Nepheline Structure Research Articles

Atypical components in the composition of nepheline from rocks of the Khibiny massif according to IR spectroscopy data

Nepheline is one of the main rock-forming minerals of the rocks of the Khibiny alkaline massif. Nepheline composition changes depending on the crystallization temperature, which makes it an indicator of rock formation conditions. In addition, nepheline is one of the main concentrators of hydrocarbon gases. The study of nepheline from different rocks of the Khibiny massif by local IR spectroscopy with Fourier transformation (micro-FTIR) has revealed the presence of water and acetylene in its structure. The water content estimated according to the method of statistical quantitative determination of water in nominally anhydrous minerals varies in the range from 0 to 0.78 wt.%. The highest water content has been observed in high-temperature nepheline of early formations (foyaite of the central and outer parts of the massif), and the lowest – in relatively low-temperature nepheline of later rocks of the Main Ring Fault (urtite, apatite-nepheline rock and poikilitic (kalsilite)-nepheline syenite – rischorrite). The study reveals that water enters the structure of nepheline in the form of H2O molecules that occupy vacancies in the potassium sites. The water content in nepheline is controlled by the number of vacancies in its structure. The presence of acetylene has been determined on a qualitative level. Additional studies are required to assess its quantitative content.

Development of single matrix aluminum silicate fluorescent powder activated by Bi3+, Eu3+, and Eu2+ and use in white LEDs

In the field of luminescent materials, the search for good substrates and the use of a single substrate to excite near-ultraviolet chips to produce white LED devices has always been of great interest. This study utilized energy transfer between Bi3+ and Eu3+ to design KAlSiO4: 0.02Bi3+, 0.04Eu3+ red emission phosphors with a potassium nepheline structure, which were calcined in air with a high-temperature solid-state method. Under 286 nm excitation, energy transfer between Bi3+ and Eu3+ up to 85.9 %. Through DFT theoretical calculations, it was found that the bandgap of the matrix was 4.86 eV, KAlSiO4: 0.02Bi3+, 0.04Eu3+ has excellent heat quenching resistance, and the emission intensity at 150 °C is 92.9 % of that seen at room temperature. Second, by doping with Eu2O3 and calcining in a reducing atmosphere, KAlSiO4: 0.02Eu2+, 0.1Li+ broadband blue emitting phosphors were obtained. Finally, KAlSiO4: 0.02Bi3+, 0.04Eu3+ was mixed with KAlSiO4: 0.02Eu2+, 0.1Li+ to prepare a near-ultraviolet white LED device. The color rendering index of the device reached 87.1, the color coordinates were (0.3328, 0.3015), and the color temperature was 5422 K. These results proved that KAlSiO4: 0.02Bi3+, 0.04Eu3+ samples have potential for use in near-ultraviolet white LEDs and that KAlSiO4 is a good substrate in the field of luminescence. This work provides a new breakthrough in the use of a single substrate to prepare multiple colored fluorescent powders and mix them together to produce white LED devices.

Iron in Nepheline: Crystal Chemical Features and Petrological Applications

Nepheline is a nominally anhydrous aluminosilicate that typically contains an impurity of ferric iron replacing aluminum in tetrahedral sites. However, previous researchers noted the constant presence of ferrous iron in the chemical composition of nepheline from the rocks of the Khibiny and Lovozero massifs (Kola Peninsula, Russia). We have carried out microprobe, spectroscopic, chemical and crystal chemical studies of nepheline from the Lovozero massif. We have established the presence of molecular water in nepheline, and also that the incorporation of ferrous iron into nepheline crystal structure is associated with the simultaneous increasing of the coordination number from four to five (or six) due to the inclusion of the ‘additional’ water molecules that form point [FeO4(H2O)n]-defects (where n = 1, 2) in the tetrahedral framework. The nepheline iron content is closely related to the presence of small needle-like aegirine inclusions. The total iron content in nepheline saturated with aegirine needles is approximately an order of magnitude lower than in nepheline free from aegirine inclusions. Most likely the aegirine inclusions in nepheline are formed as a result of the decomposition of the nepheline–“iron nepheline” solid solution. We propose that this process is triggered by the oxidation of ferrous iron in the crystal structure of nepheline.

