In situ X-ray observations of phase transitions in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel to 40 GPa using multianvil apparatus with sintered diamond anvils

 Phase transitions in MgAl₂O₄ spinel have been studied at pressures 22–38 GPa, and at temperatures up to 1600 °C, using a combination of synchrotron radiation and a multianvil apparatus with sintered diamond anvils. Spinel dissociated into a mixture of MgO plus Al₂O₃ at pressures to 25 GPa, while it transformed to the CaFe₂O₄ (calcium ferrite) structure at higher pressures via the metastably formed oxide mixture upon increasing temperature. Neither the e-phase nor the CaTi₂O₄-type MgAl₂O₄, which were reported in earlier studies using the diamond-anvil cell, were observed in the present pressure and temperature range. The zero-pressure bulk modulus of the calcium-ferrite-type MgAl₂O₄ was calculated as K=213 (3) GPa, which is significantly lower than that reported by Yutani et al. (1997), but is consistent with a more recent result by Funamori et al. (1998) and that estimated by an ab initio calculation by Catti (2001). Received: 2 April 2002 / Accepted: 29 July 2002 Acknowledgements The authors thank Y. Higo, Y. Sueda, T.␣Ueda, Y. Tanimoto, A. Fukuyama, K. Ochi, F. Kurio and T. Kawahara for help in the in situ X-ray observations at SPring-8 (No: 2000A0061-CD-np and 2000B0093-ND-np). We also thank W.␣Utsumi, J. Ando and O. Shimomura for advice and encouragement during this study, and N. Funamori and an anonymous reviwer for comments on the article. The present study is partly supported by the grant-in-aid for Scientific Research (A) of the Ministry of Education, Science, Sport and Culture of the Japanese government (no: 11694088).     

Anelasticity and microcreep in polycrystalline MgO at high temperature: an exploratory study

 The frequency dependence of the shear modulus and dissipation in polycrystalline MgO has been determined at high temperature using both microcreep (ɛ = 10⁻⁴) and seismic frequency forced-oscillation (ɛ = 10⁻⁵) measurements. The frequency-dependent and time-dependent data have been described in terms of the elastic, anelastic and viscous components of deformation using the Andrade model. The forced-oscillation measurements show that for temperatures above 700 °C the shear modulus begins to decrease dramatically and the modulus becomes frequency-dependent with increasing temperature. This is accompanied by an increase in dissipation, which also becomes frequency-dependent. The microcreep measurements resolve this frequency-dependent behaviour into an anelastic regime from 700–1050 °C, and a viscoelastic regime from 1100–1300 °C. At 1300 °C, the seismic frequency shear wave speed is ∼60% of the extrapolated low-temperature frequency-independent value, and the dissipation has risen to Q ⁻¹ = 10⁻¹ from 10⁻³ at temperatures below 600 °C. The mechanism by which this frequency-dependent rheology occurs appears to be diffusional creep, which produces viscous slip on the grain boundaries. It is proposed that the anelastic behaviour is due to viscous slip occurring on segments of grain boundaries, with the viscous deformation being accommodated by elastic distortion of adjacent unslipped regions of the grain boundary. At higher temperatures, slippage occurs across the entire grain boundary and viscoelastic behaviour begins to occur. Received: 11 April 2002 / Accepted: 9 January 2003 Acknowledgements Samples were precision-ground by Andrew Wilson, and polished sections prepared by Harri Kokkonen, who also did the SEM work. Uli Faul calculated the volume fraction of grain sizes. The density measurements were done by Lara Weston.     

A ternary feldspar-mixing model based on calorimetric data: development and application

A mixing model for high structural state ternary feldspars in the NaAlSi₃O₈–KAlSi₃O₈–CaAl₂Si₂O₈ system is presented based exclusively on calorimetric and volumetric measurements. Comparisons with existing mixing models, which are based on phase-equilibrium experiments, reveal distinct differences. The incorporation of K into Ca-rich plagioclase and of Ca into K-rich alkali feldspar is more strongly limited by our calorimetry-based model, whereas the stability field of Na-rich feldspars is broadened. Natural feldspar assemblages from well-studied magmatic and high-grade metamorphic rocks (i.e. a teschenite sill in Scotland, the Klokken syenogabbro in Greenland, and a granulite-facies metapelite in Sri Lanka) were used to test the mixing models. It was found that the new model largely eliminates discrepancies between observed and predicted feldspar compositions that were present in earlier attempts. The reasons for the problems associated with phase-equilibrium based mixing models are discussed.     

