Mineral and Rock Physics [MR]

MR31A MCC:Level 1 Wednesday

Thermoelastic Properties of Minerals at High Pressures and Temperatures: Experiment Versus Theory I Posters

Presiding: I Kantor, Bayerisches Geoinstitut, University Bayreuth; S Sinogeikin, University of Illinois

MR31A-0112

Pressure-temperature Stability Studies of Talc and 10-Å Phase Using X-ray Diffraction.

* Gleason, A E (aegleason@lbl.gov) , Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 United States
Kunz, M (mkunz@lbl.gov) , Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 United States
Parry, S (stephen.parry@manchester.ac.uk) , School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom
Pawley, A (alison.pawley@manchester.ac.uk) , School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom
Caldwell, S A (wacaldwell@lbl.gov) , Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 United States
Clark, S M (smclark@lbl.gov) , Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 United States
Clark, S M (smclark@lbl.gov) , School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL United Kingdom

We investigated the behavior of talc plus water to 10-Å phase formation. This phase is capable of transporting water from subducted oceanic lithosphere into the Earth's mantle. Knowing more about the stability field of 10-Å phase across the slab-mantle interface can help to determine the Earth's water budget. X-ray diffraction measurements of natural talc plus water at combined temperatures of $27C-$400C and pressures of 0-15 GPa were made at the Advanced Light Source beamlines 12.2.2 and 11.3.1. Elevated temperatures and pressures were achieved with a resistively heated, membrane driven diamond-anvil cell. Data were collected along isotherms and modeled using a monoclinic unit cell. Our data suggest that water is incorporated into the talc structure at room temperature while at elevated pressures. Elevated temperature data show pressure-temperature path dependence and time-dependence for in situ data collection. All temperature data suggest a change in the compression mechanism of talc starting at approximately 2 GPa.

MR31A-0113

P-V single-crystal- and P-V-T powder X-ray diffraction study of metamict zircon (ZrSiO4)

* Kunz, M (mkunz@lbl.gov) , Lawrence Berkeley Laboratory, ALS, MS 4R 0230 1 Cyclotron Rd, Berkeley, CA 94720 United States
Simoncic, P (psimoncic@ucdavis.edu) , University of California at Davis, Thermochemistry Facility 4440 Chemistry Annex 1 Shields Avenue, Davis, CA 95616 United States

Zircon (ZrSiO4) is of interest due to its capabilities to retain radioactive elements such as U and Th in the crystal structure. This makes it an ideal material for U-Pb dating in geology. Zircon is also an interesting model phase to study the mechanism and properties of structural damage inflicted by self-radiation (metamictization). This is important in view of the attempts for chemical inertialization of radioactive isotopes using ceramic nuclear waste forms. While there are numerous studies on the metamictization of zircon at ambient conditions and elevated temperatures, the effect of pressure on the radiation induced structural damage is not well understood as yet. Diverging results are published on the compressibility of non-metamict zircon. Here we present a structural study of metamict zircon (~900 ppm U, ~1.5 x 1018 alpha decay events g$^{-1}$) at elevated pressure and simultaneously elevated pressure and temperature and compare it with literature data of non-metamict zircon (Hazen and Finger, 1979; Kolesov et al, 2001, Van Westrenen et al. 2004). Data were collected on a single crystal (P-V-data, structure refinement) between 0 a 7 GPa. A powder diffraction experiment was performed in a heatable diamond anvil cell at pressures up to 10 GPa and temperatures up to 573 K sample (P-V-T-data). Room temperature single-crystal compressibility data were fitted to a 3rd-order Birch Murnaghan equation of state. This gives a room-temperature compressibility of K$_{0}$ = 205.7(5) GPa, K\'{}= 5.3(4), V$_{0}$ = $262.609(9) Å^{3}$. When constraining K\'{} to 4, the K$_{0}$ increases to 209.6(3) GPa with a V$_{0}$ of 262.585(8) Å^{3}$. The a-axis is much more compressible (Ka$_{0}$ = 180.6(4) GPa) than the c-axis (Kc$_{0}$ = 304.4(18) GPa). This is in qualitative accordance with the behavior observed for non-metamict zircon, however, the axial compressibilities are slightly more isotropic in our metamict zircon compared to a non-metamict sample. The anisotropy in compressibility is reflected in the observed change in bond-length. Between 0 and 7 GPa, we observe a decrease in bond length of 1.7 % for the Zr-O(e) bond, oriented perpendicular to the c-axis, whereas the Zr-O(c), oriented sup-parallel to c, is compressed only by 0.7 %. The four Si-O bonds compress by 1 % in the same pressure range. The softer behavior of the a-b-plane relative to the c-axis is somewhat surprising in view of the well known larger swelling of the c-axis upon metamictization. The single-crystal diffraction peaks show a reversible decrease in peak width (FWHM) with increasing pressure from $0.085(5) at ambient condition to $0.070(5) at 7 GPa. A simultaneously loaded quartz crystal showed no variation in peak width. A decrease in peak-width with increasing pressure is unusual and may be related to a stronger relative compression of the metamict portions within the crystal compared to the intact bulk structure. The analysis of temperature dependent compressibilities is ongoing and will be presented together with the P-V data. Hazen and Finger (1979): American Mineralogist, 64, 196 - 201. Kolesov et al. (2001): European Journal of Mineralogy, 13, 939 - 948. Van Westrenen et al. (2004): American Mineralogist, 89, 197 - 203.

MR31A-0114

Aggregate Elastic Moduli and Equation of State of B2 Phase of NaCl to 73 GPa by Simultaneous Synchrotron X-ray Diffraction and Brillouin Scattering Measurements.

* Lakshtanov, D L (lakshtan@uiuc.edu) , University of Illinois at Urbana-Champaign, 1301 W Green St., Urbana, il 61801 United States
Sinogeikin, S V (sinogeik@uiuc.edu) , University of Illinois at Urbana-Champaign, 1301 W Green St., Urbana, il 61801 United States
Sanchez-Valle, C (csanchez@ens-lyon.fr) , University of Illinois at Urbana-Champaign, 1301 W Green St., Urbana, il 61801 United States
Prakapenka, V (prakapenka@cars.uchicago.edu) , GSECARS, University of Chicago, University of Chicago, Chicago, il 60637 United States
Shen, G (gshen@hpcat.aps.anl.gov) , HPCAT, Advanced Photon Source, Argonne National Lab, Argonne, IL 60439 United States
Gregoryanz, E (e.gregoryanz@gl.ciw.edu) , Carnegie Institution of Washington, Geophysical Lab, 5251 Broad Branch Rd NW, Washington, DC 20015 United States
Bass, J D (jaybass@uiuc.edu) , University of Illinois at Urbana-Champaign, 1301 W Green St., Urbana, il 61801 United States

NaCl is not only the most studied ionic compound in nature, it is also a widely used quasi-hydrostatic pressure medium and pressure calibrant in high-pressure experiments up to the Megabar range. At pressures of about 30 GPa NaCl undergoes a reconstructive phase transition from the rocksalt (B1) structure to CsCl (B2) structure with more than a 4% increase in density. There have been several attempts to constrain the EOS of NaCl B2 phase by cross calibration of the NaCl unit cell volume with other pressure standards such as Au and MgO (Sata et al., 2002) or ruby fluorescence (Heinz and Jeanloz, 1984). While the resulting equations of state adequately describe the data within the pressure range of direct measurements, extrapolation of the data to higher pressures can lead to serious errors in pressure determination. This is due to the significant trade-off between the inferred bulk modulus (K$_{T}$) and its pressure derivative (K$_{T}$'), as well as the inability to directly measure the zero-pressure volume (V$_{0}$). Thus, finite strain extrapolations of the P-V data beyond the experimental pressure range are rather uncertain. We have recently installed a Brillouin spectrometer on station 13-BM-D at GSECARS, APS. This allowed us to simultaneously measure isotropic acoustic velocities by Brillouin spectroscopy and unit cell volumes by angle-dispersive x-ray diffraction on a polycrystalline sample of the B2 phase of NaCl. A plate of NaCl was loaded into a DAC with Ne as a pressure medium. Ruby chips,Au,and Pt were also in the sample chamber for pressure measurement and cross-calibration. Simultaneous velocity and density measurements were performed at pressures from 35 to 73 GPa. Thus, at each pressure we directly obtained the isotropic adiabatic bulk and shear moduli of NaCl (B2). Direct knowledge of the bulk modulus as a function of pressure allows us to extrapolate the unit cell parameters of NaCl to ambient pressure with a high degree of accuracy, and to tightly constrain the P-V EOS of NaCl (B2). Our data extends the usage of NaCl as an independent pressure marker in high-pressure experiments, providing a consistent pressure scale as a reference for comparing results obtained by various experimental techniques in the megabar pressure range.