Immobilization mechanism of cesium in geopolymer: Effects of alkaline activators and calcination temperature

Geopolymer is always regarded as a promising material for the immobilization of radioactive waste. In the present study, the stabilization of Cs in geopolymers activated by NaOH and Na2SiO3 solutions and calcined at various temperatures was studied via toxicity characteristic leaching procedure (TCLP), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope and energy dispersive spectroscopy (SEM-EDS), solid-state nuclear magnetic resonance (SSNMR), and N2 adsorption-desorption isotherm. For both NaOH-activated and Na2SiO3-activated geopolymers, the leaching concentrations of Cs decreased with the increase of calcination temperature. Specifically, most of the amorphous substance was crystallized to nepheline at 1000 °C for NaOH-activated geopolymer, and Cs+ can be incorporated into the structure of nepheline, contributing to the reduction of Cs leaching concentration. However, the amorphous structure was still maintained for Na2SiO3-activated geopolymer even after calcination at 1000 °C. It has been deduced that the main structure of Na2SiO3-activated geopolymer after calcination at 1000 °C should be in short-range order and Cs+ can be locked in a micro “crystal” structure. In addition, the change of specific surface area was not fully consistent with the decreasing trend of Cs leaching concentration. Therefore, the inner structure and the specific surface area of geopolymer should have a combined effect on the leaching behavior of Cs. This study can provide new insights into the application of geopolymer to immobilize radionuclides.

Synthesis, Thermal Properties and Electrical Conductivity of Na-Sialate Geopolymer

This work aims to study the thermal behavior of basic-geopolymers derived from metakaolin (clay). The geopolymers were characterized by different techniques: thermal analysis (DTA, TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and impedance spectroscopy. Some physicochemical properties of the products were also determined: the phases obtained after geopolymer heat treatment and their electrical properties. The results obtained after drying and heat treatment showed that the products kept their initial shapes, but revealed variable colors depending on the temperatures at which they were treated. The products obtained are amorphous between 300 up to 600 °C with peaks relating to the presence of nanocrystallites of muscovites and zeolite, thus at 900 °C it is quite amorphous but only contains nanocrystallites of muscovites. From the temperature of 950 °C, we notice that the geopolymer has been transformed into a crystalline compound predominated by the Nepheline (NaAlSiO4) with the presence of a crystalline phase by minor peaks of Muscovite, this crystalline character has been increased at 1100 °C to obtain a whole phase crystalline of a Nepheline. The treatment of this geopolymer for one hour at 1200 °C shows an amorphous phase again corresponding to corundum (α-Al2O3). This indicates that the dissolution of the grains by the liquid phase induces the conversion of the material structure from sialate [–Si–O–Al–O] to sialate siloxo [–Si–O–Al–O–Si–O–] and the formation of a new crystalline phase (α-Al2O3). This development of sialate to sialate-siloxo was confirmed by IR spectroscopy. As mentioned above, from 300 to 900 °C, Na-sialate geopolymer exhibits the same disorder structure of nepheline. The crystal structure of nepheline is characterized by layers of six-membered tetrahedral rings of exclusively oval conformation. The rings are built by Regularly alternating tetrahedral AlO4 and SiO4. Stacking the layer’s parallel to the c axis gives a three-dimensional network containing channels occupied by Na cations. This topology favors easy movement of Na+ ions throughout the structure. For this reason, ionic migration in nepheline is widely reported. The refinement of Na-Sialate geopolymer at room temperature gives bulk high ionic conductivity of about 5 × 10−5 S cm−1 and this is due to the probable joint contribution of H+ and Na+ ions. Above 200 °C, Na+ seems to remain the only charge carrier with a low activation energy of about Ea = 0.26 eV. At higher temperatures, the characteristic frequencies become so close that it is impossible to distinguish the contributions. A total resistance comprising both grain and grain boundaries contribution is then determined.

