A computer simulation study of Al/Si ordering in gehlenite and the paradox of the low transition temperature

The ordering of Al and Si in gehlenite is considered using computer simulation. The enthalpy of ordering ΔH per 2Al+2Si atoms is found to be 0.52 eV. It is dominated by the nearest neighbour interaction, but the analysis is carried out to fifth neighbours. The nearest neighbour interaction differs significantly from that for other materials. The structure does not have a connected network of ordering sites, which mainly accounts for the unobservably low transition temperature for Al/Si ordering. Two alternatives are given for the likely ordering pattern.     

Thermodynamics of sodium feldspar II: Experimental results and numerical calculations

The free energy, entropy and enthalpy of sodium feldspar in thermal equilibrium and in metastable states are derived from investigations of the heat capacities of albite, analbite, ordered and disordered Or₃₁. The lattice strains of all stable and metastable states are calculated from the two-order parameter theory published in the preceding paper. This approach also allows one to distinguish between the influence of Al, Si order and of the displacive lattice distortion on the thermodynamic behaviour of albite. The thermal cross-over between high albite and low albite is found to be a smooth function of temperature.     

Computer modelling of Al/Si ordering in sillimanite

Computer simulation is used to investigate the effect of Al/Si disordering over the tetrahedral sites on the lattice energy and the lattice constants of the mineral sillimanite Al₂SiO₅. A methodology for an atomistic assessment of the energy of the reaction 2(Si-O-Al)→(Si-O-Si)+(Al-O-Al) and its various contributions is established. This ordering energy is 0.97 eV for nearest neighbour sites in the ab-plane and 0.56 eV for those separated in the c-direction. The large difference is due to a greater constraint on the atomic relaxation in the ab-plane and shows the structural dependence of the ordering energy. Its magnitude appears to be determined by a complicated balance between Coulomb and short-range repulsive energy involving strain over many bonds, both in the ordered and disordered structures. There is also a significant interaction between second neighbour sites whereas the contribution of more distant neighbours is negligible. The lattice energies of most of the 154 configurations studied show a linear behaviour as a function of short-range order, specified by the number of Al-Al pairs. The ordering temperature T_c, estimated on the basis of a statistical mechanical model of disordering, and the calculated ordering energies are in semi-quantitative agreement with experimental values.     

Atomic movements in plagioclase feldspars: Kinetic interpretation

The diffraction, n.q.r. and optical data on plagioclase feldspars are used to derive kinetic interpretations of structural changes induced by laboratory heat treatment and by geological processes. For anorthite, the Si, Al configuration is essentially ordered except for unusual transient processes. Cooperation between Ca atoms, and random nucleation, produces a domain texture in the primitive structure which is highly sensitive to temperature. The rapid inversion from the primitive to the body-centered structure is explained by increasingly rapid “rattling” of the Ca ions in the interstices of the semi-flexible alumino-silicate framework. The weakening of “b” reflections at higher temperatures is ascribed to incipient Si, Al disorder associated with irregular vibration of the alumino-silicate framework and the Ca atoms. Quenching phenomena are explained by variation of the domain boundary texture inherited from disorder at high temperature. For albite, the Si, Al configuration changes sluggishly from an ordered to a disordered pattern, and vice versa. Kinetic data are reinterpreted using a model in which the cell dimensions depend on local rather than distant order: the major change in distant order is deduced to occur at 450–600° C. Sodic plagioclase grown at high temperature shows distant disorder of the atoms, but cell dimensions suggest development of strong local order for calcic compositions. Low-entropy plagioclases of intermediate composition show complex intergrowths and domain structures because of kinetic barriers to atomic diffusion. X-ray diffraction data for slowly-cooled specimens are consistent with nucleation of albite- and anorthite-like regions from a high-temperature disordered phase. Electrostatic energy calculations show that Na and Ca atoms, although they face smaller energy barriers for diffusion, cannot form domains until the Si and Al atoms have moved jointly. The Si, Al ordering patterns of low albite and anorthite are topologically incompatible in a continuous framework if oxygen is not to be bonded to two Al. Therefore domains of low-albite and anorthite must be separated by disordered boundaries. For intermediate compositions, An₁₅-An₇₅, domains remain small. The anorthite-like domains probably form at higher temperatures than the albite-like domains. The latter tend to be about the same size for all bulk compositions. The atomic positions are influenced by neighboring atoms. Upon heating rapidly, Si and Al atoms remain in position and provide a memory for reformation of an identical structure upon cooling. The framework changes shape, and some Na, Ca atoms inter-diffuse to yield a quasi-homogeneous structure with a diffraction pattern which qualitatively approaches that of high albite. Upon prolonged heating at high temperature, Si, Al atoms inter-diffuse producing nonquenchable changes to the high-albite structure. At Na-rich bulk compositions, some domains of low albite grow into large lamellae while others remain small in contact with anorthite domains producing alternate lamellae of intermediate structure type; hence the peristerite intergrowth. A similar but opposite process could cause an intergrowth of lamellae of anorthite structure interposed with an intermediate type structure. A unique low plagioclase series is not expected. Plagioclases of intermediate composition trend towards slightly different endproducts depending on the details of the cooling history. Breaks and bends in plots of physical properties, and intergrowths for certain specimens, depend on special compositional, growth and annealing factors. The intergrowth responsible for iridescence of intermediate plagioclase is ascribed to Na, K segregation prior to development of the complex domain structure. Prolonged annealing at high temperature in a dry environment is suggested. It is futile to attempt to describe low entropy plagioclases in terms of classical thermodynamics: only a kinetic interpretation based on atomic and sub-microscopic textural factors can be viable.     