MR31A-0115

Elastic wave velocities of Fe-bearing ringwoodite under pressure and temperature conditions of the mantle transition region

* Higo, Y (higo@sci.ehime-U.ac.jp) , Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho,Matsuyama 790-8577,Japan , Matsuyama, 790-8577 Japan
Inoue, T (inoue@sci.ehime-U.ac.jp) , Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho,Matsuyama 790-8577,Japan , Matsuyama, 790-8577 Japan
Irifune, T (irifune@dpc.ehime-U.ac.jp) , Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho,Matsuyama 790-8577,Japan , Matsuyama, 790-8577 Japan
Li, B (bli@notes.cc.sunysb.edu) , Mineral Physics Institute, State University of New York, Stony Brook, NY 11794, New York, 11794 United States
Liebermann, R C (Robert.Liebermann@stonybrook.edu) , Mineral Physics Institute, State University of New York, Stony Brook, NY 11794, New York, 11794 United States

Ringwoodite (high pressure polymorph of olivine) is considered to be the most abundant mineral at depths between 520km and 660km in the mantle transition region, and it is important to accurately determine the elastic wave velocities in order to discuss the mineralogy and composition of the mantle transition region. In this study, We have developed ultrasonic interferometry conjunction with in situ X-radiation techniques (X-ray diffraction and X-radiography) in a DIA-type cubic anvil high-pressure apparatus, in order to obtain high-quality elastic wave velocity data under the conditions of the mantle transition region and measured the elastic wave velocity of iron-bearing ringwoodite under high temperature and high pressure. The specimen was hot-pressed at 19GPa and 1473K in a 3000-ton high pressure apparatus (ORANGE-3000: GRC at ehime university). High-pressure ultrasonic experiments were performed using Kawai-type high-pressure apparatus SPEED-1500 at BL04B1. The unit cell constants of pressure marker (NaCl, Au) and sample (ringwoodite) were measured for estimation of pressure and sample length. Also, X-radiography was used in this study for direct measurement of sample length at high pressure and high temperature. The ultrasonic signals were generated and received by a LiNbO3 transducer (10 Y-cut), which can produce both longitudinal and shear waves at the same time. The waveforms were very clear and the each echoes both P-wave and S-wave were identifiable at high temperature and high pressure. P-wave and S-wave velocities increasing with increasing pressure. And P-wave and S-wave velocities decrease with increasing temperature. Especially, S-wave velocity has large temperature dependence, which velocity (at 18 GPa, 1673 K) slower than those of ambient condition. The results of our high-pressure experiment, including the elastic moduli and their pressure dependence, effect of iron on the elastic moduli, as well as their implication for the mantle transition zone, will be presented.

MR31A-0116

Elasticity of Hot-Pressed Minerals of the Earth's Transition Zone Determined by Resonance Ultrasound Spectroscopy

* Isaak, D (disaak@apu.edu) , Institute of Geophysics & Planetary Physics, University of California, Los Angeles, CA 90095-1567
Gwanmesia, G (ggwanmes@desu.edu) , Physics and Pre-Engineering Department, Delaware State University, Dover, DE 19901
Triplett, R (talion@netzero.net) , Physics and Pre-Engineering Department, Delaware State University, Dover, DE 19901
Fisher, E (efisher@apu.edu) , Department of Mathematics and Physics, Azusa Pacific University, Azusa, CA 91702
Falde, D (lcwr4c00@aol.com) , Department of Mathematics and Physics, Azusa Pacific University, Azusa, CA 91702
Wang, L (lipwang@notes.cc.sunysb.edu) , The Mineral Physics Institute, Earth and Space Sciences Building, State University of New York, Stony Brook, NY 11794-2100

Models showing how processes in Earth's interior influence conditions near the surface must be constrained by accurate laboratory data. Understanding the effect of structure, temperature, and pressure on elastic properties of minerals is of special interest to Earth scientists. Accordingly, we have initiated a collaborative project to accurately and precisely measure the elasticity of several transition zone mineral phases using resonant ultrasound spectroscopy (RUS) on hot-pressed polycrystalline aggregates of these minerals. To date, we have studied the elasticity of CaTiO3 perovskite (density=4.02 g cm$^{-3}$) from room temperature to 500 K and Mg2SiO$_{4}$ forsterite (density=3.227 g cm$^{-3}$) at ambient conditions. For CaTiO3 perovskite, we find $K_{S}$=173.9 GPa, $G$=104.7 GPa, ($\partial K_{S}/\partial T$)$_{P}$=-0.0188 GPa K$^{-1}$, and ($\partial G/\partial T$)$_{P}$=-0.0175 GPa K$^{-1}$. With the exception of the ($\partial K_{S}/\partial T$)$_{P}$ result, these values are consistent with those from elasticity studies using alternative techniques. At room temperature, we obtain $K_{S}$=129.4 GPa and $G$=80.9 GPa for Mg2SiO$_{4}$ forsterite which are in excellent agreement with results from single-crystal studies. We confirm that the hot-pressed specimens exhibit isotropic elastic behavior; there is no evidence that texturing and preferred orientation adversely affect the elasticity data. The significance of these studies will be discussed. We also expect to report on new measurements of temperature dependences of elasticity for both forsterite and wadsleyite hot-pressed polycrystalline aggregates.

MR31A-0117

Ab Initio Treatment of the CaSiO3-MgSiO3 Solvus.

* Jung, D Y (daniel.jung@erdw.ethz.ch) , Institute for Mineralogy and Petrography, ETH Zürich, Sonneggstr.5, Zürich, 8092 Switzerland
* Jung, D Y (daniel.jung@erdw.ethz.ch) , Laboratory for Crystallography, ETH Zürich, ETH Hönggerberg Wolfgang-Pauli-Strasse 10, Zürich, 8093 Switzerland
Oganov, A R (a.oganov@mat.ethz.ch) , Laboratory for Crystallography, ETH Zürich, ETH Hönggerberg Wolfgang-Pauli-Strasse 10, Zürich, 8093 Switzerland
Schmidt, M W (max.schmidt@erdw.ethz.ch) , Institute for Mineralogy and Petrography, ETH Zürich, Sonneggstr.5, Zürich, 8092 Switzerland