Investigation of Glass Preparation by using Processed Nepheline Syenite from kaleibar area

Abstract: In this study, the structure of Nephline (Neph) (including two minerals, syenite and microcline) was studied, using hydrothermal and milling methods by the 2.% mol. soda (NaNeph) and 30 wt.% lime(CaNeph), respectively. According to the X-ray diffraction and Raman spectra, the structure of the microcline phase in the (CaNeph) sample remained stable but the structure of Nepheline was somewhat degraded. The main characteristic XRD peaks of Syenite in the NaNeph sample at 28 o stayed compared to the CaNeph one. According to Raman spectroscopy the structural order of alumina-silicate rings in CaNeph sample was higher than Neph due to the milling and entering of the alkaline earth elements into the ring structure .Finally, Nepheline bearing glass samples with chemical composition of 6 wt% Al2O3 -65.5 wt.% SiO2- 13 wt.% Na2O- CaO 16.wt% were used. The amounts of used Nepheline in the glasses compositions were 25 to 38% by weight (depending on the final composition). The glass transition temperatures were 25 ,37 o C increased in GCaNeph and GNaNeph samples respectively. The chemical resistance of GCaNeph and GNaNeph glasses in the present work was 4 times higher than GNeph. The density of GCaNeph glass in the present work was 0.9 g/cm3 higher than GNeph.

Effect of Ti4+ on the structure of nepheline (NaAlSiO4) glass

In this study, the effect of Ti4+ on the structure of nepheline glass (NaAlSiO4) is investigated as SiO2 is systematically replaced with TiO2. Traditionally, TiO2 is considered to be a nucleating agent for silicate crystallization but can also be incorporated into the glass network in relatively large amounts as either a network former or modifier depending on its coordination with oxygen. To determine the effect of Ti4+ on the structure of nepheline glass, X-ray and neutron pair distribution function (PDF) analysis paired with Empirical Potential Structure Refinement (EPSR) were conducted and are supplemented with Raman spectroscopy, electron probe microanalysis, and X-ray absorption spectroscopy (including Extended X-ray Absorption Fine Structure, EXAFS). Through these methods, it has been found that up to 15 mol% (16 wt%) TiO2 can incorporate into the glass network as a four-fold coordinated species, with a minor contribution of higher coordinated Ti. Between NaAlTi0.1Si0.9O4 and NaAlTi0.2Si0.8O4, EXAFS suggests a local structure change in the second coordination sphere of Ti, which changes from Ti-Ti to Ti-Al. Raman spectroscopy also suggests that as Ti content increases, the Na environment becomes more ordered. These results suggest that the Ti activity coefficient and its isotopic fractionation for magnetite and other Ti-bearing minerals should be fairly constant in polymerized melts, such as metaluminous and peraluminous rhyolites.

Mechanoluminescence of NaAlSiO4: Eu, Dy phosphor for developing impact sensor

Mechanoluminescent materials are used for detecting stress distribution in solids. The NaAlSiO4: Eu, Dy phosphor was prepared by solid state reaction method. XRD analysis shows that hexagonal nepheline structure with space group P63. The impulsive excitation technique is used for mechanoluminescence (ML) measurement. The ML intensity initially increases with time, attains a peak value and decreases with time. The time corresponding to ML intensity does not change significantly with increasing impact velocity. The peak ML intensity increases linearly with square of impact velocity. This measurements made suitability of NaAlSiO4: Eu, Dy phosphor for developing stress and damage sensors.

Nepheline structural and chemical dependence on melt composition

Nepheline crystallizes upon slow-cooling in some melts concentrated in Na2O and Al2O3, which can result in a residual glass phase of low chemical durability. Nepheline can incorporate many components often found in high-level waste radioactive borosilicate glass, including glass network ions (e.g., Si, Al, Fe), alkali metals (e.g., Cs, K, Na, and possibly Li), alkaline-earth metals (e.g., Ba, Sr, Ca, Mg), and transition metals (e.g., Mn, and possibly Cr, Zn, Ni). When crystallized from melts of different compositions, nepheline composition varies as a function of starting melt composition. Five simulated high-level nuclear waste borosilicate glasses shown to crystallize large fractions of nepheline on slow-cooling were selected for study. These starting melt compositions contained a range of Al2O3, B2O3, CaO, Na2O, K2O, Fe2O3, and SiO2 concentrations. Compositional analyses of nepheline crystals in glass by electron probe micro-analysis (EPMA) indicate that nepheline is generally rich in silica, whereas boron is unlikely to be present in any significant concentration, if at all, in nepheline. Also, several models are presented for calculating the fraction of vacancies in the nepheline structure.

