Effect of pressure on the crystal structure of perovskite-type MgSiO3

The crystal structure of perovskite-type MgSiO₃ has been studied up to 96 kbar, using a miniature diamondanvil pressure cell and by means of single-crystal four-circle diffractometry. The observed unit cell compression gives a bulk modulus of K _o=2.47 Mbar, assuming K′_o=4. The unit cell compression is controlled mainly by the tilting of SiO₆ octahedra. The effect of pressure is to change Mg polyhedron towards 8-fold coordination rather than 12-fold coordination. The polyhedral bulk moduli of SiO₆ and MgO₈ are 3.8 Mbar and 1.9 Mbar, respectively.     

High-Pressure crystal chemistry of spinel (MgAl2O4) and magnetite (Fe3O4): Comparisons with silicate spinels

High-pressure crystal structures and compressibilities have been determined by x-ray methods for MgAl₂O₄ spinel and its isomorph magnetite, Fe₃O₄. The measured bulk moduli, K, of spinel and magnetite (assuming K′=4) are 1.94±0.06 and 1.86±0.05 Mbar, respectively, in accord with previous ultrasonic determinations. The oxygen u parameter, the only variable atomic position coordinate in the spinel structure (Fd3m, Z=8), decreases with pressure in MgAl₂O₄, thus indicating that the magnesium tetrahedron is more compressible than the aluminum octahedron. In magnetite the u parameter is unchanged, and both tetrahedron and octahedron display the 1.9 Mbar bulk modulus characteristic of the entire crystal. This behavior contrasts with that of nickel silicate spinel (γ-Ni₂SiO₄), in which the u parameter increases with pressure because the silicon tetrahedron is relatively incompressible compared to the nickel octahedron.     

Elasticity of diopside

The thirteen single-crystal elastic moduli for diopside as determined by the acoustic technique based on Brillouin scattering are: c₁₁=2.23, c₂₂=1.71, c₃₃=2.35, c₄₄=0.74, c₅₅=0.67, c₆₆=0.66, c₁₂=0.77, c₁₃=0.81, c₁₅=0.17, c₂₃=0.57, c₂₅=0.07, c₃₅=0.43, c₄₆=0.073. The Reuss bound of the adiabatic bulk and shear moduli calculated from these data are K _s=1.08 Mbar and G=0.651 Mbar. The room-pressure isothermal bulk modulus, K _T , and the pressure derivative of the bulk modulus, K′ _T have also been determined on a four-circle diffractometer, from a single crystal mounted in a gasketed opposed-anvil diamond cell, giving values of K _T =1.13 Mbar and K′ _T =4.8. The principal axes of the strain ellipsoid, calculated from the elastic moduli and observed in the static compression data, are identical, and the linear compressibilities are in reasonable agreement. The single-crystal elastic moduli can be correlated with the structural features of diopside.     

High-pressure crystal chemistry of phenakite (Be2SiO4) and bertrandite (Be4Si2O7(OH)2)

Compressibilities and high-pressure crystal structures have been determined by X-ray methods at several pressures for phenakite and bertrandite. Phenakite (hexagonal, space group R \(\bar 3\) ) has nearly isotropic compressibility with β=1.60±0.03×10^?4 kbar^?1 and β=1.45±0.07×10^?4 kbar^?1. The bulk modulus and its pressure derivative, based on a second-order Birch-Murnaghan equation of state, are 2.01±0.08 Mbar and 2±4, respectively. Bertrandite (orthorhombic, space group Cmc2₁) has anisotropic compression, with β _a =3.61±0.08, β _b =5.78±0.13 and β _c =3.19±0.01 (all ×10^?4 kbar^?1). The bulk modulus and its pressure derivative are calculated to be 0.70±0.03 Mbar and 5.3±1.5, respectively. Both minerals are composed of frameworks of beryllium and silicon tetrahedra, all of which have tetrahedral bulk moduli of approximately 2 Mbar. The significant differences in linear compressibilities of the two structures are a consequence of different degrees of T-O-T bending.     