The lower mantle of the Earth extends from about 670 km to 2980 km and consists mainly of MgSiO3-perovskite (~ 75 vol%), (Mg,Fe)O magnesiowüstite (~ 20 vol%) and CaSiO3-perovskite (~ 5 vol%). It is possible to calculate thermodynamic properties, structures and energetics of the individual minerals at extreme conditions of the mantle using {\it ab initio} methods, such as density functional theory. To obtain a more realistic picture of the lower mantle it is necessary to not only investigate chemically pure minerals, but to consider minerals as solid solutions, as they are in nature. The density functional theory with the generalized gradient approximation (GGA) and the projector augmented wave (PAW) method, as implemented in the VASP code, was used to calculate the structure and stability of CaSiO3 perovskite in the pressure range of the Earth's mantle (0-150 GPa), no post-perovskite structure has been found $[$1$]$. Here we focus on the two perovskite solvus. We use a subregular solid solution model together with point defect calculations to model the solvus at different pressures in the lower mantle regime. Additionally, the effect of the different symmetries ({\it Pbnm} and {\it I}4/{\it mmm}) of the perovskites has to be included. This is important especially for the Ca-perovskite, since the energy differences of the two phases are very small and thus likely to have an influence on the solvus. We investigated the solvus at different pressures of the lower mantle. At pressures and temperatures of the lower mantle, the solvus in the (Ca,Mg)SiO3 system remains wide open and solubilities of Ca in Mg-perovskite and Mg in Ca-perovskite low. From these results in the simple system it is highly unlikely that Ca-perovskite will disappear (i.e. fully dissolve in Mg-perovskite) with depths in the lower mantle. Information of the solubility of Ca in MgSiO3 in more complex systems will elucidate the mineralogical composition of the lower mantle of the Earth. This is the first work to treat this subject with {\it ab initio} methods. Presently, calculations on the Ca-Mg-perovskite solvus with aluminium impurities are in progress. $[$1$]$ Jung D.Y., Oganov A.R. (2005) Phys. Chem. Minerals 32, 146-153

MR31A-0118

Elasticity of polycrystalline MgSiO3-orthoenstatite to 1373K

* Kung, J (jkung@mail.ncku.edu.tw) , Mineral Physics Institute, Stony Brook University, ESS building Stony Brook Univsity, Stony Brook, 11794 United States
* Kung, J (jkung@mail.ncku.edu.tw) , Cheng Kung University, Department of Earth Sciences, 1 University Rd, Tainan, 70101 Taiwan
Jackson, I (Ian.Jackson@aun.edu.au) , Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACt, 0200 Australia
Weston, L (lara.weston@anu.edu.au) , Research School of Earth Sciences, Australian National University, Mills Road, Canberra, ACt, 0200 Australia
Liebermann, R (Robert.liebermann@sunysb.edu) , Mineral Physics Institute, Stony Brook University, ESS building Stony Brook Univsity, Stony Brook, 11794 United States

Orthopyroxene is an important mineralogical constituent of petrological models of the Earth's upper mantle. To construct the mineralogical models for Earth's mantle, the pressure and temperature derivatives of the bulk and shear moduli are essential. The elasticity data for orthoenstatite (OEN) are, mainly, available at high pressure conditions (Flesch et al., 1998; Angel and Jackson, 2002; Kung et al. 2004). Some selected elastic constants (Cij) of the OEN phase were measured up to high temperature, room pressure by Jackson et al. (2004), however, they are insufficient to calculate the bulk elasticity properties for the aggregates. In this study, P and S wave sound velocities have been measured on dense isotropic polycrystalline MgSiO3-OEN in an internally heated gas-medium high-pressure apparatus. Two different runs were carried out up to 973K and 1373K at 300 MPa. Both P and S wave velocities from two runs decrease linearly as function of temperature and show highly agreement below 973K. The temperature derivatives of velocities above 1100K are very different from those below 973K. We speculated that the changes of temperature dependence at high temperature are related to the velocity softening" observed in MgSiO3 due to the phase transition (Jackson et al.,2004). We will present our new measurements in the meeting. Ref: Flesch et al. (1998) Am. Miner. 83(5-6), 444-450. Angel and Jackson (2002) Am. Mineral. 87(4), 558-561. Kung et al. (2004) Phys. Earth Planet. Inter., 147(1), 27-44. Jackson et al. (2004) Am. Mineral. 89(1), 239-244.

MR31A-0119

Elastic Constants of Brucite (Mg(OH)2) and Diaspore (AlO(OH)) to 12 GPa by Brillouin Scattering

Jiang, F (fumingj@princeton.edu) , Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
Speziale, S (speziale@uclink.berkeley.edu) , University of California, Berkeley, Department of Earth and Planetary Science, 307 McCone Hall, Berkeley, CA 94720 United States
Majzlan, J (Juraj.Majzlan@minpet.uni-freiburg.de) , Albert-Ludwig-University of Freiburg, Institute of Mineralogy, Petrology and Geochemistry, Albertstrasse 23b, Freiburg, D-79104 Germany
* Duffy, T S (duffy@princeton.edu) , Princeton University, Department of Geosciences, Princeton, NJ 08544 United States

Hydroxides such as brucite, Mg(OH)2, and diaspore, AlO(OH), serve as analogs for the more complex hydrogen-bearing minerals found in subduction zones. The single-crystal elastic constants of these two minerals were determined by Brillouin scattering up to 12 GPa in a diamond anvil cell. A 16:3:1 methanol-ethanol-water and 4:1 methanol-ethanol mixture were used as pressure media and ruby as pressure calibrant. Two platelets of brucite and three platelets of diaspore were measured at room pressure and over nine elevated pressures. Brillouin spectra were recorded in 37 directions with a 5-degree step for each plane. All individual elastic stiffness constants were retrieved by fitting the velocity data to the Christoffel's equation. The individual elastic constants, aggregate bulk and shear moduli, were then fitted to the finite Eulerian stain equations to obtain their pressure derivatives. For brucite, aggregate moduli and their pressure derivatives are $K_{T0}$=35.9(9) GPa, $G_{0}$=31.3(2) GPa, $(\partial K_{T}/\partial P)_{T0}$=8.9(3), $(\partial G/\partial P)_{0}$=4.33(4) for the Reuss bound. Diaspore aggregate moduli and their pressure derivatives are $K_{T0}$=147.0(7) GPa, $G_{0}$=114.8(5) GPa, $(\partial K_{S}/\partial P)_{T0}$=4.1(1), $(\partial G/\partial P)_{0}$=1.6(1). For diaspore, both individual and aggregate elastic moduli define nearly linear modulus pressure trend within the measured pressure range. For Brucite, there are clear deviations from linear modulus - pressure trends. In comparison with diaspore, brucite exhibits strong anisotropy. The ratio of the linear compressibility of brucite along the c and a axes decreased from 4.7 to 1.3 over the examined pressure range. The shear anisotropy $(C_{66}/C_{44})$ decreased from 2.6 at ambient condition to 1.3 with increase of pressure to 12 GPa. Compression curves constructed from our Brillouin data for both brucite and diaspore are in good agreement with previous x-ray diffraction data.