Effect of Zirconium on the Structure and Congruent Crystallization of a Supercooled Calcium Aluminosilicate Melt

The influence of zirconium as a nucleating agent on the congruent crystallization and relevant physical properties of a supercooled calcium aluminosilicate melt of a composition close to CaAl2SiO6 has been investigated up to 6 mol% ZrO2. Zirconium marginally affects rheological and structural properties, decreasing the viscosity of the Zr‐free melt by no more than 0.25 log unit and, as observed by Raman spectroscopy, not changing significantly the polymerization state of the material. Whereas the Zr‐free melt crystallizes congruently and heterogeneously from the sample surface to yield yoshiokaite, a stuffed derivative of the nepheline structure, addition of zirconia promotes instead bulk crystallization of tetragonal ZrO2 and then of yoshiokaite. The latter process takes place in two stages: dissolved Zr first promotes homogeneous precipitation of zirconia before yoshiokaite crystallizes congruently from a Zr‐depleted volume of melt around zirconia precipitates. This process makes zirconium, and probably other poorly soluble oxides, valuable to control congruent crystallization in silicate glass‐ceramics. From the recorded thermograms, an enthalpy of crystallization of 40 and 46 kJ/mol has been determined at 1060 and 1140 K, respectively, for CaAl2SiO6 yoshiokaite, a very low value that is likely due to the extensive atomic disorder of crystals precipitating at high degrees of supercooling.

Modulated structure of nepheline

The incommensurately modulated structure of a natural nepheline of composition K(0.54)Na(3.24)Ca(0.03)Al(3.84)Si(4.16)O(16) has been determined in superspace. The compound crystallizes in the trigonal centered superspace group X3(00γ)0 with γ = 0.2048 (10), X = (0, 0, 0, 0), (1/3, 2/3, 0, 2/3), (2/3, 1/3, 0, 1/3), a = 17.2889 (8) and c = 8.3622 (10) Å. The structure is characterized by a framework of corner-connected (Al,Si)O(4) tetrahedra. The additional cations are incorporated in two different types of channels of the framework. All atoms in the structure are displacively modulated with amplitudes below 0.1 Å. The modulation can be well described taking into account harmonics of first order only. Atomic positions in the smaller channels of the framework are fully occupied by Na(+). Cationic positions in the larger channel are occupationally modulated, yet the variation of electron density as a function of the internal coordinate t is very small and indicates that the incorporation of different types of cations (K(+), Na(+), Ca(2+)) and vacancies is realised in a highly disordered way. Average T-O distances indicate a nearly complete Al/Si ordering in the tetrahedral framework. A large part of the O atoms are approximated by split-atom positions, which are additionally affected by occupational modulation resulting in a high degree of disorder in the modulated structure. Occupational probabilities for the split-atom positions are complementary. Occupational modulations of the cations in the larger channels and the O atoms of the tetrahedral framework are coupled and correlations between occupational and displacive modulations exist.

Incorporation of water in the structure of nepheline from the data of NMR and IR spectrometry

Incorporation of water in the structure of nepheline from the data of NMR and IR spectrometry

Elastic behavior and pressure-induced structural evolution of nepheline: Implications for the nature of the modulated superstructure

The elastic behavior and the pressure-induced structural evolution of a natural nepheline (K0.54 Na3.24Ca0.03Al4Si4O16) were investigated by in-situ single-crystal X-ray diffraction up to 7.5 GPa with a diamond anvil cell under hydrostatic conditions. As observed in previous studies, at room conditions the diffraction pattern of nepheline includes satellite reflections, whereas the structure refinement to the Bragg reflections confirms that the O1 site is displaced from the triad at (2/3, 1/3, z). The reflection conditions confirm that the space group of the average structure of nepheline remains as P63 throughout the pressure range investigated, and no significant compression of the T-O bonds was measured up to 7.5 GPa. As pressure was increased to around 1 GPa the integrated intensities of the satellites decreased slightly, and at 1.8 GPa no significant intensity of the satellites was detected. Over the same pressure range the O1 site moved toward the triad and thus the tilts of the T1 and T2 tetrahedra decreased. The presence of the subsidiary non-Bragg reflections is therefore related to the split of the O1 site. When the satellites disappear at pressures above 2 GPa, the O1 site is on the triad at (2/3, 1/3, z), corresponding to a straight T1-O1-T2 bond. Below 2 GPa the structure responds to increased pressure by tilting of all four tetrahedra and above 2 GPa by tilting of the T3 and T4 tetrahedra alone. The change in compression mechanism arising from the changes in the O1 position is associated with changes in the compression of the unit-cell axes and the unit-cell volume. The volume can be described by fourth-order Birch-Murnaghan equation-of-state with parameters V0+ = 723.57(4) Å3, KT0 = 47.32(26) GPa, K’ = 2.77(24), and K’’ = 0.758(79) GPa-1. The elastic behavior along the a- and c-axis can be described with a “linearized” fourth-order Birch-Murnaghan equations-of-state, with the following refined parameters: a0 = 9.9911(2) Å, KT0(a) = 43.1(3) GPa, K’(a) = 2.5(3), and K’’(a) = 0.68(8) GPa-1 for the a-axis and c0 = 8.3700(1) Å, KT0(c) = 58.6(3) GPa, K’(c) = 4.0(3), and K’’(c) = 0.85(11) GPa-1 for the c-axis. The pressure-induced structural evolution in nepheline up to 7.5 GPa appears to be completely reversible. The recovery of the modulation upon complete pressure release points to the framework of nepheline having an instability corresponding to a rigid-unit mode with a wave vector corresponding to the observed positions of the satellite reflections

