O₆)–diopside/hedenbergite (DiHd; Ca(Mg, Fe) Si₂O₆) exchange equilibrium between clinopyroxene and liquid is temperature sensitive. When compositional corrections are made to the calibrated equilibrium constant the resulting geothermometer is (i) 10⁴ T=6.73−0.26^* ln [Jd^px*Ca^liq*Fm^liqDiHd^px*Na^liq*Al^liq] −0.86^* ln [Mg^liqMg^liq+Fe^liq]+0.52^*ln [Ca^liq] an expression which estimates temperature to ±27 K. Compared to (i), the equilibrium constant for jadeite formation is more sensitive to pressure resulting in a thermobarometer (ii) P=−54.3+299^* T10⁴+36.4^* T10⁴ ln [Jd^px[Si^liq]^2*Na^liq*Al^liq] +367^*[Na^liq*Al^liq] which estimates pressure to ± 1.4 kbar. Pressure is in kbar, T is in Kelvin. Quantities such as Na^liq represent the cation fraction of the given oxide (NaO_0.5) in the liquid and Fm=MgO+FeO. The mole fractions of Jd and diopside+hedenbergite (DiHd) components are calculated from a normative scheme which assigns the lesser of Na or octahedral Al to form Jd; any excess Al^VI forms Calcium Tschermak’s component (CaTs; CaAlAlSiO₆); Ca remaining after forming CaTs and CaTiAl₂O₆ is taken as DiHd. Experimental data not included in the regressions were used to test models (i) and (ii). Error on predictions of T using model (i) is ±40 K. A pressure-dependent form of (i) reduces this error to ±30 K. Using model (ii) to predict pressures, the error on mean values of 10 isobaric data sets (0–25 kbar, 118 data) is ±0.3 kbar. Calculating thermodynamic properties from regression coefficients in (ii) gives V^Jd _f of 23.4 ±1.3 cm³/mol, close to the value anticipated from bar molar volume data (23.5 cm³/mol). Applied to clinopyroxene phenocrysts from Mauna Kea, Hawaii lavas, the expressions estimate equilibration depths as great as 40 km. This result indicates that transport was sufficiently rapid that at least some phenocrysts had insufficient time to re-equilibrate at lower pressures. Received: 16 May 1994/Accepted: 15 June 1995     

Synthesis, structure, thermodynamic properties, and stability relations of K-cymrite, K[AlSi3O8]·H2O

Single-phase K-cymrite, K[AlSi₃O₈]·H₂O, has been synthesized in the P-T range 3≤P(GPa)≤4 and 350≤T(°C)≤650, and characterized by a variety of techniques like SEM, FTIR, and ²⁹Si MAS-NMR. Its thermal expansivity and compressibility have been measured up to 375?°C and 6.0?GPa, respectively. Within the uncertainty of the microchemical determination of H₂O by Karl-Fischer titration, it invariably contains 1?mol of H₂O per mol of KAlSi₃O₈. Under the SEM, it appears a small idiomorphic prisms. It is optically negative, with n _o=1.553(1) and n _e=1.521(1). FTIR spectrum identifies the water in its structure as molecular H₂O. Its lattice constants are a=5.3348(1)?Å, c=7.7057(1) Å, V= 189.924 ų, the space group being P6/mmm. The ²⁹Si MAS-NMR suggests a weak short-range order of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural with cymrite (BaAl₂Si₂O₈·H₂O), the structure comprising double tetrahedral sheets with H₂O molecules residing in their cavities, K serving as an interlayer cation. Whereas cymrite, with its ordered tetrahedral Al/Si distribution, shows a Pm symmetry, the weak short-range Al/Si order in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[AlSi₃O₈]·H₂O (KCym)=K[AlSi₃O₈] (Kfs)+H₂O, executed in this study, confirm the earlier results of Thompson (1994) and supplement her data. A simultaneous treatment of those reversals, together with the thermodynamic data for Kfs and H₂O available in the literature, helps derive the standard enthalpy of formation (?4233±9.4?kJ/mol) and standard entropy (276.3±10.2 J/K·mol) for K-cymrite. The computed phase relations of KCym in the KAlSi₃O₈-H₂O binary are shown in Figure 4 for three different values of a_{H 2}O. Given a 5?°C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphic environment, KCym will be expected to grow by hydration of Kfs, unless the a_{H 2}O had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend critically on the rate of uplifting and on the a_{H 2}O prevailing during that process.     

Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia

We studied a high-pressure amphibole-bearing migmatite cropping out along the northeastern coast of Sardinia, a few kilometres northeast of Olbia, in order to improve our knowledge about its evolution using pressure–temperature (P–T) pseudosections. Thermodynamic calculations with PERPLE_X were undertaken in the system Na₂O–K₂O–CaO–FeO–MnO–MgO–Al₂O₃–TiO₂–SiO₂–H₂O using a haplogranitic melt model. Calculations were conducted for average compositions of the protolith and the tonalitic leucosome with different contents of H₂O to construct various pseudosections in the P–T range 2–20 kbar and 550–900°C. We demonstrate that the molar ratios Na/K and Si/Al of the anatectic melt are of high geothermobarometric value. Particularly the melt field in the P–T pseudosection for the protolith, contoured by Si/Al isolines, shows a significant decrease in this ratio with rising pressure. This ratio is only weakly dependent on variable H₂O contents of the amphibole-bearing migmatite as demonstrated by contoured T–H₂O pseudosections. The application of the Na/K and Si/Al ratios in melt yielded P–T conditions close to 13 kbar and 700°C, assigned as the conditions of partial melting. Evaluation of contoured P–T pseudosections for the leucosome composition resulted in conditions of about 10.5 kbar and 700°C for the crystallization of amphibole in the leucosome melt, and 9 kbar and 680°C for complete crystallization of this melt.