A TEM and RUM study of the inherent displacive flexibility of the fresnoite framework structure type

 A careful electron diffraction study has been made of the incommensurately modulated room-temperature phases of the fresnoites Ba₂TiGe₂O₈ (BTG) and Ba₂TiSi₂O₈ (BTS) and used to determine their (3+1)- and (3+2)-dimensional superspace group symmetries. The primitive primary modulation wave vectors in both materials are found to occur close to the same position in the parent Brillouin zone, near ∼0.3〈110〉 _p ^*+1/2c _p ^st . A rigid unit mode (RUM) analysis of the inherent displacive structural flexibility of the ideal fresnoite framework structure type is then carried out in an attempt to understand the significance of this particular modulation wave vector. Six zero-frequency RUM modes and two close to zero frequency quasi-RUM (Q-RUM) modes are found to exist for any modulation wave vector. These RUM modes are all primarily associated with rotations of the constituent TO₄ (T=Si or Ge) tetrahedra and TiO₅ square pyramids around in-plane i.e. perpendicular to c rotation axes. A seventh RUM mode involving rotation of the constituent rigid polyhedra around c combined with shifts in the basal plane is found but only at a very specific modulation wave vector q∼0.30〈110〉 _p ^* , in close agreement with the condensed RUM mode found in the electron diffraction study. It is the condensation of just such a RUM mode that appears to play a major role in the various incommensurately modulated structures observed in Ba₂TiGe₂O₈, Ba₂TiSi₂O₈ and Sr₂TiSi₂O₈, respectively. Received: 26 November 2001 / Accepted: 25 May 2002 Present address: Y. Tabira Materials Characterization Laboratory Mitsui Mining & Smelting Co. Ltd. 1333-2 Haraichi, Ageo, Saitama 362-0021, Japan     

Structure and chemistry of (111) twin boundaries in MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel crystals from Mogok

The atomic scale structure and chemistry of (111) twins in MgAl₂O₄ spinel crystals from the Pinpyit locality near Mogok (Myanmar, formerly Burma) were analysed using complementary methods of transmission electron microscopy (TEM). To obtain a three-dimensional information on the atomic structure, the twin boundaries were investigated in crystallographic projections and Using conventional electron diffraction and high-resolution TEM (HRTEM) analysis we have shown that (111) twins in spinel can be crystallographically described by 180° rotation of the oxygen sublattice normal to the twin composition plane. This operation generates a local hcp stacking in otherwise ccp lattice and maintains a regular sequence of kagome and mixed layers. In addition to rotation, no other translations are present in (111) twins in these spinel crystals. Chemical analysis of the twin boundary was performed by energy-dispersive X-ray spectroscopy (EDS) using a variable beam diameter (VBD) technique, which is perfectly suited for analysing chemical composition of twin boundaries on a sub-nm scale. The VBD/EDS measurements indicated that (111) twin boundary in spinel is Mg-deficient. Quantitative analyses of HRTEM (phase contrast) and HAADF-STEM (Z-contrast) images of (111) twin boundary have confirmed that Mg²⁺ ions are replaced with Be²⁺ ions in boundary tetrahedral sites. The Be-rich twin boundary structure is closely related to BeAl₂O₄ (chrysoberyl) and BeMg₃Al₈O₁₆ (taaffeite) group of intermediate polysomatic minerals. Based on these results, we conclude that the formation of (111) twins in spinel is a preparatory stage of polytype/polysome formation (taaffeite) and is a result of thermodynamically favourable formation of hcp stacking due to Be incorporation on the {111} planes of the spinel structure in the nucleation stage of crystal growth. The twin structure grows as long as the surrounding geochemical conditions allow its formation. The incorporation of Be induces a 2D-anisotropy and exaggerated growth of the crystal along the (111) twin boundary.     