Exchange of Na and K between water vapor and feldspar phases at high temperature and low vapor pressure

In order to determine whether gas (steam) containing a small amount of dissolved alkali chloride is effective in promoting base exchange of Na⁺ and K⁺ among alkali feldspars and coexisting brine or brine plus solid salt, experiments were carried out at 400–700°C and steam densities ranging down to less than 0.05. For bulk compositions rich in potassium, the low pressure results are close to previous high-pressure results in composition of the fluid and coexisting solid phase. However, when the bulk composition is more sodic, alkali feldspars are relatively richer in potassium at low pressure than at high pressure. This behaviour corresponds to enrichment of potassium in the gas phase relative to coexisting brine and precipitation of solid NaCl when the brine plus gas composition becomes moderately sodic.

The gas phase is very effective in promoting base exchange between coexisting alkali feldspars at high temperature and low water pressure. This suggests that those igneous rocks which contain coexisting alkali feldspars out of chemical equilibrium either remained very dry during the high-temperature part of their cooling history or that the pore fluid was a gas containing very little potassium relative to sodium.     

藏北可可西里中新世钾质火山岩矿物化学及温压计算

对藏北可可西里中新世钾质火山岩的主要斑晶矿物化学成份进行了电子探针分析,结果表明,火山岩中的辉石为普通辉石、紫苏辉石和古铜辉石,角闪石为钙质角闪石,云母为钛铁黑云母和富铁黑云母,长石为中长石、更长石、钠透长石和透长石,不透明矿物主要为钛铁矿和磁铁矿,可见钛铁矿?磁铁矿固溶体。根据火山岩中单斜辉石?熔体温压计估算的辉石斑晶的形成温度和压力分别为1 065～1 100 ℃和5.3～9.1 kbar。二辉石地质温度计结果表明,辉石的共结温度为976～1 020 ℃。而钛铁矿?磁铁矿固溶体平衡计算结果表明,钛?磁铁矿的平衡温度为841～974 ℃,logfO2为－13.71～－10.87。研究结果显示,可可西里火山岩岩浆房的埋深深度小于30 km,中?酸性系列火山岩是岩浆房在相对低压条件下的中上部地壳部位,经分离结晶后形成。     

Ar40 diffusion in homogenous orthoclase and an interpretation of Ar diffusion in K-feldspars

Ar⁴⁰ diffusion in a natural, non-perthitic orthoclase has been studied isothermal heating experiments between 500° and 800°C under both vacuum and hydrothermal (2 kbar) conditions. The sample is a one-phase K-feldspar without detectable chemical of structural inhomogeneities as verified by heating experiments, chemical and microprobe analyses, and cell refinements. The orthoclase does not disorder detectably and is stable for the duration of the heating interval. Diffusion coefficients were calculated using an isotropic model for spherical grains. Agreement of diffusion coefficients obtained on grain-sizes which differed by a factor of four indicate that the effective dimension for Ar⁴⁰ diffusion is the actual particle size. A series of experiments at 700°C show that Ar⁴⁰ loss may be described by the ideal spherical model and that the diffusion coefficient does not change with time. The Arrhenius relation is obeyed with a single activation energy and the diffusion coefficients are described by: D = (0·00982) exp — (43800/RT). Agreement of experiments conducted under vacuum and hydrothermally (up to 2 kbar) indicate that pressure and H₂O do not significantly affect Ar⁴⁰ loss. Relatively small amounts of alkali exchange between the feldspar and hydrothermal salt solutions do not affect the loss behavior.The simple behavior obtained for this orthoclase is attributed of the use of a simple technique within the region of sample stability and to the homogeneous nature of the feldspar. Effects due to sample instability and to the use of perthites are discussed. The new data are compared to those for homogeneous feldspars showing that the orthoclase gives diffusion coefficients which are as low as those sanidine. It is suggested that perthitization of feldspars in nature may reduce the effective grain size for diffusion and thereby allow diffusional loss of Ar⁴⁰ at relatively low temperatures.     