MR31A-0120

Brillouin Scattering and Synchrotron X-Ray Measurements at GSECARS, Advanced Photon Source: Simultaneous Measurements of Sound Velocities and Density

* Bass, J D (jaybass@uiuc.edu) , University of Illinois Urbana-Champaign, Geology Department 1301 W Green St, Urbana, IL 61801 United States
Sinogeikin, S V (sinogeik@uiuc.edu) , University of Illinois Urbana-Champaign, Geology Department 1301 W Green St, Urbana, IL 61801 United States
Lakshtanov, D (lakshtan@uiuc.edu) , University of Illinois Urbana-Champaign, Geology Department 1301 W Green St, Urbana, IL 61801 United States
Prakapenka, V (prakapenka@cars.uchicago.edu) , GSECARS, Dept of Geophysical Sciences University of Chicago, Chicago, IL 60637 United States
Shen, G (shen@cars.uchicago.edu) , GSECARS, Dept of Geophysical Sciences University of Chicago, Chicago, IL 60637 United States

A Brillouin spectrometer have been installed and successfully tested at the GSECARS Sector 13 (BMD line) of the Advanced Photon Source (APS). It is now possible to perform simultaneous measurements of density (by x-ray diffraction) and sound velocities (by Brillouin scattering) on transparent single crystal and polycrystalline materials at high pressure and temperatures. This allows the determination of an absolute pressure scale. The equation of state and acoustic velocities and, hence, elastic moduli of materials as a function of pressure and temperature can now be determined without resort to a secondary pressure standard, such as the ruby fluorescence scale. We describe here the design of the system and the first experimental results obtained from this unique facility. Some of the important design characteristics for the system are: 1) it is user-friendly, and a general user can operate the system without being an expert. 2) it requires minimal set-Up time and does not require frequent alignment of the optics; 3) all control and adjustment of optics normally performed during a Brillouin experiment can be performed from outside the hutch; 4) it is flexible with respect to sample position and compatible with powder and single-crystal x-ray diffraction techniques, while not interfering with the other experimental techniques used on the beamline. We have measured a single crystal elasticity of NaCl (B1) and MgO to 30 GPa, and aggregate velocities and elastic moduli of NaCl (B2) to over 70 GPa. High P and T measurements were also performed, demonstrating the potential of this system for PVT equations of state measurements over a broad range of conditions.

MR31A-0121

A novel thermodynamic model of Mg2SiO$_{4}$ with a superior representation of experimental data predicts negligible layering in mantle convection

Jacobs, M H (Jacobs@geo.uu.nl) , Dept. Theoretical Geophysics, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
* de Jong, B H (bernard@geo.uu.nl) , Petrology Group, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands
van den Berg, A P (berg@geo.uu.nl) , Dept. Theoretical Geophysics, Faculty of Geosciences, Utrecht University, Budapestlaan 4, Utrecht, 3584CD Netherlands

We present a new thermodynamic database for Mg2SiO$_{4}$. This novel database has three characteristics (1) thermodynamic properties are anomaly free in the complete temperature-pressure space and experimental data are represented within their experimental uncertainties in accordance with Calphad criteria (2) it discriminates between experimental data (3) it includes thermo-mechanical properties and matches them against tomographic results within experimental uncertainty. Recently [1], we showed that large differences exist between experimental data on ambient volume and between thermal expansivity data for γ-Mg2SiO$_{4}$, possibly related to hydration effects. We demonstrated that a thermodynamic technique based on polynomial parameterizations of 1 bar thermodynamic properties cannot discriminate between the different ambient volume data and thermal expansivity data for γ-Mg2SiO$_{4}$, hampering the accurate prediction of bulk sound velocities in the transition zone to within tomographic accuracy. We therefore developed a computational technique based on an extended form of Kieffer's [2] approach to model the vibrational density of states of a substance, a key property to derive the Helmholtz energy. This canonical thermodynamic framework, which uses input parameters from Raman and infrared spectroscopic data, constrains thermodynamic properties tighter compared to methods based on polynomial parameterizations of thermal expansivity, heat capacity and isothermal bulk modulus. We shall present recent results on the application of this approach to the Mg2SiO$_{4}$ system [3]. We discovered that anharmonicity in Mg2SiO$_{4}$ (α) affects the heat capacity (C$_{P}$), and position and slope of the α-β phase boundary. For γ-Mg2SiO$_{4}$ our thermodynamic analysis prefers the ambient volume measured by Inoue et al. [4] and thermal expansivity measured by Suzuki [5]. Our analysis reveals that experimental data for MgO and MgSiO3 are represented to within experimental uncertainty by assuming that these substances behave quasi-harmonically. The predicted Clapeyron slope of the post- spinel phase boundary is -(2.00.5) MPa/K. These results, have been included in a numerical model of convection in the Earth's mantle revealing no layered convection in the transition zone. Our model includes the recently discovered post-perovskite phase (P$\approx$125 GPa) based on ab-initio results and V-P-T measurements by Murakami et al. [6]. The convection results indicate that the post- perovskite layer at the bottom of the mantle is a time-dependent phenomenon strongly affected by core temperature of a cooling earth. References [1] M.H.G. Jacobs and B.H.W.S. de Jong, Geochim. Cosmochim. Acta (2005), in press. [2] S.W. Kieffer, Rev. Geophys. Space Physics, 17 (1979) 35-59. [3] M.H.G. Jacobs, B.H.W.S. de Jong and A.P. van den Berg, Calphad (2005), submitted. [4] T. Inoue, Y. Tanimoto, T. Irifune, T. Suzuki, H. Fukui and O. Ohtaka, Phys. Earth Planet. Int. 143-144 (2004) 279-290. [5] I. Suzuki, J. Phys. Earth, 27 (1979) 53-61. [6] M. Murakami, K. Hirose, K. Kawamura, N. Sata and Y. Ohishi, Science, 304 (2004) 855-855

MR31A-0122

Acoustic wave propagation at a cobalt/helium interface - high pressure elasticity of hexagonal metals.

* Crowhurst, J (crowhurst1@llnl.gov) , Lawrence Livermore National Laboratory, 7000 E avenue, Livermore, CA 94551 United States
Antonangeli, D (antonangeli3@llnl.gov) , Lawrence Livermore National Laboratory, 7000 E avenue, Livermore, CA 94551 United States
Brown, J M (brown@ess.washington.edu) , Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195 United States
Goncharov, A (goncharov@ciw.edu) , Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd, Washington, DC 20015 United States
Farber, D (farber2@llnl.gov) , Lawrence Livermore National Laboratory, 7000 E avenue, Livermore, CA 94551 United States
Aracne, C (chantel@llnl.gov) , Lawrence Livermore National Laboratory, 7000 E avenue, Livermore, CA 94551 United States

The velocity of an acoustic wave propagating at the interface formed by a single crystal of cobalt in contact with liquid helium has been measured using impulsive stimulated light scattering. Dispersion curves of velocity versus in-plane propagation direction to 10 GPa have been obtained. The data is used to determine the pressure dependence of the elastic tensor element c44 assuming only the density of cobalt. The additional elements c12 and c13 are also obtained by assuming the values for c11 and c33 that were determined in a separate study using inelastic x-ray scattering [1]. Absolute values of elastic constants and pressure derivatives are compared to the results of first principles theoretical calculations in the generalized gradient and local density approximations. The validity of such calculations for predicting the high pressure elasticity of metals of hexagonal symmetry is discussed. [1] D. Antonangeli, M. Krisch, G. Fiquet, D. Farber, C. Aracne, J. Badro, F. Occelli, and H. Requardt, Phys. Rev. Lett. 93, 215505 (2004).

MR31A-0123

Phase stability and thermal equation-of-state of iron at moderate pressures

* Gao, L (liligao2@uiuc.edu) , Lili Gao, 247 NHB, 1301 W.Green st, Urbana, IL 61801 United States
Li, J (jackieli@uiuc.edu) , Lili Gao, 247 NHB, 1301 W.Green st, Urbana, IL 61801 United States

Iron is the most abundant element in the cores of terrestrial planets. The melting temperature of iron places a critical constraint on the temperature at the Earth's inner-outer-core boundary. The elastic properties of iron are important for modeling the geodynamic processes of the core. The phase stability and thermal equation-of-state of iron have been widely studied. Most studies have been carried out using diamond anvil cells (DAC) The data coverage at moderate pressures and high temperatures is relatively poor. In this work, we have determined the phase boundaries for solid-solid transition and melting of iron to 25 GPa and about 2400 K, using a large-volume press and synchrotron x-ray diffraction at SPring-8, Japan. Our new data also allow derivation of thermal equation-of state of iron. These results can be applied to models of proto-core of the Earth, and the current Martian core.