Mechanisms and geochemical significance of Si–Al substitution in zeolite solid solutions

Rock-forming zeolites often exhibit complex solid solutions reflecting isomorphous substitutions between Si and Al in tetrahedral framework sites, between charge-balancing extraframework cations, and between water molecules and vacancies. Although the number of moles of charge on extraframework cations in a zeolite must equal the moles of Al in order to maintain charge balance, the relationships between Si–Al and extraframework substitutions vary considerably across this mineral group. Review of available compositional data suggests that there are three main modes of Si–Al substitution in zeolites: 1) coupled CaAl–NaSi substitution; 2) coupled substitution of a single extraframework cation plus Al for Si; and 3) completely uncoupled substitution among extraframework cations and Si and Al on tetrahedral sites. Among zeolites that exhibit the latter two modes of solid solution, Si–Al substitution can be described by an SiO₂ (± H₂O) compositional exchange vector from a hypothetical, pure-silica endmember composition. Recent calorimetric, structural, and theoretical investigations suggest that Si–Al substitution follows a non-ideal, athermal solution model characterized by no excess enthalpies of mixing and negative excess entropies of mixing. Because Si–Al exchange in these minerals can be explicitly or implicitly described by exchange of an SiO₂ component, the Si/Al ratio in their framework can be predicted solely as a function of temperature, pressure, and the chemical potential of SiO₂. Application of this model leads to calculated Si/Al ratios in stilbite (coexisting with albite), analcime, and chabazite consistent with observed mineral compositions and parageneses in very low-grade metamorphic environments. Coexistence of silica polymorphs with zeolites containing SiO₂·nH₂O exchange vectors potentially provides a means of performing thermobarometric calculations in very low-grade metamorphic and diagenetic environments.     

Al/Si interdiffusion in albite: effect of pressure and the role of hydrogen

The effect of pressure on the rate of Al/Si disorder in albite has been determined at temperatures from 800° C to 1050° C and at pressures up to 24 kbar, using dried samples in welded Pt containers, in piston-cylinder devices and internally-heated gas apparatus. In the piston-cylinder device with NaCl medium, the effect of pressure is profound. A pure low albite from Clear Creek, California reaches the equilibrium state of disorder at 850° C and 22 kbar in 10 h, whereas at 6 kbar it has not equilibrated in three weeks, and at one bar it probably cannot be disordered at 850° C in the laboratory. The enhancement of Al/Si interdiffusion takes place under dry conditions: any H₂O penetrating the samples would have produced melting, and none was observed. Hydrogen, however, is produced by dissociation of moisture in the pressure medium and can penetrate the Pt sample capsules. If the samples are deprived of hydrogen by replacing NaCl with glass or by embedding the samples in a hydrogen getter such as Fe₂O₃ or ZnO, the order-disorder reaction is greatly inhibited.A mechanism is suggested in which OH^– groups are formed by hydrogen hopping in albite. The smaller charge on the tetrahedral complex induces transient coordination of Al greater than four at elevated pressures, facilitating Al/Si interchange during the bond-breaking process. This results in an exponential pressure dependence of diffusion. It is also suggested that the presence of hydrogen at high pressures can greatly effect the mechanical properties and reactivity of deep crustal and mantle rocks. In nature, as well as in the laboratory, equilibration at elevated pressures may be as much dependent on the availability of hydrogen as on time or temperature.     