The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O

Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H₂O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, Al^VI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe³⁺ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H ⁰ _f,?Ann =?5125.896±8.319 [kJ/mol] and S ⁰ _Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H ⁰ _f,Ann =?5130.971±7.939 [kJ/mol] and S ⁰ _Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H ₂ (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.     

Beginning of melting in the granite system Qz-Or-Ab-An-H2O

The beginning of melting in the system Qz-Or-Ab-An-H₂ O was experimentally reversed in the pressure range kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H₂O. In the granite system Qz-Or-Ab-An-H₂O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system Qz-Or-An-H₂O) is approximately 50° C. With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by zoisite+kyanite+quartz and zoisite+muscovite-paragonite_ss +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600° C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures. The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas of granitic or granodioritic composition are usually not saturated with water.     

A high-pressure neutron diffraction study of the ferroelastic phase transition in RbCaF3

The fluoroperovskite phase RbCaF₃ has been investigated using high-pressure neutron powder diffraction in the pressure range ~0–7.9 GPa at room temperature. It has been found to undergo a first-order high-pressure structural phase transition at ~2.8 GPa from the cubic aristotype phase to a hettotype phase in the tetragonal space group I4/mcm. This transition, which also occurs at ~200 K at ambient pressure, is characterised by a linear phase boundary and a Clapeyron slope of 2.96 × 10^?5 GPa K^?1, which is in excellent agreement with earlier, low-pressure EPR investigations. The bulk modulus of the high-pressure phase (49.1 GPa) is very close to that determined for the low-pressure phase (50.0 GPa), and both are comparable with those determined for the aristotype phases of CsCdF₃, TlCdF₃, RbCdF₃, and KCaF₃. The evolution of the order parameter with pressure is consistent with recent modifications to Landau theory and, in conjunction with polynomial approximations to the pressure dependence of the lattice parameters, permits the pressure variation of the bond lengths and angles to be predicted. On entering the high-pressure phase, the Rb–F bond lengths decrease from their extrapolated values based on a third-order Birch–Murnaghan fit to the aristotype equation of state. By contrast, the Ca–F bond lengths behave atypically by exhibiting an increase from their extrapolated magnitudes, resulting in the volume and the effective bulk modulus of the CaF₆ octahedron being larger than the cubic phase. The bulk moduli for the two component polyhedra in the tetragonal phase are comparable with those determined for the constituent binary fluorides, RbF and CaF₂.     

Zoned ternary feldspars in the Klokken intrusion: exsolution microtextures and mechanisms

The microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the Klokken intrusion, described in the preceding paper, were determined by TEM and their origin and evolution deduced. The feldspars normally have a plagioclase core and an alkali feldspar rim; cores become smaller and rims larger and the An content of both decrease with distance from the contact of the intrusion. The following microtextural sequence was observed. The inner plagioclase cores are homogeneous oligoclase-andesine with Albite growth twins only, but are crypto-antiperthitic towards the outer core. At first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on Albite-twin composition planes. With further increase in bulk Or they are homogeneously distributed in the plagioclase. Thicker, through-going plates in platelet-free areas are found, which induce Albite twins in the surrounding plagioclase. The microtextures in the rims are regular cryptomesoperthitic, with (¯601) lenses or lamellae, depending on the bulk Or-content, of low sanidine in Albite-twinned low oligoclase-andesine. Albite and Pericline twins in plagioclase in an M-twin relationship, together with lenticular low sanidine, were found in only one small area. The overall diffraction symmetry of the mesoperthites is monoclinic, showing that exsolution started in a monoclinic feldspar, whereas that of the antiperthites is triclinic. The intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres.The cryptomesoperthites are very regular where Or-rich and probably arose by spinodal decomposition. The platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere. Near the intermediate zone they coarsened to give larger plates which induced Albite-twins in the plagioclase. Because of the zoning, microtextures that were initiated in areas of given composition, can propagate laterally into zones of different composition. A diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly-cooled rocks.CRPG contribution 730     

The high-pressure phase diagram of synthetic epsomite (MgSO4·7H2O and MgSO4·7D2O) from ultrasonic and neutron powder diffraction measurements

