Alkali Sites Research Articles

Variable water content of nepheline from Somma-Vesuvio, Italy

The water content of nephelines from volcanic, subvolcanic and metamorphic parageneses of Somma-Vesuvio, Italy was studied by polarized FTIR microspectrometry on oriented (0001) crystal plates. The H(in2)O content varies from 0.05 to 0.39 wt% and is mainly controlled by the number of vacancies in the alkali site. Most samples show an inhomogeneous distribution of the H2O molecules, oriented with their H-H-axis perpendicular to [0001]. The absorption behaviour in the region of the OH-stretching fundamentals is characterized by four spectra types. The “high temperature spectrum” of a thermally treated nepheline reported by Beran and Rossman (1989) also occurs in some natural samples from the metamorphic paragenesis of Somma-Vesuvio.

The effects of ferromagnesian components on the paragonite‐muscovite solvus: a semiquantitative analysis based on chemical data for natural paragonite‐muscovite pairs

ABSTRACTChemical data for 139 natural paragonite‐muscovite (Pg‐Ms) pairs illustrate the effects of ferromagnesian components on the P‐T‐X topology of the Pg‐Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of the P‐T conditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg‐Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end‐member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure‐induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site ‐ to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end‐member paragonite with increasing pressure. Broadening of the Pg‐Ms solvus with increasing pressure reflects increasingly nonideal Na‐K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg‐phengitic‐Ms pairs, it is concluded that, at the present time, Pg‐Ms solvus thermometry is only viable for quasibinary Pg‐Ms pairs.

NMR studies of the leucite analogues X2YSi5O12, where X= K, Rb, Cs; Y = Mg, Zn, Cd

A number of leucite group materials with the formula X2YSi5O12 where X=K, Rb, Cs and Y=Mg, Zn, Cd have been synthesized by dry and hydrothermal crystallization of glass starting materials. 29Si MASNMR has been used to obtain structural information, such as the number of distinct tetrahedral sites, degree of cation ordering, and estimates of the mean T-O-T bond angles of the tetrahedra. X-ray powder diffraction gave information on cell volumes and degree of distortion from cubic symmetry for all the samples and space group and structural information for some samples. Integration of the different length-scale data obtained using these two complementary techniques allows greater reliance to be placed on the structures deduced for these leucite samples, which are only available as fine-grained powders. Hydrothermally synthesized K2MgSi5O12, K2ZnSi5O12 and Rb2ZnSi5O12 have structures with 12 distinct tetrahedral sites (T-sites) and are monoclinic P21/c while the dry-synthesized equivalents are disordered with single T-sites and are cubic, Ia3d. Most of the other members of the group have structures with 6 tetrahedral sites with Cs2CdSi5O12 being orthorhombic, Pbca. Cs2ZnSi5O12 has an intermediate “5+2” structure. Decreasing the size of the X+ cation for a given Y2+ cation gives more collapsed and distorted frameworks. 133Cs NMR was used to show that samples with 6 T-sites have 2 alkali sites. It is deduced that samples with 12 T-sites will undergo a displacive phase transition to a 6 T-site structure (possibly via a 5+2 intermediate in some cases) with no change in the framework topology or degree of T-site ordering.

Effect of hydrogen reduction temperature on the surface chemistry of pure and promoted magnetite

The first and repeated isotherms were measured in hydrogen reduced magnetites, using nitrogen, carbon monoxide and carbon dioxide as adsorbates. Based on a modification of the method developed by Emmett and Brunauer (P.H. Emmett and S. Brunauer, J. Am. Chem. Sot., 59 (1937) 310–315), we were able to estimate the fraction of the total surface area covered by iron, alkali and oxygen sites. In pure magnetite, as the reduction temperature increases, iron covers a higher percentage of the total surface area at the expense of oxide containing sites. By contrast, in promoted magnetites, the fraction of the surface covered by iron remains constant with reduction temperature. There is, however, an increase in the alkali surface area fraction at the expense of the oxygen-containing sites. Volumetric measurements suggest that in pure magnetite the adsorption of carbon monoxide is linear and that one of every two adsorbed carbon monoxide molecules remains chemisorbed on the iron.The latter indicates that a “bridge” type of adsorption predominates over chemisorption. Alkali promotion seemsto inhibit the development of the linearly adsorbed carbon monoxide molecule, and enhances the amount of carbon monoxide chemisorbed. This effect increases with increasing reduction temperature and promoter addition.

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).