Thermodynamics of sodium feldspar I: Order parameter treatment and strain induced coupling effects

The thermodynamic behaviour of Na-rich feld-spars is derived from a Landau-type free energy expression which contains two order parameters. One order parameter describes the structural distortion during the displacive phase transition between monalbite and analbite, the other the Al, Si order. The structural phase transition \(C2/m - C\bar 1\) in sodium feldspar under thermal equilibrium is due to both order parameters and is accompanied by a crossover between a thermal regime with low Al, Si order (high-albite) and a thermal regime with high Al, Si order (low-albite). The crossover is a smooth function of temperature. Simple formulae are given for the calculation of thermodynamic potentials for thermal equilibrium and for metastable phases with temperature independent Al, Si order.     

Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry

Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria:

1. edenite+4quartz=tremolite+albite
2. edenite+albite=richterite+anorthite

For A and B all possible within-site and cross-site interactions among the species □?K?Na?Ca?Mg?Fe²⁺?Fe³⁺?Al?Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400–1000°C and 1–15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.     

Determination of the origin and magnitude of Al/Si ordering enthalpy in framework aluminosilicates from ab initio calculations

Ab initio total energy calculations based on a new optimised oxygen pseudopotential has been used to determine the enthalpy of disorder for the exchange of Al and Si in tetrahedral coordination in simple derivative aluminosilicate structures based on the high temperature tridymite structure. The problem has been studied as a function of defect interaction, and defect concentration, and the results indicate that the energy for Al/Al neighbouring tetrahedra can be assigned primarily to two effects, the first, a coulombic effect, associated with the disturbed charge distribution, and the second associated with the strain related to misfit due to the very different dimensions of the Si and Al containing tetrahedra. In practice each of these effects contributes approximately 0.2 eV per Al−Al neighbour to the overal disorder enthalpy. These simple results were obtained after a careful study of possible chemical interaction between adjacent Al/Si containing tetrahedra which showed that chemical interaction was effectively absent. Since individual Al/Si tetrahedra proved to be discrete entities that are individually heavily screened by the shared oxygens it follows that coulombic and strain effects in disorder effectively account for the whole of the disorder enthalpy. The complete set of results have been used to establish new criteria for the structure and disorder enthalpies of the feldspar group of minerals and their long period derivatives. Received: October 7, 1996 / Revised, accepted: March 17, 1997     

Kinetics of Hydrothermal Reactions of Minerals in Near-critical and Supercritical Water

This work presents new experimental results on the kinetics of mineral dissolution in near-critical and supercritical water in a temperature range (T) from 25 to 400℃ and a constant pressure of 23 MPa. Kinetic experiments were carried out by using a flow reactor (packed bed reactor) of an open system. The dissolution rates of albite and magnetite were measured under these experimental conditions. Na, Al and Si release rates for albite dissolution in water were measured as a function of the temperature and flow velocity in the reaction system. The maximum release rates of Na, AI and Si of albite dissolution in the hydrothermal flow systems under different flow velocities were always obtained at 300℃, that is to say, the maximum albite dissolution rates in the flow systems, regardless of different flow rates, were repeatedly measured at 300℃. Results indicate a wide fluctuation in albite dissolution rates occurring close to the critical point of water. The dissolution rates increased when the temperatu     

偏岭石结构研究

用广角度X射线散射 (WAXS)和由它得到的径向分布函数 (RDF)研究了偏岭石的结构。偏岭石是一种以非晶组份为主的部分有序的中间序性物质。构成偏岭石的结构单元是四面体 [MO₄ ](M =Si,Al)和八面体 [AlO₆](O =O,OH)。偏岭石结构中近邻四面体与四面体共顶连接构成M-O层,八面体与八面体共棱或共面连接构成Al-O层;而四面体与八面体通过不规则的共顶连接构成近程层状结构。它的近程结构将不随序性或结晶性变化。它的微结构及其随热处理温度的变化主要决定于结构基元之间的关系和序性。偏岭石是结构不同于变高岭石的一种粘土.     

Structure energies of the alkali feldspars

Structural energetics of the alkali feldspars have been studied using a “lattice” or structure energy model. Electrostatic energies, U _e,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 U _e 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 U _e 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 U _evalues 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.     

