Anna M. Dymshits

Crystal chemistry of sodium in the Earth’s interior: The structure of Na2MgSi5O12 synthesized at 17.5 GPa and 1700 °C

Abstract The crystal structure and chemical composition of a crystal of Na2MgSi5O12 garnet synthesized in the model system Mg3Al2Si3O12-Na2MgSi5O12 at 17.5 GPa and 1700 °C have been investigated. Quantitative analysis leads to the following formula: Na1.98Mg1.00Si5.01O12. Na2MgSi5O12 garnet was found to be tetragonal, space group I41/acd, with lattice parameters a = 11.3966(6), c = 11.3369(5) Å, V = 1472.5(1) Å3. The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 Å for both the sites), whereas Si is ordered at both the Y (mean: 1.793 Å) and Z sites (means: 1.630 and 1.624 Å). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na2MgSi5O12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages

Microsoft excel spreadsheets for calculation of P-V-T relations and thermodynamic properties from equations of state of MgO, diamond and nine metals as pressure markers in high-pressure and high-temperature experiments

We present Microsoft Excel spreadsheets for calculation of thermodynamic functions and P-V-T properties of MgO, diamond and 9 metals, Al, Cu, Ag, Au, Pt, Nb, Ta, Mo, and W, depending on temperature and volume or temperature and pressure. The spreadsheets include the most common pressure markers used in in situ experiments with diamond anvil cell and multianvil techniques. The calculations are based on the equation of state formalism via the Helmholtz free energy. The program was developed using Visual Basic for Applications in Microsoft Excel and is a time-efficient tool to evaluate volume, pressure and other thermodynamic functions using T-P and T-V data only as input parameters. This application is aimed to solve practical issues of high pressure experiments in geosciences and mineral physics. The program to calculate P-V-T properties of pressure markers is presented.The program was developed using VBA module in MS Excel.The calculation scheme is based on the formalism of equations of state.Thermodynamic and P-V-T properties of MgO, diamond and 9 metals is calculated.

Conditions of magmatic crystallization of Na-bearing majoritic garnets in the earth mantle: Evidence from experimental and natural data

Results of experimental study at 7.0–8.5 GPa and 1300–1900°C of the systems pyrope Mg3Al2Si3O12 (Prp)-Na2MgSi5O12 (NaGrt) modeling solid solutions of Na-bearing garnets, Prp-jadeite NaAlSi2O6 (Jd) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp-Na2CO3 are presented. Prp-Na2MgSi5O12 join is a pseudobinary that results from the decomposition of NaGrt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na2O) and excess Si (3.05–3.15 f.u.). Maximal Na2O concentrations (1.5 wt % Na2O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al → Na + Si type with the appearance of Na2MgSi5O12 component as a mechanism of such garnet formation. The Prp-Jd join is also pseudobinary that results from the formation of two series of solid solutions: (1) garnet (Prp-NaGrt-majorite) and (2) pyroxene (Jd-clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest Na2O contents (>0.8 wt %) are formed. In spite of quite a wide field of garnet crystallization (20–100 mol % Prp), garnets with significant sodium concentration (>0.3 wt % Na2O) are formed in a Jd-rich part of the system (20–50 mol % Prp). In the Prp-Na2CO3 system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9 wt % Na2O. Our experiments demonstrate that melt alkalinity, as well as PT-parameters control the crystallization of Na-bearing majoritic garnets. The results obtained provide evidence for the fact that the majority of natural diamonds with inclusions of Na-bearing majoritic garnets containing <0.4 wt % Na2O were formed in alkaline silicate (carbonate-silicate) melts at a pressure of <7 GPa. Only a small portion of garnets with higher sodium concentrations (>1 wt % Na2O) could be formed at a pressure of >8.5 GPa. 1 This article was translated by the authors.

Thermal equation of state of majoritic knorringite and its significance for continental upper mantle

The P-V-T equation of state (EoS) for majoritic knorringite Mg3.19Cr1.60Si3.19O12 at pressures to 17 GPa and temperatures to 1673 K was obtained from in situ X-ray diffraction experiments using a Kawai-type multi-anvil apparatus. Fitting of the room-temperature P–V data to a third-order Birch-Murnaghan EoS yielded an isothermal bulk modulus, K0,300 = 154 (4) GPa, and a pressure derivative, K′0,300 = 5.4 (1.2). When fitting a high-temperature Birch-Murnaghan EoS using all of the P-V-T data at a fixed V0 = 1549.08 A3, we find that K0,300 = 157 (2) GPa, K′0,T = 4.5 (6), (∂K0,T/∂T)P = −0.019 (4) (GPa K−1), a = 3.00 (14) × 10−5 K−1, and b = 0.65 (24) × 10−8 K−2, where α = a + bT is the volumetric thermal expansion coefficient. Fitting the Mie-Gruneisen-Debye EoS with the present data with a Debye temperature fixed at θ0 = 731 K yielded a Gruneisen parameter, γ0 = 1.34 at q = 1.0 (fixed). The thermoelastic parameters of pure knorringite were estimated and were compared with the previous data on other garnet compositions. The presence of Cr2O3 in pyrope garnets in the upper mantle decreases P- and S-velocities by 1.6% and the density increases by 1.7% (for 20 mol.% knorringite end member) compared to pure pyrope. The results show the importance of accounting knorringite end-member for accurate estimation of mantle garnet acoustic velocities.