Electronic structure of alkali-pnictide compounds

The authors present an investigation of the electronic structure and the crystal binding in a number of metallic and semiconducting alkali-pnictide compounds based on self-consistent linear-muffin-tin-orbital (LMTO) calculations. They show that at all compositions the electronic structure is dominated by the strong attractive potential of the pnictide anions. In compounds with the 'octet' composition A3B (A=alkali metal, B=Bi, Sb) they find a narrow gap separating the highest occupied anion band from the lowest empty cation band. The large difference in valence leads to a very large negative excess volume and a high electronic pressure on the alkali sites. This large electronic pressure leads to a lowering of the (n-1)d states relative to the ns states, especially for the heavy alkalis. As a consequence, the ionic gap in the octet compounds is very narrow and varies in a non-monotonic way in the series (Li, Na, K, Rb, Cs)3-(Sb, Bi). At the equiatomic composition, the alkali-Sb compounds contain infinite spiral Sb chains stabilized by strong (pp sigma ) interactions. The bands close to the Fermi level are formed by bonding, non-bonding, and antibonding Sb p states, with the Fermi level falling into the gap between the non-bonding and the antibonding band. The Bi-rich alkali-Bi compounds are metallic, but the electronic density of states still bears the signature of the strong Bi potential.

Magic angle spinning NMR observation of sodium site exchange in nepheline at 500° C

Dynamics in minerals at time scales from seconds to microseconds are important in understanding mechanisms of displacive phase transitions, diffusion, and conductivity. High resolution, magic-angle-spinning (MAS) NMR spectroscopy can directly show the rates of exchange among sites, potentially providing less model-dependent information than more traditional NMR relaxation time measurements. Here we use a newly developed high temperature MAS probe (Doty Scientific, Inc.) to observe the exchange of Na+ among the alkali sites in a high-Na nepheline at temperatures as high as 500° C. Observed exchange rates are consistent with correlation times derived from cation diffusivity.

Solubility of jarosite solid solutions precipitated from acid mine waters, Iron Mountain, California, U.S.A. Solubilité des solutions solides de jarosite précipitées à partir des eaux minières acides, Iron Mountain, Californie, U. S. A

Seven samples of acid mine drainage (initial pH 0.8 to 1.4) from the Hornet and Richmond massive sulfide ore deposits at Iron Mountain, West Shasta district, California, were stored for 1 1 to 13 years at 22 ± 3 °C. A yellow precipitate (Munsell color 2.5 Y 7/8) which formed in each sample is identified by powder X-ray diffraction, atomic absorption spectrophotometry, and thermogravimetric analysis as jarosite solid solution (jss) having the general formula K(x)Na(y)(H₃O)(j.x-y)Fe₃(SO₄)₂(OH)₆, where .53 < x < .83, .03 < y < . 1 5, and .10 <(1-x-y) <.32. It is assumed that at the time of collection in 1974-76, the mine waters were somewhat undersaturated with respect to jss ; subsequent oxidation of aqueous ferrous iron during storage has caused saturation and precipitation of jss. The jss crystals are 5 to 10 micrometers in diameter and rhombohedral in form. The well crystallized nature of the precipitate suggests that the oxidized mine waters have approached thermodynamic equilibrium with jss and therefore can provide useful data for the evaluation of thermodynamic properties of К-Н₃О-Na substitution in jarosite. Because of the common occurrence of 15 to 25 mole percent hydronium substitution on the alkali site in jarosites, it is necessary to consider the hydronium content of jarosites in any attempt at rigorous evaluation of jarosite solubility or of the saturation state of natural waters with respect to jarosite. A Gibbs free energy of 3293.5 ± 2. 1 kJ mol⁻¹ is recommended for a jarosite solid solution of composition К₇₇Na₀₀₃(H₃O) ₂₀Fe₃(SO₄)₂(OH)₆. Solubility determinations for a wider range of natural and synthetic jarosite solid solutions will be necessary to quantify the binary and ternary mixing parameters in the (K-Na-H₃O) system. In the absence of such studies, molar volume data for endmember minerals indicate that the K-H30 substitution in jarosite is probably closer to ideal mixing than either the Na-K or Na-H₃O substitution.