Thin amorphous films (1–2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos,Arizona

 Olivine grain boundaries and phase boundaries in xenoliths from San Carlos have been investigated by high-resolution transmission electron microscopy (HREM) and analytical electron microscopy (AEM). Thin amorphous intergranular layers with variable width (1–2 nm) were detected along olivine grain boundaries. The Al₂O₃, TiO₂ and CaO concentrations of the amorphous layers increase with increasing width of the layer. The composition of the amorphous intergranular layers depends on the interface type – grain or phase boundary. Morphology, amorphous state and chemical composition of the intergranular layer suggest the presence of a melt film at olivine grain boundaries. Since the composition of the amorphous phase strongly depends on the type of interface, the melt must have been generated at the grain boundary. Also, the melt chemistry is different from the composition of partial melts produced from possible hydrous phases, such as phlogopite or amphibole, and from the host basanite. The mobility of very thin melt films is assumed to be very limited due to the strong interface forces between the melt and the grain boundary. It is concluded that grain boundary melting occurred at the interfaces due to decompression during uplift. The melt wetted olivine grain boundaries as well as olivine-opx phase boundaries. The thin amorphous layers formed melt microsystems. Mixing of melts from different microsystems is suggested to occur in wider melt films, melt veins or melt pockets thus creating a magmatic melt that could be extracted from its source. Received: 6 November 1995 / Accepted: 24 January 1996     

High pressure behaviour of lead feldspar (PbAl2Si2O8)

An in situ high pressure powder diffraction study, using high-brilliance synchrotron radiation, on lead feldspar (PbAl₂Si₂O₈) was performed. Two samples, with Q _od=0.68 and 0.76, were loaded in a diamond anvil cell and were compressed up to 11 GPa. Up to P=7.1 GPa the only phase present is lead feldspar. In the range 7.1–9.4 GPa sudden changes in the position of the reflections suggest the transformation of lead feldspar to a new phase (probably feldspar-like). The absence of split that would be compatible with triclinic symmetry rules out the monoclinic-triclinic transition, that was reported for the structurally similar strontium feldspar. At P>9.4 GPa some new extra reflections not indexable in the feldspar cell are present as well. During decompression the lead feldspar was the only phase present at P<6 GPa. Peak enlargement was observed with pressure, probably preliminary to amorphization. However almost complete amorphization was observed only after fortuitous shock compression at ∼18 GPa; the crystallinity was recovered at room pressure after decompression. The bulk modulus for lead feldspar was K=71.0(9) and 67.6(1.2) GPa for the two samples, in the range reported for feldspars. The cell parameters show a compression pattern which is similar to that observed in anorthite, with Δa/a ₀>Δc/c ₀>Δb/b ₀; comparison with the high temperature behaviour shows that for lead feldspar the strain tensor with pressure is more isotropic and the deformation along a is less prominent. A turnover in the behaviour of the β angle with pressure suggests a change in the compression behaviour at P∼2 GPa. Rietveld refinement of the Pb coordinates was performed in a series of spectra with pressure ranging from 0.6 to 6.5 GPa. The combined analysis of cell parameters and Pb coordinates with pressure showed that the compression of the structure is mainly achieved by an approach of Pb atoms along a *. Received: 21 July 1998 / Revised, accepted: 13 October 1998     

Thermodynamic analysis of Fe and Mg partitioning between plagioclase and silicate liquid

 Thermodynamic analysis of Fe- and Mg-bearing plagioclase and silicate liquid was carried out based on reported element partitioning data between plagioclase and silicate liquid in reduced conditions, solution properties of ternary feldspar, standard state properties of plagioclase endmembers and solution properties of multicomponent silicate liquid. Derived mixing properties of Fe- and Mg-bearing plagioclase are in harmony with estimated results from synthetic experiments in the systems CaAl₂Si₂O₈-CaFeSi₃O₈ and CaAl₂Si₂O₈-CaMgSi₃O₈. Based on the determined solution properties of the plagioclase, a computer program to calculate the element partition relationships between Fe- and Mg-bearing plagioclase and multicomponent silicate liquid was developed. The FeO, MgO and MgO/(MgO + FeO) in plagioclase predicted from known liquid compositions and pressure are in agreement with measurements within 0.2 wt%, 0.1 wt% and 0.1 (mol ratio), respectively. The Fe³⁺ content in plagioclase crystallized at high oxygen fugacity can be estimated with this program. The Fe³⁺/total Fe ratio in plagioclase crystallized near the quartz-fayalite-magnetite buffer ranges from 0 to 0.5, which is consistent with previous study on natural plagioclase in submarine basalt. Derived solution properties of the Fe- and Mg-bearing plagioclase are also used to calculate equilibrium composition relationship between olivine and plagioclase. Change of X _Fo in olivine coexisting with plagioclase affects MgO and FeO contents in plagioclase greatly. The present model predicts X _Fo of coexisting olivine from the chemical composition of plagioclase to ±0.1 accuracy at given pressure and temperature. Received: 27 March 1998 / Accepted: 30 September 1999     