Rigid Unit Modes in disordered nepheline: a study of a displacive incommensurate phase transition

Calculations of the Rigid Unit Modes (RUMs) allowed in the nepheline structure are used to explain the diffuse scattering previously seen in electron diffraction experiments. The RUM calculations also show that the modulation wavelength for incommensurate nephelines is essentially determined by the framework topology. X-ray diffraction is used to measure the intensity of the diffuse scattering as a function of temperature. The diffuse intensity increases sharply at 308 K. This effect is interpreted as being due to the softening of a phonon mode, indicating a phase transition. Measurements of this phase transition below the transition temperature are made using hard mode infrared spectroscopy.

Monoclinic and Hexagonal Nepheline Structures of (Na3/4/K1/4)AlGeO4

Hexagonal (Na3/4K1/4)AlGeO4 crystallizes in the space group P63 with unit-cell parameters a = 10.164 (2), c = 8.540 (2) Å and Z = 8 [wR(F 2) = 0.066 for all 3060 independent reflections]. Monoclinic (Na3/4K1/4)AlGeO4 crystallizes in the space group P21 with unit-cell parameters a = 10.0477 (4), b = 8.5764 (4), c = 10.2118 (4) Å, β = 119.035 (1)° and Z = 8 [wR(F 2) = 0.120 for all 3194 independent reflections measured on a twinned crystal]. Both structures belong to the large family of stuffed tridymites, with the Al and Ge atoms occupying tetrahedral sites, and the alkali atoms occupying the cavities of the tetrahedral framework. Hexagonal (Na3/4K1/4)AlGeO4 is isostructural with the silicate mineral nepheline (Na3/4K1/4)AlSiO4, while monoclinic (Na3/4K1/4)AlGeO4 corresponds to a minor distortion of the nepheline structure. Chemical analysis by electron microprobe and structure determination of flux-grown single crystals indicate that the hexagonal form with the chemical formula (Na0.78K0.19)Al0.97Ge1.03O4 may be stabilized by an alkali deficiency similar to that found in hexagonal natural nephelines. In contrast, all alkali sites are fully occupied in the monoclinic form of composition (Na0.75K0.25)AlGeO4 and the lower symmetry eliminates the oxygen disorder present in the hexagonal form.

Thermodynamics of feldspathoid solutions

We have developed models for the thermody-namic properties of nephelines, kalsilites, and leucites in the simple system NaAlSiO4−KAlSiO4−Ca0.5AlSiO4−SiO2−H2O that are consistent with all known constraints on subsolidus equilibria and thermodynamic properties, and have integrated them into the existing MELTS software package. The model for nepheline is formulated for the simplifying assumptions that (1) a molecular mixing-type approximation describes changes in the configurational entropy associated with the coupled exchange substitutions □Si?NaAl and □Ca? Na2 and that (2) Na+ and K+ display long–range non-convergent ordering between a large cation and the three small cation sites in the Na4Al4Si4O16 formula unit. Notable features of the model include the prediction that the mineral tetrakalsilite (“panunzite”, sensu stricto) results from anti-ordering of Na and K between the large cation and the three small cation sites in the nepheline structure at high temperatures, an average dT/dP slope of about 55°/kbar for the reaction $$$$ over the temperature and pressure ranges 800–1050 °C and 500–5000 bars, roughly symmetric (i.e. quadratic) solution behavior of the K–Na substitution along joins between fully ordered components in nepheline, and large positive Gibbs energies for the nepheline reciprocal reactions $$$$ and $$$$ and for the leucite reciprocal reaction $$$$