Contrasting oxygen diffusion in nepheline, diopside and other silicates and their relevance to isotopic systematics in meteorites

Oxygen self-diffusion coefficients have been determined in natural nepheline Na 3(Na, K)[Al 4Si 4O 16] and synthetic diopside CaMg(SiO 3) 2, at one atmosphere, by monitoring the rate of 18O exchange between CO 2 gas and minerals of pre-determined grain size (10, 12.5, and 15 μm radius). Oxygen self-diffusion in nepheline between 1000° and 1300°C fits a linear Arrhenius relation of the form: D = 5.9 × 10 −9 exp[(−25.0 ± 2.5) × 10 3 cal RT ] ; and for diopside between 1150° and 1350°C: D = 6.3 exp[(−96.7 ± 5.8) × 10 3 cal RT ] . The activation energy for oxygen diffusion in nepheline is similar to that of melilite (31.9 ± 0.1 kcal/mol), but the oxygen diffusivity is lower by two orders of magnitude. The diffusion of oxygen in diopside is similar to that of nepheline at 1375°C; however, the activation energy is almost four times higher. This contrast suggests a difference in the crystal chemical and structural controls of the minerals. Both nepheline and melilite are characterized by strongly distorted alkali sites within their frameworks, which could account for their relatively low activation energies and high diffusivities. More importantly, nepheline and melilite have sheet-like structures and high anionic porosities which may provide ready migration paths for oxygen. The concentration of open space between the silicate sheets in melilite may explain the greater diffusivity of oxygen in melilite than in nepheline. Diopside, forsterite, anorthite, spinel are characterized by high activation energies (80 to 100 kcal) and are refractory with very stable bonds and low anion porosities. Furthermore, these minerals are much denser and closer packed than nepheline or melilite making it difficult for oxygen to migrate within the crystalline structure and have access to the cation-oxygen bonds. Oxygen diffusion studies constrain interpretations of post-solidus oxygen exchange among minerals and may be used to explain the stable isotope systematics observed in Ca-Al rich inclusions of carbonaceous chondrites. The slow exchange and diffusion of oxygen in diopside suggests that the isotopic composition of clinopyroxenes in Ca-Al rich inclusions of carbonaceous chondrites has not changed appreciably since their incorporation into the meteorite. However, minerals such as nepheline and melilite, which readily exchange oxygen, record the isotopic composition of the last nebular gas to which they were exposed.

Low-temperature alteration of volcanic glass: Hydration, Na, K, 18O and Ar mobility

The low-temperature alteration of siliceous volcanic glass by meteoric water involves considerable hydrogen exchange for Na + or K + ions. This can occur with little alteration of the other major- or trace-element composition of the glass. As much as 40% of the alkali sites can be occupied by H + ion. Most water in these glasses is present as molecular water rather than as hydroxyl groups. This hydration is accompanied by oxygen isotope exchange resulting in an oxygen isotopic enrichment to over 20‰ (relative to SMOW) for this suite of samples. The high degree of alkali mobility during this alteration process affects the KAr age dating method. In these glasses, Ar appears to be less mobile than alkali ions, resulting in discrepant KAr ages for glass with significant hydration and oxygen isotopic exchange.

Photon-induced ESR centers associated with p-block elements in oxide glasses

ESR of Tl 2+ in DMSO-H 2O glass and alkali silicate glasses were studied to examine the probing ability of the (Tl +) + ESR center for alkali sites and the chemistry of “inert pair” characteristic of p-block elements in a lower valency state. Tl 2+ reflected sensitively the distortion of the coordination sphere of alkali sites even in the “weak” network of DMSO-H 2O glass. Two distinct sites were detected for alkali sites in silicate glasses; one is in highly anisotropic environments (α) and the other in nearly spherical symmetry (β). It was found that “inert pair” is not inert stereochemically in α sites.

Upper-mantle amphiboles: a review

AbstractTextural and chemical data are reviewed for amphiboles occurring in Cr-diopside series and Al-augite series xenoliths; as megacrysts in a variety of igneous environments; and also as minor phases in tectonic slices of upper-mantle peridotite. New data are given for pargasite and associated phases in a lherzolite from Tanzania and in kelyphite replacing garnet in a S. African kimberlite xenolith. It is concluded that only pargasites and titaniferous pargasites occurring in Cr-diopside series blocks have formed under upper-mantle conditions. Although amphibole is present in the upper mantle, as suggested by Oxburgh (1964), its relative paucity suggests it is not a major alkali and water storage site; phlogopite is a more likely candidate, particularly in the deeper parts of the upper mantle.

