Michael T. Vaughan

Yield strength at high pressure and temperature

Yield strength is measured at high pressure and temperature using a large volume, high pressure apparatus (SAM85) with synchrotron radiation. A macroscopic deviatoric stress is manifest as a uniform deviatoric strain that is oriented by the geometry of the pressurizing medium. Microscopic deviatoric stress is identified as the elastic broadening of diffraction lines. The deviatoric stress reaches the yield point as evidenced by the uniformity, the saturation, and the temperature dependence of the deviatoric stress. Yield strengths, which correspond to the stress saturation level at a few per cent strain, are determined for NaCl and MgO up to 8 GPa and 1200°C. The results are consistent at room temperature with previous diamond anvil studies and demonstrate the effect of pressure on yield strength. These data demonstrate the feasibility of determining high pressure, high temperature yield strengths for mantle phases.

The relationship of elasticity and crystal structure in andalusite and sillimanite

The nine elastic constants of andalusite and sillimanite have been determined, using the technique of Brillouin scattering. They are, in megabars, for andalusite: c11=2.334, c22=2.890, c33=3.801, c44=0.995, c55=0.878, c66=1.123, c23=0.977, c13=1.162, c12=0.814; for sillimanite: c11=2.873, c22=2.319, c33=3.884, c44=1.224, c55=0.807, c66=0.893, c23=1.586, c13=0.834, c12=0.947. Both structures are characterized by chains of edge-linked coordination octahedra extending parallel to the crystallographic c direction, cross-linked by polyhedra of lower coordination. In each structure the stiffness measured parallel to c is greater than that measured normal to c. The shear moduli can be directly correlated with the relative rigidity of the cross-linking structures.

In situ high P‐T X ray diffraction studies on three polymorphs (α, β, γ) of Mg2SiO4

Unit cell volumes of the beta (β) and spinel (γ) phases (Mg2SiO4) have been measured under simultaneous high pressure and high temperature using synchrotron X ray radiation, in a cubic anvil apparatus. With volume-temperature data at constant pressure, we determine the average volume thermal expansion coefficients of the β phase from 724 to 872 K at 7.6 GPa to be 2.28(±0.45)×10−5/K and of the γ phase from 759 to 962 K at 9.8 GPa to be 1.71(±0.14)×10−5/K. Thermodynamic relations are used to constrain the temperature derivative of the isothermal bulk modulus (KT) from the high-pressure thermal expansion data: (∂KT/∂T)P is found to be −2.7(±0.5)×10−2 GPa/K for the β phase and −2.8(±0.3)×10−2 GPs/K for the γ phase. Unit cell volumes of the olivine (α) phase, back-transformed from the β and γ phases at high temperatures, have been measured under pressure at temperatures above the Debye temperature; using thermal pressure equations, we find (∂KT/∂T)P for the α phase to be −2.1(±0.2)×10−2 GPa/K. These new data on the temperature derivatives of the bulk modulus for the three phases are consistent with an upper mantle containing 60 to 65% olivine and the absence of a velocity signature for the β to γ phase transition near 520 km depth.

Strength and water weakening of mantle minerals, olivine, wadsleyite and ringwoodite

Rheological properties of olivine and its high pressure polymorphs, both dry and hydrous, have been studied by monitoring x-ray diffraction peak broadening as a function of pressure, temperature and time. The measurements were carried out up to a pressure of 10 GPa and a temperature of 600°C for olivine and wadsleyite, and a pressure of 20 GPa and a temperature of 1000°C for ringwoodite. The observations indicate that olivine is much weaker than the other two minerals for this range of conditions, and furthermore, water weakens olivine dramatically but only slightly weakens wadsleyite and ringwoodite. When the temperature increases from 25°C to 400°C the yield strength in the olivine drops by 39% without water and 62% with water, however there is hardly any change in strength for wet or dry wadsleyite and ringwoodite.

Phase Transition and Thermal Expansion of MgSiO3 Perovskite

Results from in situ x-ray diffraction experiments with a DIA-type cubic anvil apparatus (SAM 85) reveal that MgSiO3 perovskite transforms from the orthorhombic Pbnm symmetry to another perovskite-type structure above 600 kelvin (K) at pressures of 7.3 gigapascals; the apparent volume increase across the transition is 0.7%. Unit-cell volume increased linearly with temperature, both below (1.44 x 10-5 K–1) and above (1.55 x 10–5 K–1) the transition. These results indicate that the physical properties measured on the Pbnm phase should be used with great caution because they may not be applicable to the earths lower mantle. A density analysis based on the new data yields an iron content of 10.4 weight percent for a pyrolite composition under conditions corresponding to the lower mantle. All current equation-of-state data are compatible with constant chemical composition in the upper and lower mantle; thus, these data imply that a chemically layered mantle is unnecessary, and whole-mantle convection is possible.

Single crystal elastic properties of protoenstatite: A comparison with orthoenstatite

AbstractThe single-crystal elastic properties of Li-Sc bearing protoenstatite [(Mg1.6, Li0.2, Sc0.2)Si2O6], a high temperature polymorph of orthoenstatite, have been obtained at 22° C and 1 atm from Brillouin scattering measurements. The elastic moduli are (in Mbar):

X-ray strain analysis at high pressure: Effect of plastic deformation in MgO

\begin{gathered} C_{{\text{11}}} = {\text{2}}{\text{.13(2), }}C_{{\text{22}}} = {\text{1}}{\text{.52(1), }}C_{{\text{33}}} = {\text{2}}{\text{.46(2),}} \hfill \\ C_{{\text{44}}} = {\text{0}}{\text{.81(1), }}C_{{\text{55}}} = {\text{0}}{\text{.44(1), }}C_{{\text{66}}} = {\text{0}}{\text{.67(1),}} \hfill \\ C_{{\text{12}}} = {\text{0}}{\text{.76(3), }}C_{{\text{13}}} = {\text{0}}{\text{.59(4), }}C_{{\text{23}}} = {\text{0}}{\text{.70(3)}}{\text{.}} \hfill \\ \end{gathered}

Large volume high pressure research using the wiggler port at NSLS

These data have been corrected for changes in phonon direction and scattering geometry due to refraction of the laser light at the crystal surface. A description of the method used to make these corrections is outlined in detail. Comparison with the orthoenstatite moduli leads us to conclude that kinking of the tetrahedral chains in the pyroxene structure is directly related to stiffness along the c direction (C33): a high degree of kinking reduces this modulus; extended chains increase it. The stacking sequence of octahedral layers in the a direction, which alters distant rather than near neighbor environments, has a marked effect on C55 (rigidity in the a–c plane), and C66 (rigidity in the a–b plane). Shear rigidity in the b–c plane (C44) is, however, unaffected.