Compression mechanisms of coesite

 The structure of coesite has been determined at ten pressures up to a maximum of 8.68 GPa by single-crystal X-ray diffraction. The dominant mechanism of compression is the reduction of four of the five independent Si–O–Si angles within the structure. There is no evidence of the fifth linkage, Si1–O1–Si1, deviating from 180°. Some Si–O bond distances also decrease by up to 1.6% over the pressure range studied. The pattern of Si–O–Si angle reduction amounts to a rotation of the Si2 tetrahedron around the [001] direction. This rotation induces significant internal deformation of the Si1 tetrahedron. Comparison of the experimental data with rigid-unit distance least-squares simulations of coesite suggests that this pattern of compression, the anomalous positive values of both s₂₃ and K′′ in the equation of state of coesite, its high elastic anisotropy and the unusual straight Si1–O1–Si1 linkage within the structure are all consequences of the connectivity of the tetrahedral framework. Received: 11 July 2002 / Accepted: 14 January 2003 Acknowledgements The help of Christian Baerlocher of ETH Zurich in providing both the DLS-76 software and advice in its use is gratefully acknowledged, as are discussions with Paul Ribbe of Virginia Tech and the comments of two anonymous reviewers. The data analysis was supported by the National Science Foundation under grant EAR-0105864 to N.L. Ross and R.J. Angel.     

A quantum-mechanical study of the (110) surface of sphalerite (ZnS) and its interaction with Pb<Superscript>2+</Superscript> species

 The structure of the (110) surface of sphalerite has been studied using mainly quantum-mechanical (QM) methods. The experimentally observed puckering of the surface is well reproduced by periodic plane wave density functional calculations. Water adsorption on this surface is modelled using this QM approach and is used to judge the accuracy of less computationally demanding cluster methods. A cluster model, thus validated, is used to study the possible modes of interaction of Pb²⁺ ions with the (110) surface. It is found that adsorption of hydrated PbO and PbOH⁺ is energetically feasible and leads to Pb–O distances compatible with REFLEXAFS data. Received: 22 April 2002 / Accepted: 23 October 2002 Acknowledgements We thank EPSRC for support of this research.     

Study on Al2O3 content and phase stability of aluminous-CaSiO3 perovskite at high pressure and temperature

 In order to clarify Al₂O₃ content and phase stability of aluminous CaSiO₃-perovskite, high-pressure and high-temperature transformations of Ca₃Al₂Si₃O₁₂ garnet (grossular) were studied using a MA8-type high-pressure apparatus combined with synchrotron radiation. Recovered samples were examined by analytical transmission electron microscopy. At pressures of 23–25 GPa and temperatures of 1000–1600 K, grossular garnet decomposed into a mixture of aluminum-bearing Ca-perovskite and corundum, although a metastable perovskite with grossular composition was formed when the heating duration was not long enough at 1000 K. On release of pressure, this aluminum-bearing CaSiO₃-perovskite transformed to the “LiNbO₃-type phase” and/or amorphous phase depending on its Al₂O₃ content. The structure of this LiNbO₃-type phase is very similar to that of LiNbO₃ but is not identical. CaSiO₃-perovskite with 8 to 25 mol% Al₂O₃ was quenched to alternating lamellae of amorphous layer and LiNbO₃-type phase. On the other hand, a quenched product from CaSiO₃-perovskite with less than 6 mol% consisted only of amorphous phase. Most of the inconsistencies amongst previous studies could be explained by the formation of perovskite with grossular composition, amorphous phase, and the LiNbO₃-type phase. Received: 11 April 2001 / Accepted: 5 July 2002     