Structure energies of the alkali feldspars

Structural energetics of the alkali feldspars have been studied using a “lattice” or structure energy model. Electrostatic energies, Ue,for 20 well-refined, non-intergrown alkali feldspars were calculated using Bertaut's (1952) summation procedure and average about −13,400 kcal/mol; the repulsive energies of the alkali site in each structure (∼15 kcal/mol) were calculated using repulsive parameters for K-O and Na-O interactions estimated from bulk modulus data for NaF and KF and the exponential form of the repulsive potential. Using a procedure in which the position of the alkali cation was varied while the oxygen cage was kept fixed, structure energy gradients for the alkali sites of high albite and a hypersolvus Ab42Or58 structure were computed. In both cases, a broad structure energy well, elongated approximately parallel to c and subparallel to the observed split Na positions, was found. In both structures there is a single energy minimum corresponding closely with the observed single alkali positions. Comparison of Ue values for the alkali feldspars with different K/Na ratios shows that intermediate compositions are predicted to be less “stable” than either endmember and that the potassic end-member is predicted to be less “stable” than the sodic one, assuming that all other factors contributiong to the free energies of each phase are approximately the same. Comparison of Ue values for the high albite and low sanidine structures with different Al/Si distributions and a fixed tetrahedral framework indicates that the ordered charge distributions are 63.0 and 54.8 kcal/mol, respectively, more “stable” than the disordered distributions. Smaller, more realistic energy differences were obtained by using Uevalues averaged from four separate calculations with a +3 charge on a different T site in each and with +4 charges on the other T sites. If, in addition, the charges on cations and oxygen are reduced to half their nominal formal charges, in agreement with Pauling's electroneutrality principle and the results of recent molecular orbital calculations on silicates, the predicted electrostatic energy differences are reduced to 3.6 and 1.6 kcal/mol, respectively. These calculations also indicate that the T1O site in the high albite structure energetically favors Al and that the Al/Si distribution determines the Na position within the alkali site.

A nepheline-alkali feldspar geothermometer

A nepheline-alkali feldspar geothermometer has been developed which is based on the thermodynamics of an Na—K exchange reaction between nepheline and alkali feldspar. The activities are formulated in terms of site occupancies, and the regular solution model is used to represent non-ideal mixing of the cations on each site. The distribution of Na and K on the alkali sites in nepheline is calculated from published nepheline-brine ion-exchange data. The standard Gibbs energy of the geothermometer reaction is calculated from experimental data on coexisting nephelines and alkali feldspars (Hamilton and MacKenzie, 1960, 1965). The geothermometer is applied to nepheline syenites from the Precambrian Igdlerfigssalik intrusion, S.W. Greenland, and gives temperatures which suggest that nepheline and alkali feldspar continue to equilibrate with cooling after they crystallise from the magma.

Alkali feldspars from part of the Galway granite, Ireland

SummaryThe structural states of alkali feldspars from the eastern end of the Galway Granite show transitions from orthoclase to microcline as a result of late deformation. Analyses of twenty samples for SiO2, Al2O3, TiO2, Fe2O3, CaO, Na2O, K2O, S, Cu, Zn, Ga, Rb, Sr, Ba, and Pb agree with previously determined orders of entry of elements into the ‘alkali site’. A relationship between this order of entry and the melting points of feldspar end-members is discussed.

Metastable Solid Solution with Nepheline-type Structure in the CaO–Al2O3–SiO2 System

Abstract The metastable nepheline-type solid solution in the CaO–Al2O3–SiO2 system shows the best crystal development upon devitrification on the CaAl2O4–CaAl2Si2O8 join. The crystallization occurs through the devitrification of glass at relatively low subsolidus temperatures, around 1000°C in the air. The X-ray powder diffraction patterns of the solid solution can be indexed on the basis of a hexagonal unit cell of the nepheline structure without exception. The cell parameter a of the solid solution is similar to that of nepheline, while its parameter c is smaller than that of nepheline and corresponds to that of high-temperature tridymite. When the cell parameters of the solid solution on the join are plotted against the composition, there is a break in the slope at Ca6Al12Si4O32, though the cell volume varies continuously. The nepheline-type solid solution on the join may be expressed as Ca8−(1⁄2)x\Box(1⁄2)xAl16−xSixO32, where \Box means the interstitial vacant site. The relationship between the cell parameter and the composition suggests that there should be two kinds of Ca-sites in the structure of the present solid solution, and that these kinds correspond to the alkali sites of two different sizes in the nepheline structure.

Possible splitting of energy levels of atoms or ions in octahedral crystals due to sets of equivalent neighbouring defects

Divalent ions when doped in alkali halide single crystals will go in alkali sites and form associate pairs with neighbouring cation vacancies or other impurity defects. As there will be sets of equivalent positions for such associated pairs, their energy levels will be degenerate. The reducible representations of these associated pairs involving first to sixth nearest neighbours have been reduced to the corresponding irreducible representations of the point group Oh, for cases having rock-salt structure as well as for cases having cesium chloride structure. It is pointed out that if there is a rapid hopping of the vacancies or the defects in the crystal one may expect because of the possible interactions in the system, a splitting of the energy levels of the pairs as indicated by the different irreducible representations. It is stated that this kind of splitting may be expected also in systems in which associated magnetic pairs are formed by two neighbouring spins.