MR31A-0124

High-Pressure Experiments on FeO up to 200 GPa

* Sata, N (sata@jamstec.go.jp) , Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technolgy, 2-15 Natsushima-cho, Yokosuka-shi, KAN 237-0061 Japan
Hirose, K (kei@geo.titech.ac.jp) , Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, TOK 152-8551 Japan
Ohishi, Y (ohishi@spring8.or.jp) , Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Mikazuki-cho, HYO 679-5198 Japan
Shen, G (shen@cars.uchicago.edu) , Consortium for Advanced Radiation Sources, University of Chicago, 9700 South Cass Avenue, Argonne, IL 60439 United States

Even if FeO itself supposed to be not exiting in both the Earth's mantle and the core, understandings of its properties are important as an end-member of both mantle and core minerals. Goals of this study are investigation of stable phase of FeO and measurements of its density under the Earth's core conditions. High-pressure and high-temperature x-ray diffraction experiments were carried out up to around 200 GPa at 1500 K using a laser heated diamond anvil cell with synchrotron x-ray. Experimental procedures are as follows: increasing target pressure, heating at 1500 K, quench to room temperature, and increasing next target pressure. X-ray diffractions were measured before, during, and after heating at each target pressure. Using NaCl pressure medium, the cubic B1 phase was observed up to around 150 GPa at 1500K. Over 150 GPa, new diffraction peaks were observed up to around 200 GPa . These peaks can be indexed by another cubic cell. However, using Ar pressure medium, the cubic B1 phase was observed up to around 180 GPa and not observed the other cubic phase. These suggest that possibility of reactions between FeO and NaCl over 150 GPa and the cubic B1 phase is still stable at least up to 180 GPa.

MR31A-0125

High Pressure Study of K-Na Hollandite Solid Solution

* Liu, J (jun.liu@uni-bayreuth.de) , Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany, Bayreuth, 95440 Germany
Boffa-Ballaran, T (Tiziana.Boffa-Ballaran@Uni-Bayreuth.DE) , Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany, Bayreuth, 95440 Germany
Dubrovinsky, L (Leonid.Dubrovinsky@uni-bayreuth.de) , Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany, Bayreuth, 95440 Germany
Frost, D (Dan.Frost@Uni-Bayreuth.DE) , Bayerisches Geoinstitut, Universität Bayreuth, D-95440 Bayreuth, Germany, Bayreuth, 95440 Germany

(Na, K)AlSi3O$_{8}$ aluminosilicates hollandite-type materials with their dense structure, in which all Si and Al are in six-fold coordination, are considered as a possible repository of potassium and sodium in the Earth's lower mantle. Phase relations in the system KAlSi3O$_{8}$ - NaAlSi3O$_{8}$ have been examined by other workers at pressures of 5 - 23 GPa and temperatures of 700 - $1200C, indicating that the maximum solubility of NaAlSi3O$_{8}$ component into KAlSi3O$_{8}$ hollandite-type structure at $1000C is about 40 mol%. At higher Na content the high pressure phase appears to be that of the calcium-ferrite type structure. However, in the last few years there have been a number of reports of natural occurrences of NaAlSi3O$_{8}$ hollandite in shock-induced melt veins of chondrite. Aim of our research is, therefore, to extent the study of the phase relation of the K-Na system at higher temperature and to determine the physical-chemical properties and high-pressure behaviour of silicate hollandite-type structures containing K and Na in different concentrations. A series of synthesis experiments has been done with the multi-anvil presses at the Bayerisches Geoinstitut in the pressure range 13-25 GPa and $1700C, using K$_{x}$Na$_{1-x}$AlSi3O$_{8}$ (x = 1, 0.7, 0.5, 0.4) glasses as the starting materials. X-ray powder diffraction analysis and electron microprobe measurements of the run products show that at 20GPa, pure K$_{0.7}$Na$_{0.3}$AlSi3O$_{8}$ hollandite has been synthesized, with the lattice parameters of a = 9.3133 (5) Å, c = 2.7226 (2) Å, V = 236.15 (2) Å3. At 22 GPa, pure K$_{0.5}$Na$_{0.5}$AlSi3O$_{8}$ hollandite has been synthesized, with the lattice parameters of a = 9.2984 (4) Å, c = 2.71920 (17) Å, V = 235.103 (15) Å3. The pressure stability field of K$_{x}$Na$_{1-x}$AlSi3O$_{8}$ hollandite appears therefore larger than in the study of privious work. High-pressure X-ray powder and single-crystal diffraction and Raman spectroscopy experiments for this composition are in progress. Preliminary experiments using a NaAlSi3O$_{8}$ end-member glass yield no hollandite-type structure at 22.5GPa and $1700C. Further synthesis experiments are planned to determine the experimental P-T stability field of NaAlSi3O$_{8}$ hollandite.

MR31A-0126

A High-Pressure Phase Transition of Calcite-III

* Catalli, K C (krystle@mit.edu) , University of California, Santa Cruz, Department of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States
Williams, Q , University of California, Santa Cruz, Department of Earth Sciences 1156 High Street, Santa Cruz, CA 95064 United States

We document the presence of a high-pressure phase transition in metastable calcite-III using infrared spectroscopy. The post-calcite-III transition initiates at a pressure of 15.5 ( - 2) GPa, and is completed between 25 and 30 GPa. The transition is particularly apparent in the $\nu$$_{4}$-in-plane bending vibration of the carbonate group, in which two new peaks gradually supplant the doublet associated with calcite-III. Furthermore, both the $\nu3-asymmetric and $\nu$$_{1}$-symmetric stretches of the carbonate group in the high-pressure phase appear at considerably lower frequencies than the extrapolated positions of the corresponding calcite-III peaks. The geometry of the carbonate unit within the high-pressure phase is likely closer to trigonal symmetry than in the calcite-III structure, and the C-O bond is probably longer than in the lower pressure calcite-III phase.

MR31A-0127

Comparison of Quantum and Classical Monte Carlo on a Simple Model Phase Transition

Cohen, D E (dcohen@gl.cw.edu) , Geophysical Laboratory Carnegie Institution of Washington, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States
* Cohen, R E (cohen@gl.ciw.edu) , Geophysical Laboratory Carnegie Institution of Washington, 5251 Broad Branch Rd., N.W., Washington, DC 20015 United States

Most simulations of phase transitions in minerals use classical molecular dynamics or classical Monte Carlo. However, it is known that in some cases, quantum effects are quite large, even for perovskite oxides [1]. We have studied the simplest model of a phase transition where this can be tested, that of interacting of double wells with an infinite- range interaction. The energy is $E = \sum_i (-A x_i^2 + B x_i^4 + \xi <x> x_i)$ . We used the same parameters used in a study of vibrational spectra and soft- mode behavior [4], $A=$0.01902, $B=$0.14294, $\xi=$0.025 in Hartree atomic units. This gives T$_c$ of about 400 K. We varied the oscillator mass from 18 to 100. Classical Monte Carlo and path integral Monte Carlo (PIMC) were performed on this model. The maximum effect was for the lightest mass, in which PIMC gave a 75K lower T$_c$ than the classical simulation. This is similar to the reduction in T$_c$ observed in PIMC simulations for BaTiO3 at zero pressure [1]. We will explore the effects of varying the well depths. Shallower wells would show a greater quantum effect, as was seen in the high pressure BaTiO3 simulations, since pressure reduces the double well depths [5]. [1] Iniguez, J. & Vanderbilt, D. First-principles study of the temperature-pressure phase diagram of BaTiO3. Phys. Rev. Lett. 89, 115503 (2002). [2] Gillis, N. S. & Koehler, T. R. Phase transitions in a simple model ferroelectric-- -comparison of exact and variational treatments of a molecular-field Hamiltonian. Phys. Rev. B 9, 3806 (1974). [3] Koehler, T. R. & Gillis, N. S. Phase Transitions in a Model of Interacting Anharmonic Oscillators. Phys. Rev. B 7, 4980 (1973). [4] Flocken, J. W., Guenther, R. A., Hardy, J. R. & Boyer, L. L. Dielectric response spectrum of a damped one-dimensional double-well oscillator. Phys. Rev. B 40, 11496-11501 (1989). [5] Cohen, R. E. Origin of ferroelectricity in oxide ferroelectrics and the difference in ferroelectric behavior of BaTiO3 and PbTiO3. Nature 358, 136-138 (1992).