Mineral Surface after Reaction with Aqueous Solution at High Temperatures and Pressures

This work presents new experimental results on surface chemistry of reacting minerals and interface kinetics between mineral and aqueous solutions. These experiments were carried out using a flow reactor (packed bed reactor) of an open system as well as a continuous stirred tank reactor, CSTR. The authors measured reaction rates of such minerals as zeolite, albite and carbonate (rhodochrosite, dolomite) in various solutions, and tested corresponding mineral surface by using SEM, XPS, SIMS, etc. This paper mainly presents the experimental results of zeolite dissolution in water and in low pH solutions at room temperature, and dolomite dissolution at elevated temperatures. The results show that the release rates of Si, Al and Na of zeolite are different in most cases. The incongruent dissolution of zeolite is related to surface chemical modifications. The Na, Al and Si release rates for dissolution of albite and zeolite in water and various solutions were measured as a function of temperature, flow veloci     

Structural states of Mg-cordierite I: Order parameters from synchrotron X-ray and NMR data

The hexagonal to orthorhombic phase transition in synthetic Mg-cordierite has been studied by (i) measuring the spontaneous strain associated with the transition using Synchrotron X-ray powder diffraction and (ii) measuring the degree of Al, Si order in terms of the number of Al-O-Al bonds per formula unit using solid state NMR spectroscopy. This defines the two order parametersQ andQ _od respectively, and their relationship as a function of annealing temperature and time is used to define the structural states of cordierite during the ordering sequence. The formation of modulated hexagonal cordierite within which a high degree of Al, Si order can be attained, results in a strongly non-linear relationship betweenQ andQ _od .The transition from modulated to orthorhombic cordierite is strongly first-order under all temperature conditions studied and involves a large step inQ, whileQ _od changes continuously throughout the ordering sequence with no marked discontinuity at the phase transition. The lattice distortion, traditionally defined in cordierite by the Δ index provides no full information on the degree of Al, Si order in anhydrous Mg-cordierite, and both order parameters must be used to define its structural state. Transmission electron microscopy has been used to study the mechanism of the transformation from hexagonal to modulated to orthorhombic cordierite.     

Experimental determination of the solubility of the assemblage paragonite,albite, and quartz in supercritical H2O

The concentrations of Na, Al, and Si in an aqueous fluid in equilibrium with natural albite, paragonite, and quartz have been measured between 350°C and 500°C and 1 to 2.5 kbar. Si is the dominant solute in solution and is near values reported for quartz solubility in pure H₂O. At 1 kbar the concentrations of Na and Al remain fairly constant from 350°C to 425°C but then decrease at 450°C. At 2 kbar, Na increases slightly with increasing temperature while Al remains nearly constant. Concentrations of Si, Na, and Al all increase with increasing pressure at constant temperature.The molality of Al is close to that of Na and is nearly a log unit greater than calculated molalities assuming Al(OH)⁰₃ is the dominant Al species. This indicates a Na-Al complex is the dominant Al species in solution as shown by Anderson and Burnham (1983) at higher temperature and pressure. The complex can be written as NaAl(OH)⁰₄ ± nSiO₂ where n is the number of Si atoms in the complex. The value of n is not well constrained but appears to be less than or equal to 3.The results indicate Al can be readily transported in pure H₂O solutions at temperatures and pressures as low as 350°C and 1 kbar.     

A new approach to simulating disorder in crystals

The technique of calculating lattice dissociation energies using static, minimum lattice energy, ionic models has been extended to allow for multiple occupancy of the ionic sites. A particular lattice site can have a fraction x of an ionic species A and a fraction y of an ionic species B, where the position of each can be relaxed separately along with the unit cell dimensions until an equilibrium is reached. Various degrees of long and short range order can be modelled. This technique has been applied to the mineral sillimanite, Al₂SiO₅, to calculate the effect on the lattice energy of (Al, Si) ordering over the tetrahedral sites. It is found using this method that (Al, Si) ordering with space group Pbmn stabilizes the material by 29.25 kcal/mol (Al^iv-O-Al^iv), with respect to the completely disordered material.     