We present an ultrasonic and neutron powder diffraction study of crystalline MgSO₄·7H₂O (synthetic epsomite) and MgSO₄·7D₂O under pressure up to ~3 GPa near room temperature and up to ~2 GPa at lower temperatures. Both methods provide complementary data on the phase transitions and elasticity of magnesium sulphate heptahydrate, where protonated and deuterated counterparts exhibit very similar behaviour and properties. Under compression in the declared pressure intervals, we observed three different sequences of phase transitions: between 280 and 295 K, phase transitions occurred at approximately 1.4, 1.6, and 2.5 GPa; between 240 and 280 K, only a single phase transition occurred; below 240 K, there were no phase transformations. Overall, we have identified four new phase fields at high pressure, in addition to that of the room-pressure orthorhombic structure. Of these, we present neutron powder diffraction data obtained in situ in the three phase fields observed near room temperature. We present evidence that these high-pressure phase fields correspond to regions where MgSO₄·7H₂O decomposes to a lower hydrate by exsolving water. Upon cooling to liquid nitrogen temperatures, the ratio of shear modulus G to bulk modulus B increases and we observe elastic softening of both moduli with pressure, which may be a precursor to pressure-induced amorphization. These observations may have important consequences for modelling the interiors of icy planetary bodies in which hydrated sulphates are important rock-forming minerals, such as the large icy moons of Jupiter, influencing their internal structure, dynamics, and potential for supporting life.     

An electron microscopic study of anorthoclase spherulites

Spherulites consisting of fibrous alkali feldspar and silica minerals are produced by devitrification of rhyolite glass under hydrothermal conditions. The alkali feldspars (Ab_72.5Or_23.0An_4.5, Ab_81.7Or_14.0An_4.3) in spherulites from two localities in Japan consist of triclinic anorthoclase showing fine cross-hatched twinning and monoclinic sanidine showing fine cross-hatching not attributable to twinning. The cross-hatching, which corresponds to albite and pericline twinning, is produced in the process of transition from a monoclinic to a triclinic phase. The spherulite may develop at a temperature lower than about 200°C because the co-existing silica mineral is not quartz, but metastable tridymite. According to the phase diagram of the alkali feldspars by MacKenzie (1952), the alkali feldspars should have been triclinic during growth. However, the textures show that the alkali feldspar grew as a disordered monoclinic phase. Because of the high growth rate, the Al/Si disordered structure was produced during growth and afterwards transformed into a triclinic structure with cross-hatched twinning.     

The pressure quench formation of rapakivi texture

Chemical and textural data for rapakivi granites are combined with experimentally determined phase equilibria to provide constraints on the growth of mantled feldspars by a pressure quench mechanism. In water saturated melts of granitic composition the mantled texture develops in response to a decrease in pressure of as little as 1–0.5 kbar. In water undersaturated melts a similar mechanism is operable over a wider range of pressures and a restricted range of water contents. Water undersaturated granites containing 6–10 wt % water undergoing a change in pressure from 5–2 kbar will develop mantling as will a more calcic granodiorite containing 1–5wt% water undergoing a pressure change from 10-1 kbar. The mantling process is interpreted as a reflection of the reduction of pressure accompanying the emplacement of a magma.     

In situ determination of crystal structure for high pressure phase of ZrO2 using a diamond anvil and single crystal X-ray diffraction method

Combining a miniature diamond-anvil pressure cell with a single crystal four-circle diffractometer, the crystal structure of a synthetic ZrO₂ has been studied in situ up to 51 kbar at room temperature. The space group of the unquenchable orthorhombic high pressure phase is Pbcm. The directions of the b and c axes are preserved through the transition and the transformation is displacive. The coordination configurations of the Zr atoms and oxygen atoms are the same in the high pressure and low pressure phases. The orthorhombic high pressure phase has a higher entropy than that of low pressure monoclinic phase.     