MR31A-0128

Effect of Phase Transition of Intermediate Plagioclase on Elastic Wave Velocity

* Kono, Y (d03ta007@ynu.ac.jp) , Yokohana National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
Ishikawa, M (ishikawa@ed.ynu.ac.jp) , Yokohana National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan
Arima, M (arima@server2.edhs.ynu.ac.jp) , Yokohana National University, 79-7, Tokiwadai, Hodogaya-ku, Yokohama, 240-8501 Japan

We measured compressional (Vp) and shear (Vs) wave velocities of plagioclases with albite, labradorite, and bytownite composition up to 700C at 1 GPa with a piston cylinder high-pressure apparatus. The Vp and Vs measurements were conducted with the pulse reflection method. Simultaneous Vp and Vs measurements were carried out using 10Y-cut LiNbO3 transducer. The discontinuous change in temperature derivative of Vp and Vs at 400C is observed in the labradorite while the albite and bytownite show linear Vp and Vs change with elevating temperature up to 700C. The temperature derivative of Vp (dVp/dT) and Vs (dVs/dT) is -0.9x10$^{-4}$ km s$^{-1}$ C$^{-1}$ and -1.2x10$^{-4}$ km s$^{-1}$ C$^{-1}$ in the albite, respectively. In the bytownite dVp/dT is -1.2x10$^{-4}$ km s$^{-1}$ C$^{-1}$ and dVs/dT is -0.9x10$^{-4}$ km s$^{-1}$ C$^{-1}$. The dVp/dT and dVs/dT in the labradorite is -0.7x10$^{-4}$ and -1.2x10$^{-4}$ km s$^{-1}$ C$^{-1}$ below 400C, respectively. Above 400C dVp/dT and dVs/dT increase to -3.0x10$^{-4}$ and -3.3x10$^{-4}$ km s$^{-1}$ C$^{-1}$, respectively. The XRD analyses of the labradorite before and after high P-T measurements show that the discontinuous change in temperature derivative of Vp and Vs in the labradorite would be the result of phase transition of plagioclase. The present data suggest that Vp and Vs of intermediate plagioclase above 400C could not be evaluated by the simple mixing theory using the low temperature Vp and Vs values of end-member plagioclases (albite and anorthite).

MR31A-0129

High-pressure Phase Relation In The MgAl2O4-Mg2SiO4 System

* Kojitani, H (hiroshi.kojitani@gakushuin.ac.jp) , Gakushuin University, Dept. of Chemistry, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan
Hisatomi, R (03142013@gakushuin.ac.jp) , Gakushuin University, Dept. of Chemistry, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan
Akaogi, M (masaki.akaogi@gakushuin.ac.jp) , Gakushuin University, Dept. of Chemistry, 1-5-1 Mejiro, Toshima-ku, Tokyo, 171-8588 Japan

High-pressure and high-temperature experiments indicate that high-pressure phases of oceanic basalts contain Al-rich phases. MgAl2O$_{4}$ with calcium ferrite-type crystal structure is considered as a main component of such the Al-rich phases. Since the calcium ferrite-type MgAl2O$_{4}$ can be synthesized at only the maximum pressure of a Kawai-type high-pressure apparatus with tungsten carbide (WC) anvils, the amount of a synthesized sample is very limited. Therefore, the crystal structure of the calcium ferrite-type MgAl2O$_{4}$ has been hardly known in detail due to these difficulties in sample synthesis. In our high-pressure experiments in the MgO-Al2O3-SiO2 system, it was shown that Mg2SiO$_{4}$ component could be dissolved in the MgAl2O$_{4}$ calcium ferrite. In this study, we tried to synthesize a single phase MgAl2O$_{4}$ calcium ferrite sample and to make the Rietveld refinement of the XRD pattern of the sample. The high-pressure phase relations in the MgAl2O$_{4}$-Mg2SiO$_{4}$ system were studied to know the stability field of the MgAl2O$_{4}$-Mg2SiO$_{4}$ calcium ferrite solid solutions. Lattice parameters-composition relation of the MgAl2O$_{4}$-Mg2SiO$_{4}$ calcium ferrite solid solutions was also determined. High-pressure and high-temperature experiments were performed by using a Kawai-type high-pressure apparatus at Gakushuin University. WC anvils with truncated edge length of 1.5 mm were used. Heating was made by a Re heater. Temperature was measured by a Pt/Pt-13%Rh thermocouple. Starting materials for the phase relation experiments were the mixture of MgO, Al2O3 and SiO2 with bulk compositions of MgAl2O$_{4}$:Mg2SiO$_{4}$ = 90:10, 78:22, 70:30 and 50:50. The starting materials were held at 21-27 GPa and 1600 C for 3 hours and then were recovered by the quenching method. The MgAl2O$_{4}$ calcium ferrite sample for the Rietveld analysis was prepared by heating MgAl2O$_{4}$ spinel at 27 GPa and about 2200 C for one hour. Powder X-ray diffraction (XRD) profiles of obtained samples were measured by using a X-ray diffractometer at Gakushuin University (RINT 2500V, Cr Kα, 45 kV, 250 mA). Composition analysis of the recovered samples was made using SEM-DES. The RIETAN-2000 program was used to perform the Rietveld refinement. The results of the high-pressure phase relation experiments show that stability field of single phase of MgAl2O$_{4}$-Mg2SiO$_{4}$ solid solutions spreads at lower pressure than that of pure MgAl2O$_{4}$ calcium ferrite. The lowest pressure at which the calcium ferrite solid solution can be synthesized is about 23 GPa. The maximum solubility of Mg2SiO$_{4}$ component is about 35%. Lattice parameters of pure MgAl2O$_{4}$ calcium ferrite were determined as a = 9.9495(6) Å, b = 8.6466(5) Å, c = 2.7901(2) Å ({\it Pbnm} space group) by the Rietveld refinement. Obtained atomic positions for calcium ferrite-type MgAl2O$_{4}$ are very similar to those of CaFe2O$_{4}$ calcium ferrite. Lattice parameters of MgAl2O$_{4}$-Mg2SiO$_{4}$ calcium ferrite solid solutions with various compositions indicate that c-axis does not change with the composition and that a- and b-axes have a linear increase and decrease trend with increasing Mg2SiO$_{4}$ component, respectively.