新疆拜城波孜果尔东矿区侵入岩矿物学特征

新疆拜城县波孜果尔东矿区侵入岩为富含铌、钽、铀、钍、稀土、锆、铷、铯、锂等有用元素的含矿岩体.通过偏光显微镜、电子探针(EPMA)分析, 对拜城波孜果尔东矿区侵入岩的矿物学特征进行了详细的研究, 并对岩浆形成的构造背景进行了初步探讨, 这对成岩、成矿作用研究有一定参考意义.研究结果表明, 波孜果尔东矿区侵入岩为黑云母碱性花岗岩, 主要造岩矿物包括石英、钠长石、钾长石、钠铁闪石、锂云母和黑云母等.其中, 黑云母为锂铁叶云母, 以富Si富Fe、低Al贫Mg为特征; 锂云母以富Si、高Li低Al为特征, 它们为黑云母-锂铁云母系列的不同成员, 均属三八面体型.钠铁闪石以富含Fe2+为特征.副矿物包括烧绿石、星叶石、氟铈矿、独居石、钍石、萤石、锆石、铌铁矿等.岩石形成于非造山的板内构造环境, 且具高温、无水、低氧逸度的成岩特点.      

Study of the weathering of albite at room temperature and pressure with a fluidized bed reactor

A continuous reactor based on the fluidized bed technique was developed in order to study the kinetics and the mechanisms of the initial stages of weathering of albite. Simultaneous determination of Si, Al and Na and the observed low concentrations of the dissolved elements which were always at levels below saturation with respect to possible secondary precipitates, indicate that formation of a residual layer of a few tens of angstroms occurred at the surface of the feldspar. The composition of this layer, enriched in Si and/or Al, is strongly dependent on the pH of the aqueous solution. The formation of the layer is followed by the establishment of a quasi-steady state during which the dissolution of albite tends to become stoichiometric.     

Equilibrium thermodynamics of Al/Si ordering in anorthite

Variations in the equilibrium degree of Al/Si order in anorthite have been investigated experimentally over the temperature range 800-1535° C. Spontaneous strain measurements give the temperature dependence of the macroscopic order parameter, Q, defined with respect to the \(C\bar 1 \rightleftharpoons I\bar 1\) phase transition, while high temperature solution calorimetric data allow the relationship between Q and excess enthalpy, H, to be determined. The thermodynamic behaviour can be described by a Landau expansion in one order parameter if the transition is first order in character, with an equilibrium transition temperature, T _tr, of ～2595 K and a jump in Q from 0 to ～0.65 at T_tr. The coefficients in this Landau expansion have been allowed to vary with composition, using Q=1 at 0 K for pure anorthite as a reference point for the order parameter. Published data for H and Q at different compositions allow the calibration of the additional parameters such that the free energy due to the \(C\bar 1 \rightleftharpoons I\bar 1\) transition in anorthite-rich plagioclase feldspars may be expressed (in cal. mole^-1) as: \(\begin{gathered}G = \tfrac{1}{2} \cdot 9(T - 2283 + 2525X_{Ab} )Q^2 \\ {\text{ + }}\tfrac{1}{4}( - 26642 + 121100X_{Ab} )Q^4 \\ {\text{ + }}\tfrac{1}{6}(47395 - 98663X_{Ab} )Q^6 \\ \end{gathered}\) where X _Ab is the mole fraction of albite component. The nature of the transition changes from first order in pure anorthite through tricritical at ～An₇₈ to second order, with increasing albite content. The magnitude of the free energy of \()\) ordering reduces markedly as X _Ab increases. At ～700° C incommensurate ordering in crystals with compositions ～An₅₀–An₇₀ needs to have an associated free energy reduction of only a few hundred calories to provide a more stable structure. These results, together with a simple mixing model for the disordered ( \()\) ) solid solution, an assumed tricritical model for the incommensurate ordering and published data for ordering in albite have been used to calculate a set of possible free energy relations for the plagioclase system. The incommensurate structure should appear on the equilibrium phase diagram, but its apparent stability with respect to the assemblage albite plus anorthite at low temperatures depends on the values assigned to the mixing parameters of the $$$$ solid solution.     

Al,Si ordering in cordierite using “magic angle spinning” NMR

A method of determining the number of Al-O-Al bonds per unit cell from ²⁹Si nuclear magnetic resonance (NMR) data of synthetic cordierites with increasing Si, Al order is described. The number of Al-O-Al bonds is found to vary linearly with the logarithm of the annealing time. This may be correlated with previously published heat of solution data on similar samples (Carpenter et al. 1983) to determine the enthalpy change Δh, associated with a single Al?Si interchange in cordierite. Δh is found to be 8.1 kcal/mole. The NMR data show that the short range Al, Si order cannot be described in terms of twin domains of ordered orthorhombic cordierite. An ordering model derived from group theoretical constraints on possible Al, Si distributions within the hexagonal symmetry of the cordierite is found to provide a better fit to the NMR data.