Plastic deformation of wadsleyite: I. High-pressure deformation in compression

 We have studied the plastic deformation of Mg₂SiO₄ wadsleyite polycrystals. Wadsleyite was synthesized from a forsterite powder in a multianvil apparatus. It was then recovered and placed in a second multianvil assembly designed to induce plastic deformation by compression between two hard alumina pistons. After the deformation experiment, the microstructures are characterized by transmission electron microscopy (TEM) and large-angle convergent beam electron diffraction (LACBED). Deformation experiments have been carried out at 15–19 GPa and at temperatures ranging from room temperature to 1800–2000 °C. Five different dislocation types have been identified by LACBED: [100], 1/2〈111〉, [010], 〈101〉 and [001]. The [001] dislocations result from dislocation reactions and not from activation of a slip system. The [010] dislocations are activated under high stresses at the beginning of the experiments and further relax by decomposition into 1/2〈111〉 dislocations or by dissociation into four 1/4[010] partial dislocations. The following slip systems have been identified: 1/2〈111〉{101}, [100](010), [100](001), [100]{011}, [100]{021}, [010](001), [010]{101} and 〈101〉(010). Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP – Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). P.C. has benefited from a Congé thématique pour recherche from the University of Lille, and would like to thank warmly all the people in Bayreuth who contributed to this work by daily assistance and discussions: Nathalie Bolfan-Casanova, Daniel Frost, Jed L. Mosenfelder and Brent Poe. The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

The temperature dependence of the speciation of water in NaAlSi3O8-KAlSi3O8 melts: an application of fictive temperatures derived from synthetic fluid-inclusions

 The speciation of water dissolved in glasses along the join NaAlSi₃O₈-KAlSi₃O₈ has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration of cylinders of bubble-free anhydrous start glasses with water at 1040^° C and 2 kbar. These melts have been isobarically and rapidly (200^° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples of the same chemistry has precisely defined the glass transition temperature of these melts (±5^° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities for OH and H₂O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation data permits an analysis of the temperature dependence of the water speciation over the 113^° C range of fictive temperatures obtained for these water-saturated melts. The variation of water speciation, cast as the equilibrium constant K where K = [H₂O] [O _m ]/[OH]² is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol. The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange. Received: 8 March 1995/Accepted: 6 June 1995     

High-pressure phase transformations in the MgFe2O4 and Fe2O3–MgSiO3 systems

 The crystal structure of MgFe₂O₄ was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite undergoes a phase transformation at about 25 GPa, which leads to a CaMn₂O₄-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe₂O₄ form on the high-pressure phase transformations in the (MgSi)_0.75(Fe^III)_0.5O₃ system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe₂O₄ and Mg₂SiO₄. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure. We thus conclude for the formation of Mg₃Fe₂Si₃O₁₂ perovskite. Received: 27 March 2000 / Accepted: 1 October 2000     

Plastic deformation of wadsleyite: II. High-pressure deformation in shear

 We have studied the dislocation microstructures that develop in (Mg_0.9Fe_0.1)₂SiO₄ wadsleyite deformed by simple shear at high pressure. The experiments were performed in a multianvil apparatus with the shear assembly designed by Karato and Rubie (1997). The samples were synthesized in a separate experiment from high-purity oxides. The deformation experiments were carried out at 14 GPa and 1300 °C with time durations ranging from 1 to 8 h leading to plastic shear strains of 60 and 73%, respectively. The microstructures investigated by transmission electron microscopy (TEM) show that dislocation glide is activated under these conditions over the whole experimental time. The easy slip systems at 1300 °C involve 1/2<111> dislocations gliding in {101} as well as [100] dislocations gliding in (010) and {011}. Received: 15 July 2002 / Accepted: 14 February 2003 Acknowledgements High-pressure experiments were performed at the Bayerisches Geoinstitut under the EU IHP — Access to Research Infrastructures Programme (Contract no. HPRI-1999-CT-00004 to D.C. Rubie). The quality of the preparation of the TEM specimens by H. Schultze is greatly appreciated.     