MR31A-0130

Acoustic Anomalies in Magnesioferrite up to 700 C

* Antao, S M (sytle.antao@stonybrook.edu) , State University of New York @ Stony Brook, Mineral Physics Institute and Department of Geosciences, Stony Brook, NY 11794-2100 United States
Jackson, I (Ian.Jackson@anu.edu.au) , Australian National University, Earth Materials Research School of Earth Sciences, Canberra, ACT 0200 Australia
Weston, L (Lara.Weston@anu.edu.au) , Australian National University, Earth Materials Research School of Earth Sciences, Canberra, ACT 0200 Australia
Kung, J (jennifer.kung@stonybrok.edu) , State University of New York @ Stony Brook, Mineral Physics Institute and Department of Geosciences, Stony Brook, NY 11794-2100 United States
Li, B (Baosheng.Li@stonybrook.edu) , State University of New York @ Stony Brook, Mineral Physics Institute and Department of Geosciences, Stony Brook, NY 11794-2100 United States
Chen, J (Jiuhua.chen@stonybrook.edu) , State University of New York @ Stony Brook, Mineral Physics Institute and Department of Geosciences, Stony Brook, NY 11794-2100 United States
Hassan, I (ishmael.hassan@uwimona.edu.jm) , University of the West Indies, Department of Chemistry, Mona, KIN Kingston 7 Jamaica
Liebermann, R C (robert.liebermann@stonybrook.edu) , State University of New York @ Stony Brook, Mineral Physics Institute and Department of Geosciences, Stony Brook, NY 11794-2100 United States
Parise, J (john.parise@stonybrook.edu) , State University of New York @ Stony Brook, Mineral Physics Institute and Department of Geosciences, Stony Brook, NY 11794-2100 United States

A recent study indicated that magnesioferrite spinel, MgFe2O$_{4}$, displays two main transitions with increasing temperature: (1) a Curie transition at about $360C (determined by thermal analyses, TA, in a magnetic field), and (2) relaxation of cations towards equilibrium at about $581C as determined by Rietveld analysis (550 to $568C observed by TA). Ultrasonic experiments were carried out at 300 MPa pressure in gas-medium high-pressure apparatus to determine if these two transitions have any effect on the acoustic properties of magnesioferrite. On heating, the travel times for both the P and S waves increase gradually up to $350C. At $400C there is an abrupt decrease in the travel times, which probably represents the Curie transition that was observed by TA at about $360C. At $600C, the S wave was unobserved, and the P wave showed a sharp increase in travel time up to $700C. Beyond $700C, the P-wave signal was very weak. The deterioration of signal quality and markedly greater travel times at the higher temperatures may reflect the onset of cation disorder or alternatively partial reduction of Fe$^{3+}$ in the reducing environment of the mild steel jacket.

MR31A-0131

Thermal Expansion of Fe3S

* Chen, B (binchen2@uiuc.edu) , Department of Geology, University of Illinois at Urbana-Champaign, 245 Natural History Building, 1301 West Green Street, Urbana, IL 61820 United States
Li, J (jackieli@uiuc.edu) , Department of Geology, University of Illinois at Urbana-Champaign, 245 Natural History Building, 1301 West Green Street, Urbana, IL 61820 United States

We have investigated the thermal expansion of the iron-sulfur compound Fe3S at pressures up to 42.5 GPa and temperatures to 900 K by using synchrotron X-ray diffraction techniques. It has been well recognized that iron and sulfur are possible elements in the cores of the terrestrial planets. According to our present knowledge, Fe3S is the most iron-rich compound in the Fe-S system. The Earth's core probably has sulfur content between pure iron and Fe3S. Given higher sulfur content, Fe3S is a possible component of the Martian inner core. Hence, the equation-of-state of Fe3S is of fundamental importance for our understanding of the Earth's and planetary cores. The room temperature compression curve of Fe3S was determined by Fei et al. [1]. To date, no experimental data have been reported on the thermal expansion of Fe3S. In this study, we report in situ measurements of thermal expansion of Fe3S at high pressure by means of an externally-heated diamond-anvil cell and the energy-dispersive X-ray diffraction techniques at x17c beamline, the National Synchrotron Light Source, Brookhaven National Laboratory. The measured thermal expansion for Fe3S can be directly used for modeling the cores of the terrestrial planets and is important for interpreting current geophysical observations of the Earth's and planetary cores. [1]Fei, Y.-W. , Li, J., Bertka, C.M., and Prewitt, C.T. (2000) Structure type and bulk modulus of Fe3S, a new iron-sulfur compound. American Mineralogist, 85, 1830-1833.

MR31A-0132

Equations of State of MgAl2O4 Spinel at High Pressure

* Banigan, E (ejb45@georgetown.edu) , Georgetown University, Department of Physics, 3700 O St. NW, Washington, DC 20057 United States

We performed static first-principles calculations using the density functional theory as implemented in the ABINIT package to study the behavior of MgAl2O4 spinel, under pressure. We considered several possible structures for which we computed the enthalpy and the PV relations. The calculations showed that MgAl2O4 undergoes a phase transition sequence from spinel structure (Fd3m) to a mixture of oxides (MgO+Al2O3) to a calcium titanite-type structure (Cmcm) with phase transitions at 14.5 and 44.5 GPa, respectively. The calcium ferrite-type structure, a tetragonally distorted cubic structure, and a cation-defective hexagonal structure are all unstable in the -10 to 150 GPa pressure range. We fit Vinet equations of state for the various phases of MgAl2O4. The results were in fair agreement with previous studies; the bulk moduli, K0, of the cubic spinel and calcium titanite-type structures were determined to be 202 GPa and 220 GPa, respectively.

MR31A-0133

First principles investigation of ringwoodite's dissociation

* Yu, Y (yonggang@cems.umn.edu) , Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455 United States
Wentzcovitch, R (wentzcov@cems.umn.edu) , Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455 United States
Tsuchiya, T (takut@sci.ehime-U.ac.jp) , Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455 United States
Umemoto, K (umemoto@cems.umn.edu) , Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455 United States
Tsuchiya, J (junt@cems.umn.edu) , Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455 United States

The 660-km discontinuity is widely believed to be caused by the dissociation of (Mg,Fe)2SiO4 a-spinel into (Mg,Fe)O, magnesiowistite, and (Mg,Fe)SiO3 perovskite. We have investigated first principles the dissociation of the iron free phase, ringwoodite. The magnitude of the Clapeyron slope is still somewhat uncertain, varying between -1.2(0.8) (Katsura et al., 2003), -1.3 MPa/K (Fei, 2004) and -2.8 MPa/K (Ito and Takahashi, 1989) at relevant conditions. In general, the details of a mantle phase transition affect convection modeling and our understanding of internal processes. In particular, this transition tends to inhibit mantle flow across the 660 km discontinuity. We have performed LDA and GGA calculations trying to clarify some details of this transition. Similarly to the case of transformations between single phases, the GGA phase boundary occurs at higher pressures than the LDA and is more similar to the experimental phase boundaries, regardless of the CS sign. The higher GGA transition pressures can be rationalized by closely inspecting the relationship between GGA and LDA functional forms.

MR31A-0134

Observations of stress-strain curves of hcp-Fe at high pressures and temperatures

* Nishiyama, N (nishiyama@cars.uchicago.edu) , Center for Advanced Radiation Sources, Univ. Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
Wang, Y (wang@cars.uchicago.edu) , Center for Advanced Radiation Sources, Univ. Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
Rivers, M L (rivers@cars.uchicago.edu) , Center for Advanced Radiation Sources, Univ. Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
Sutton, S R (sutton@cars.uchicago.edu) , Center for Advanced Radiation Sources, Univ. Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States

We performed in-situ X-ray diffraction and radiography experiments of hcp-Fe within its stability field to observe some independent stress-strain curves of this material. Deformation experiments were carried out at the GSECARS 13-BM-D beamline (APS) using a deformation-DIA with a monochromatic X-ray diffraction and a radiographic imaging system. We used four tungsten carbide and two sintered diamond (SD) anvils with truncated edge length of 2 mm. Pressure medium was a mixture of boron and epoxy and a cylindrical graphite furnace was employed. The starting material is a fragment of pure bcc-Fe wire (0.5 mm in diameter and 0.5 mm long) and two deformation pistons made of alumina were situated above and below the sample. First, the cell assembly was compressed uniformly at room temperature up to about 13 GPa. After that, the sample was heated to about 700 K and we observed transformation of the sample to hcp-Fe. Several deformation cycles were repeated at high pressures and temperatures after the transformation. The incident beam was directed through an anvil-gap and impinged the sample. The diffracted X-rays went through the SD anvils, and thus we were able to observe diffraction Debye rings over the entire 360 degrees detector azimuth range. Two-dimensional diffraction patterns were collected using an X-ray CCD detector. Using distortion of the Debye rings from the true circle and single-crystal elastic moduli of hcp-Fe, differential stresses can be calculated. The sample length was measured by radiography using a wide X-ray beam. Using the radiographic data, axial strains of the sample can be determined. We observed ten independent stress-strain curves with pressures of about 12 GPa, axial strains in excess of 15 percent, three different temperatures, and some strain rates. These stress-strain curves indicate that hcp-Fe deforms elastically at the beginning of deformation. In some of these, we observed saturation of the sample stresses, which means that the deformation reaches steady state. The differential stresses at the steady state flow tend to decrease with increasing temperature and with decreasing strain rate.