Der Einfluß des Drucks auf die Zusammensetzung koexistierender Alkalifeldspäte und Plagioklase im System NaAlSi3O8-KAlSi3O8-CaAl2Si2O8-H2O

Pairs of alkali feldspars and plagioclases were synthesized at pressures of 1, 5, and 10 kbar and a temperature of 650° C, to study the composition of coexisting feldspars in relation to pressure. Data were obtained mainly from pairs of feldspars crystallized from dehydrated gels in the presence of an aqueous vapor phase. Data obtained were confirmed by exchange reactions in mixtures of synthetic alkali feldspars and plagioclases. The effects of temperature and pressure on the distribution of albite molecule in coexisting feldspars are opposed to each other. An increase in pressure of 10 kbar equals to a decrease in temperature of about 125–150° C, thus posing severe restrictions to the application of the feldspar geothermometer of Barth. Pairs of coexisting feldspars are discussed in terms of their temperature and pressure of formation.

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Dem Direktor des Mineralogisch-Petrographischen Instituts der Universität zu Köln, Herrn Prof. Dr. K. Jasmund, danke ich für sein förderndes Interesse an dieser Arbeit sowie die kritische Durchsicht des Manuskripts. Herrn Prof. Dr. M. Okrusch und Herrn Dr. V. Rudert sei ebenfalls für kritische Anmerkungen zum Manuskript gedankt.

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Die Untersuchung wurde mit Hilfe von Personal- und Sachmitteln durchgeführt, die Herrn Professor Dr. K. Jasmund von der Deutschen Forschungsgemeinschaft zur Verfügung gestellt worden waren.     

Raman spectra of analcime under pressure

Raman spectra of natural analcime have been recorded at atmospheric pressure and up to 9.4 kbar. The basic Si, Al-O network vibrations are little affected by pressure even though significant volume changes and a minor phase transition take place. However, the 3,557 cm^?1 OH-stretch mode is modified in that band splitting takes place indicating at least two O-OH hydrogen bond distances. Thus there are at least three sites of hydrogen bonding in analcime. The bonded water (H₂O) in analcime appears to remain in the mineral at high pressure. The bulk volume change, determined previously by cell dimension measurements, can be traced to reduction of the size of the “voids” in the structure. This is deduced from the fact that Si-Al-O vibrations are little affected by pressure but O-H vibrations of water molecules found in the voids are strongly pressure-dependent.     

A transferable interatomic potential for crystalline phases in the system MgO—SiO2

A central interatomic potential model is presented for compounds in the binary system MgO-SiO₂. The potential, of a simple form which consists of a Coulombic term, a Born repulsive term, and a Van der Walls term for oxygen-oxygen interactions, is designed to predict the properties of magnesium silicates containing Si in octahedral and tetrahedral coordination. This is achieved by fitting simultaneously to forsterite and MgSiO₃ ilmenite crystal structure data, and fixing the partial ionic charges using elastic data for forsterite. The potential is found to transfer successfully to γ-Mg₂SiO₄ and MgSiO₃ perovskite. The potential results in local structural errors around the bridging oxygen ions in clinoenstatite and β-Mg₂SiO₄. The predicted structure for MgSiO₃ garnet is similar to the experimentally measured structure of the MnSiO₃ analogue. Calculated elastic constants average to K=2.41 Mbar and μ=1.44 Mbar for the bulk and shear moduli of MgSiO₃ perovskite, and K=1.87 Mbar and μ=1.10 Mbar for the bulk and shear moduli of MgSiO₃ garnet.     

Quasi-hydrostatic compression of ruby to 500 Kbar

Measurements of the cell parameters of ruby to 500 Kbar under quasi-hydrostatic conditions in a diamond-anvil cell show a practically isotropic compression, the ratio c/a of the hexagonal cell parameters remaining constant. These results are consistent with previous single-crystal measurements below 100 Kbar as well as with ultrasonic elastic-constant measurements. There is no evidence for significant differences between the bulk moduli of ruby and corundum, and a review of compression and ultrasonic data yield Ko = 2.53±0.01 Mbar and Ko = 5.0 ± 0.4. Good agreement with the experimental values is found for compressions calculated from empirically or theoretically determined elastic properties.