P-V-T equations of state for iron carbides Fe3C and Fe7C3 and their relationships under the conditions of the Earth’s mantle and core

According to the geophysical data, the outer liquid core of the Earth has a deficiency of density and seis� mic velocities of 5–12%, and inner solid core has 3– 5% at the expense of the presence of one or several light elements. The most probable candidates for the role of the light element are H, C, O, S, and Si [1]. The problem of light elements may be solved using detailed thermodynamic description of solid iron and nickel compounds, as well as metal alloys, on the basis of the data on their equations of state and phase transition boundaries. The Fe–C system is of key importance for discussion of the composition of the Earth’s core. The equations of state of iron carbides at 300 K are studied at pressures up to 180 GPa [2, 3].

In situ observation of the pyroxene-majorite transition in Na2MgSi5O12 using synchrotron radiation and Raman spectroscopy of Na-majorite†

Abstract In situ X-ray diffraction study of the pyroxene to majorite transition in Na2MgSi5O12 was carried out in Kawai-type high-pressure apparatus coupled with synchrotron radiation. The phase boundary between Na-pyroxene and Na-majorite was determined over the temperature interval of 1073-1973 K and was described by a linear equation P (GPa) = 12.39 + 0.0018×T (K). The Clapeyron slope (dP/ dT) determined in this study is similar to the one predicted by computer simulations (Vinograd et al. 2011) but smoother than the one obtained by quenched experiments (Dymshits et al. 2010). The presence of sodium in the system lowers the pressure of pyroxene-to-majorite transformation. For the first time Na-majorite was characterized using Raman spectroscopy. Raman peaks of Na-majorite are broader than pyrope due to the substitution of Mg2+ for Na+ at the X site. Both Si-O symmetric stretching (A1g-n1) and O-Si-O symmetric bending (A1g-n2) modes of Na-majorite are significantly shifted to higher frequencies relative to corresponding bands of pyrope. In contrast the A1g-R (SiO4) mode of Na-majorite (342 cm-1) displays a lower frequency than that of pyrope (365 cm-1). Our results enable further understanding of the mechanisms responsible for phase transformations in the Earth’s transition zone and lower mantle.

The Mg3Al2Si3O12-Na2MgSi5O12 system at pressures of 7.0 and 8.5 GPa and a temperature of 1300–1800°C: Phase relationships and crystallization of Na-bearing majoritic garnet

The results of an experimental study of the pyrope (Mg3Al2Si3O12)-jadeite (NaAlSi2O6) system at P = 7.0 and 8.5 GPa and T = 1300–1800°C are summarized in this paper. The main phases that were obtained in the experiments are garnet, pyroxene, kyanite (in some cases corundum), and quenched melt. Garnets are characterized by a stable Na2O admixture (up to 0.6 wt % at 7.0 GPa and up to 0.8 wt % at 8.5 GPa) and the high silicon content (Si = 3.016–3.166). The maximal sodium concentrations in garnet were found at the solidus of the system, which results from an increase of the coefficient of sodium partitioning between garnet and melt during a temperature decrease.

Phase relations in the system (Mg,Ca)3Al2Si3O12-Na2MgSi5O12 at 7.0 and 8.5 GPa and 1400–1900°C

The CaO-MgO-Al2O3-SiO2-Na2O multicomponent system was experimentally studied at 7.0 and 8.5 GPa using an anvil-with-hole toroidal high-pressure apparatus to examine two binary joins: pyropegrossular and grossular-Na-majorite. These and literature data were employed to simulate the liquidus surface of the pyrope-grossular-Na-majorite system. The liquidus surface of garnet of predominantly pyrope composition is dominant in the diagram, and the garnet contains much of the Na2MgSi5O12 end member. Melting was observed in this region at temperatures above 1900°C, and the solidus of the system occurs at temperatures below 1550°C. The pyrope-grossular system shows a miscibility gap at 50–65 mol % of the pyrope component and two series of garnet solid solutions. The dominant phase at grossular and Na-majorite mixing is pyroxene, and garnet crystallizes within a fairly narrow field in the grossular-rich region. All garnets synthesized in the systems have elevated Si and Na concentrations and belong to the majorite series, for which a uniform mechanism of isomorphism (Mg, Ca) + Al = Si + Na was proved.

Thermal equation of state of NaMg0.5Si2.5O6 and new data on the compressibility of clinopyroxenes

The results of studies of the P-V-T equations of state (EOS) of Na-pyroxene using the multi-anvil technique and synchrotron radiation at pressures up to 15.3 GPa and temperatures up to 1673 K are presented. By fitting the Birch-Murnaghan EOS, the following parameters were determined: V0 = 407.2 (5) Å3, the space group P2/n, KT0 = 103 (2) GPa, KT0 = 6.2 (7), ∂KT/∂T = −0.018 (7), α = 3.38(13) + 0.65(62)T. Thus, despite the small volume of the cell, Na-pyroxene has a sufficiently high bulk modulus. This can be caused by the appearance of antipathetic bonds in Na-polyhedron, Si-tetrahedra rotation, and the ordering of Mg and Si cations in the M1 position. Thus, it is substantiated that the phase transformations in the minerals accompanied by the presence of Si in octahedral coordination are characterized by a significant change in the physical characteristics, such as density (ρ) and bulk modulus (KT). Such transformations occurring in the minerals and deep Earth can lead to significant jumps in the seismic wave velocities. Therefore, the presence of phases with silicon in sixfold coordination, such as Na-Ca majoritic garnet is of fundamental importance for understanding the Earth’s upper mantle.