MR31A-0135

Thermal Equation of State of Fe3S and Sulfur in Earth's Core

* Seagle, C T (seagle@uchicago.edu) , Dept. of Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
Campbell, A J (ajc@geol.umd.edu) , Dept. of Geology, University of Maryland, 237 Regents Dr., College Park, MD 20742 United States
Heinz, D L (heinz@uchicago.edu) , Dept. of Geophysical Sciences, University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
Heinz, D L (heinz@uchicago.edu) , The James Franck Institute, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
Shen, G (shen@cars.uchicago.edu) , Center for Advanced Radiation Sources, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States
Prakapenka, V B (prakapenka@cars.uchicago.edu) , Center for Advanced Radiation Sources, University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637 United States

The thermal equation of state (EOS) of Fe3S was investigated up to 80 GPa and 2500 K using synchrotron radiation and a laser heated diamond anvil cell at the GSECARS sector of the Advanced Photon Source, Argonne National Laboratory. Starting materials were powdered iron (Fe) and iron sulfide (FeS) intimately mixed; compositions were 5 and 15 weight % sulfur. Fe3S was formed by reaction between Fe and FeS upon laser heating, with excess Fe remaining. Fitting a third order Birch-Murnaghan EOS to the room temperature data yielded bulk modulus $K_{0}=156.3(67)$ GPa and first pressure derivative $K'_{0}=3.84(34)$. The thermal pressure was assumed to be of the form Δ P_{thermal}=α K_{T} Δ T$ where α K_{T}=$constant. The best fit to the data yielded α K_{T}=0.0110(4)$ GPa K$^{-1}$. Fe and Fe3S coexisted in the high pressure and temperature data allowing a direct comparison of the density of the two phases. A relationship between the density of Fe and Fe3S was found to be nearly linear and independent of temperature. This relationship is independent of pressure calibration and removes the uncertainties inherent in comparing equations of state. Extrapolation of the data to core mantle boundary (CMB) conditions and adjusting the volumes of the phases to account for melting suggest that 13 weight % sulfur is required to resolve the density deficit of the outer core.

MR31A-0136

Structural State and Elastic Properties of Perovskites in the Earth's Mantle

* Ross, N L (nross@vt.edu) , Virginia Tech, Dept. Geosciences 4044 Derring Hall, Blacksburg, VA 24061 United States
Angel, R J (rangel@vt.edu) , Virginia Tech, Dept. Geosciences 4044 Derring Hall, Blacksburg, VA 24061 United States
Zhao, J (jzhao@vt.edu) , Virginia Tech, Dept. Geosciences 4044 Derring Hall, Blacksburg, VA 24061 United States

Recent advances in laboratory-based single-crystal X-ray diffraction techniques for measuring the intensities of diffraction from crystals held in situ at high pressures in the diamond-anvil cell have been used to determine the role of polyhedral compression in the response of 2:4 and 3:3 GdFeO3-type perovskites to high pressure [1]. These new data clearly demonstrate that, contrary to previous belief, the compression of the octahedral sites is significant and that the evolution of the perovskite structure with pressure is controlled by a new principle; that of equipartition of bond-valence strain across the structure [2]. This new paradigm, together with the minimal information available from high- pressure powder diffraction studies, may provide the possibility of predicting the structural state and elastic properties of perovskites of any composition at mantle pressures and temperatures. Cation partioning between silicate perovskites and other phases should then be predictable through the application of a Brice-style model [3]. The geochemical implications of this type of analysis will be presented as well as the possibility for extending these measurements to higher pressures. References [1] e.g. Zhao, Ross & Angel (2004) Phys Chem Miner. 31: 299; Ross, Zhao,. & Angel (2004). J. Solid State Chemistry 177:1276. [2] Zhao, Ross, & Angel (2004). Acta Cryst. B60:263 [3] e.g Walter et al. (2004) Geochim Cosmochim Acta 68:4267; Blundy & Wood (1994) Nature 372:452

MR31A-0137

Anisotropy of Shear Strength of Silica: a Molecular Dynamics Study

* Zheng, L (zhengl@missouri.edu) , Lianqing Zheng, Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211 United States
Luo, S (sluo@gps.caltech.edu) , Sheng-Nian Luo, P-24 Plasma Physics, Los Alamos National Laboratory, Los Alamos, NM 87544 United States
Tschauner, O (olivert@physics.unlv.edu) , Oliver Tschauner, High-Pressure Science and Engineering Center and Department of Physics, University of Nevada, Las Vegas, NV 89154 United States

We investigate the shear strengths of silica glass, alpha-quartz, coesite, and stishovite using classical molecular dynamics simulations with a modified van Beest-Kramer-van Santen potential. Shear strengths along different crystallographic orientations are studied. We also explore the effects of hydrostatic pressure, temperature, and defects on the shear strength. *Work partly performed under the auspices of the U.S. Department of Energy under contract No. W-7405-ENG-36 and NNSA Cooperative Agreement DE-FC88-01NV14049

MR31A-0138

New Experimental Setup for High-Pressure High-Temperature Gigahertz Ultrasonic Interferometry

* Kantor, A P (Anastasia.Kantor@Uni-Bayreuth.DE) , Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440 Germany
Kantor, I Y , Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440 Germany
Dubrovinsky, L S , Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440 Germany
Jacobsen, S D , Carnegie Institution of Washington, 5251 Broad Branch Rd. NW, Washington, DC 20015 United States

The only direct information about Earth's interior comes from seismological observations of sound wave velocities. In order to create compositional and mineralogical model from seismological data knowledge of the elastic properties and crystal chemistry of minerals is necessary. Gigahertz ultrasonic interferometry (GUI) is a relatively new tool used to measure single-crystal compressional and shear-wave travel times, which are converted to sound velocities and elastic moduli for direct application to problems in geophysics. Although possibility of simultaneous high-pressure and high-temperature GUI measurements in diamond anvil cell was demonstrated before up to temperature of $250C, {\it in situ} pressure measurements were not possible. We developed new experimental setup for simultaneous GUI and pressure determination using a ruby fluorescence gouge. A diamond anvil cell is equipped with a miniature internal resistive heater with thermocouple fixed at a very small distance from the sample chamber. DAC is mounted at the rotating stage with 5 degrees of freedom (XYZ and two tilting degrees), that can be fixed in three different positions: on top of a {\it P}-buffer rod for compressional wave velocities measurement, on top of S-buffer rod for shear wave velocities measurement and under the microscope, equipped with laser and portable high-resolution spectrometer for ruby fluorescence measurement. DAC under high temperature could be moved between these three positions, and independent pressure, temperature, S and {\it P} wave velocities measurements could be done simultaneously at each data point. In addition to single-crystal elasticity measurements, ability of GUI for elasticity measurements of liquids was demonstrated. Compressional wave velocities in liquid argon were measured at high pressures and temperatures, showing the ability of GUI for studies equation of state of a liquid.