Ion microprobe analysis of oxygen isotope ratios in quartz from Skye granite: healed micro-cracks,fluid flow,and hydrothermal exchange

 Quartz grains in hydrothermally altered granites from the Isle of Skye are highly heterogeneous and not equilibrated in oxygen isotope ratio at the 20 μm scale. Ion microprobe analysis of one grain shows a gradient of 13‰ over 400 μm and a greater range in δ ¹⁸O than all quartz previously analyzed on the Isle of Skye. Other crystals from the same outcrop are homogeneous. Digitized cathodoluminescence images reveal patterns of magmatic zoning and brittle fracturing not otherwise detectable. The ion probe analysis correlates low δ ¹⁸O values on a micro-scale to one set of healed cracks. Thus, quartz exchanges oxygen isotopes primarily by solution and reprecipitation along fractures, in contrast to more reactive feldspar that appears to exchange from the grain boundary inward. Macroscopic models of isotope exchange are not realistic for these rocks; the flow of hydrothermal fluids was heterogeneous, anisotropic and crack controlled. Received: 23 October 1995/Accepted: 9 April 1996     

Phosphorus distribution between potassic alkali feldspar and metaluminous haplogranitic liquid at 200 MPa (H<Subscript>2</Subscript>O): the effect of undercooling on crystal–liquid systematics

To evaluate the applicability of P₂O₅ concentration in potassic alkali feldspar as a monitor of P₂O₅ in melt for undercooled systems, crystal–melt partitioning for P was evaluated via feldspar growth experiments in P-bearing ((3 wt% P₂O₅), water-saturated haplogranitic liquids at 200 MPa, with liquidus undercoolings (ΔT) of 25, 50, 100, 200, and 300°C. Increasing undercooling in the range ΔT=25–200°C shows an evolution of crystal morphologies, from euhedral and well-filled individuals at ΔT=25–50°C to radial clusters with increasingly skeletal habit at greater undercooling. Experiments at ΔT=100–200°C also document the development of P- (up to (9 wt% P₂O₅) and Si-enriched, more alkaline boundary layers adjacent to crystals. Experiments at ΔT=300°C show an additional change in crystallization fabric in which spherulites of skeletal crystals form in open (vapor) space created by the dissolution of bulk silicate, and compositional boundary layers are not observed. We interpret the changes in reaction products at ΔT=300°C to indicate conditions below a glass transition; hence, partition coefficients were not determined for this undercooling. Values of K _d(P)^Kfs/melt from experiments at ΔT=25–200°C, calculated from pairs of crystal and immediately adjacent liquid compositions (including boundary layers at higher undercooling), are mostly in the range of 0.25–0.55 and show no effective change with increased undercooling. Essentially no change in K _d(P)^Kfs/melt with undercooling apparently stems from an interplay between boundary layer composition and a change in the substitution mechanism for P in feldspar from AlPSi₋₂, common in peraluminous to metaluminous liquids near equilibrium, to increasing proportions of ([ ],P)(M⁺,Si)₋₁ with increased undercooling. Bulk glass and liquid beyond boundary layers in experiments with significant percentages of crystallization are homogeneous, and show pronounced fractionation primarily due to the removal of an orthoclase component. Because crystallization was still in progress in experiments with ΔT≤200°C, compositional homogeneity in the bulk liquid requires extremely rapid diffusion of most haplogranite components (Na, K, and Al), apparently resulting from chemical potential gradients stemming from the removal of components from the liquid by crystal growth. Similar homogeneity and bulk fractionation in experiments with ΔT=300°C requires rapid diffusive equilibration for the alkalis even at temperatures below an apparent glass transition. Unlike the haplogranite components, P is only concentrated in liquid boundary layers (ΔT≤200°C) or low-density aqueous vapor (ΔT=300°C) adjacent to crystals. Hence, the P₂O₅ contents of melt inclusions likely are not representative of bulk melt concentrations in significantly undercooled systems (ΔT≤50–100°C).     

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     

Electrons in feldspar I: on the wavefunction of electrons trapped at simple lattice defects

 The purpose of this article is to make an initial consideration of the physical properties of electrons trapped at classic hydrogenic lattice defects in feldspar. We are particularly interested to determine the radial extent of the electron wavefunctions in the ground and excited states. It is shown that for NaAlSi₃O₈, the ground-state wavefunction is expected to be confined well within a single lattice unit cell, but the first excited state is far more extensive, being spread over several unit cells. This aspect is of direct relevance to understanding the nature of various luminescence processes in the materials. Under low-energy optical stimulation (hν∼ 1.4 eV), luminescence can be a competitive process between direct electron-hole tunnelling recombination (with the charge still trapped at the defect sites), and free-to-bound recombination (after the excited state electron accesses the conduction band). We show that analysis of the thermal behaviour of the luminescence can be used to separate the two processes. Received: 6 March 2001 / Accepted: 6 September 2001