Superstructures and domain structures in natural and synthetic kalsilite

Transmission electron microscopy (TEM) investigation of natural kalsilite, (K,77Doo9Na,4b(Si209Alu9Feo12bOg, from San Venanzo, Italy, shows that it contains a pervasive domain structure related to a superstructure. The probable space group for the superstructure is P21. The relationship between the unit -cell parameters of the P6, subcell and those of the P21 supercell is a,upec ~ b,upec ~ v'3a,ub; c,upec = Cmb,and the a,upec axis is rotated 30° about the C axis with respect to the a,ubaxis. Nonperiodic distribution of elongated twin domains causes streaking of superstructure reflections normal to the c* axis. The average structure of this kalsilite sample has P6, symmetry. On the basis of previous structure refinements, this average structure has three 01 positions slightly displaced from the threefold axis, and each 01 position has a site occupancy of V3.High-temperature annealing studies indicated that there is a displacive phase transition from the P21 superstructure to the P63 substructure. The twin domains are related by threefold twin operations. The phase-transition temperature for kalsilite may range from 500 to 600°C and is dependent on the crystal composition (e.g., the proportions of Na, Ca, and vacancies). Kalsilite crystals prepared from nepheline by Na-K exchange are structurally complex. Domain structures in Na-bearing kalsilite crystals with more than 2.5 mol% NaSiAI04 are similar to those in the natural kalsilite sample but exhibit additional twin boundaries parallel to the c axis. Na-poor and Na-free kalsilite crystals are composed of (0001) domains with P63 and P31c symmetries, respectively, and merohedral twins between the neighboring P63 or P31c domains. The merohedral twins in the synthetic crystals formed during transformation of the nepheline structure to the kalsilite structure. The intergrown domains with P31c symmetry formed during the phase transition on cooling of the Na-poor kalsilite. There is no simple symmetry hierarchy among the known structures in the proposed phase diagram.

Large range of nonstoichiometry in barium ortho alumbvate prepared through gel-to-crystallite conversion

Nonstoichiometric barium ortho aluminate with the general formula Ba 1-xAl 2O 4-x (0 < x < 0.56) was synthesized by the thermal decomposition of barium aluminium oxyhydroxide prepared through gel-to-crystallite conversion method. Phase singularity was confirmed by x-ray powder diffraction. The large range of nonstoichiometry is explained in terms of Ba 2+ vacancies within the corner sharing AlO 4 network, whereby the nepheline structure extends into the “stuffed” tridymite lattice. HREM studies indicated the ordering of point defects leading to the formation of extended defects and stacking faults. The coordination of aluminium remained tetrahedral as revealed by 27Al MAS NMR. On doping with Eu(II), Ba 1-x-yEu yAl 2O 4-x+y (0.005 < y < 0.05) shows the emission maximum around 500nm, the intensity of which increases with the x-value.

Phase relations in the system NaAlSiO4-NaGaSiO4

Phase relations in the system NaAlSiO4-NaGaSiO4 to 945° C at 1 kbar P(H2O) are dominated by stability of Na(Al,Ga)SiO4 with the beryllonite-type structure. The nepheline structure is restricted to NaAlSiO4-rich compositions at moderate and high temperature. Structure-composition relationships are controlled by space-fitting requirements of both framework and cavity cations, as in related systems. The two-phase (nepheline-type+beryllonite-type) field has been delineated from the end-member NaAlSiO4 composition up to the peritectic point at about 945° C (and 60 mol% NaGaSiO4), using a volume-composition relationship for the beryllonite-type phase, phase appearance, and electron microprobe analysis. At end-member NaGaSiO4 composition, the beryllonite-type phase is stable to the melting point (902±5° C). At end-member NaAlSiO4 composition, the beryllonite-type⇌nepheline-type transformation occurs at 348±2° C, and is associated with an increase in molar volume of 2.4% and enthalpy of 5170±40J·mol−1. Thus, end-member NaAlSiO4 nepheline, and probably all sub-potassic nephelines as well, are metastable at very-low geological temperatures.

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,K0.3,□)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).