Kurt Leinenweber

Synthesis and Structure Refinement of the Spinel, γ‐Ge3N4

γ-Ge 3 N 4 is formed from α-or β-Ge 3 N 4 at pressures greater than 12 GPa and temperatures above 100°C. It has the spinel structure, symmetry Fc3m, and lattice parameter a = 8.2125(1) A. Germanium has both tetrahedral [Ge-N = 1.879(2) A] and octahedral [Ge-N = 1.996(1) A] coordination to nitrogen. The difference between the octahedral and tetrahedral bond lengths in this nitride is close to that expected from systematics, which were largely derived from oxides.

Cell assemblies for reproducible multi-anvil experiments (the COMPRES assemblies)

Abstract The multi-anvil high-pressure technique is an important tool in high-pressure mineralogy and petrology, as well as in chemical synthesis, allowing the treatment of large (millimeter-size) samples of minerals, rocks, and other materials at pressures of a few GPa to over 25 GPa and simultaneous uniform temperatures up to 2500 °C and higher. A series of cell assemblies specially designed and implemented for interlaboratory use are described here. In terms of the size of the pressure medium and the anvil truncation size, the five sizes of assemblies developed here are an 8/3, 10/5, 14/8, 18/12, and 25/15 assembly. As of this writing, these assemblies are in widespread use at many laboratories. The details of design, construction, and materials developed or used for the assemblies are presented here.

Experimental constraints on the electrical anisotropy of the lithosphere-asthenosphere system

The relative motion of lithospheric plates and underlying mantle produces localized deformation near the lithosphere–asthenosphere boundary. The transition from rheologically stronger lithosphere to weaker asthenosphere may result from a small amount of melt or water in the asthenosphere, reducing viscosity. Either possibility may explain the seismic and electrical anomalies that extend to a depth of about 200 kilometres. However, the effect of melt on the physical properties of deformed materials at upper-mantle conditions remains poorly constrained. Here we present electrical anisotropy measurements at high temperatures and quasi-hydrostatic pressures of about three gigapascals on previously deformed olivine aggregates and sheared partially molten rocks. For all samples, electrical conductivity is highest when parallel to the direction of prior deformation. The conductivity of highly sheared olivine samples is ten times greater in the shear direction than for undeformed samples. At temperatures above 900 degrees Celsius, a deformed solid matrix with nearly isotropic melt distribution has an electrical anisotropy factor less than five. To obtain higher electrical anisotropy (up to a factor of 100), we propose an experimentally based model in which layers of sheared olivine are alternated with layers of sheared olivine plus MORB or of pure melt. Conductivities are up to 100 times greater in the shear direction than when perpendicular to the shear direction and reproduce stress-driven alignment of the melt. Our experimental results and the model reproduce mantle conductivity–depth profiles for melt-bearing geological contexts. The field data are best fitted by an electrically anisotropic asthenosphere overlain by an isotropic, high-conductivity lowermost lithosphere. The high conductivity could arise from partial melting associated with localized deformation resulting from differential plate velocities relative to the mantle, with subsequent upward melt percolation from the asthenosphere.

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.

Ultrahydrous stishovite from high-pressure hydrothermal treatment of SiO2

Stishovite (SiO2 with the rutile structure and octahedrally coordinated silicon) is an important high-pressure mineral. It has previously been considered to be essentially anhydrous. In this study, hydrothermal treatment of silica glass and coesite at 350–550 °C near 10 GPa produces stishovite with significant amounts of H2O in its structure. A combination of methodologies (X-ray diffraction, thermal analysis, oxide melt solution calorimetry, secondary ion mass spectrometry, infrared and nuclear magnetic resonance spectroscopy) indicate the presence of 1.3 ± 0.2 wt % H2O and NMR suggests that the primary mechanism for the H2O uptake is a direct hydrogarnet-like substitution of 4H+ for Si4+, with the protons clustered as hydroxyls around a silicon vacancy. This substitution is accompanied by a substantial volume decrease for the system (SiO2 + H2O), although the stishovite expands slightly, and it is only slightly unfavorable in energy. Stishovite could thus be a host for H2O at convergent plate boundaries, and in other relatively cool high-pressure environments.

Crystal structures, elastic properties, and hardness of high-pressure synthesized CrB2 and CrB4

Chromium tetraboride (CrB4), a recently proposed candidate for superhard materials, has been synthesized at high pressure and temperature by a solid-state reaction. As a byproduct, chromium diboride (CrB2) also forms and co-exists with CrB4 in the final product. The comparative studies of crystal structure, elastic property, and hardness of both phases have been conducted at the same sample environment conditions. The crystal structure of CrB4 has been refined with an orthorhombic symmetry of Immm(space group no. 71) or Pnnm (space group no. 58) using X-ray diffraction data. Further simulations indicate that the structural distinction between Immm and Pnnm can be resolved by neutron diffraction, due to the high scattering cross-section of boron (11B) by neutrons. Although CrB2 and CrB4 have close bulk modulus at about 230 GPa, the measured asymptotic Vickers hardness yields 16 GPa for CrB2 but 30 GPa for CrB4, which is nearly two times that of CrB2. The dramatic enhancement in hardness in CrB4 is attributed to the strong three-dimensional Cr-B network, in contrast to the layered lattice structure of hexagonal CrB2.

Large-volume multianvil cells designed for chemical synthesis at high pressures

Two multianvil cell assemblies with octahedral edge lengths (OEL) of 18 and 25 mm have been developed, which allow sample volumes of up to 153 and 387 mm3, respectively. Pressure calibrations were performed at room temperature and 1200 °C using quenched samples, and at variable temperature using in situ synchrotron measurements. These calibrations employed 25.4 mm tungsten carbide anvils with a truncation edge length of 12 mm for the 18 mm OEL cell (18/12 configuration) and 15 mm for the 25 mm OEL cell (25/15 configuration) and yielded sample pressures of up to 10 and 7.5 GPa, respectively, at loads approaching 800 t. The measured thermal gradients at 1200 °C vary between 5–10 °C/mm (18/12) and 15–20 °C/mm (25/15). By combining large sample/reaction volumes with minimal thermal gradients, the new multianvil cell assemblies are especially useful for chemical synthesis at high-pressure, high-temperature conditions.

Stability of hydrous minerals in H2O‐saturated KLB‐1 peridotite up to 15 GPA

Stability fields of Ti‐chondrodite, Ti‐clinohumite, phase A, and Al2O3‐bearing phase E were determined in H2O‐saturated KLB‐1 peridotite at 6 to 15 GPa. Phase E contains 1.5‐9 wt. % Al2O3 and was found from 800 °C, 9 GPa to 1400 °C, 15 GPa. When a downdragged hydrous peridotite follows a hotter P‐T path than one passing through 800 °C at 4 GPa, serpentine breaks down, followed by talc, chlorite and pargasite, Beyond the pargasite‐out reaction the downdragged peridotite will be almost free of H2O bound in crystals except for a small amount in phlogopite and then K‐richterite. This means that the hydrous peridotite should encounter ‘‘a choke point’’ restricting the passage of H2O to greater depth. In subducting basalt, lawsonite is the phase most resistant to pressure induced dehydration. Therefore, dense hydrous magnesium silicates in the downdragged flow of the base of the mantle wedge could absorb H2O from dehydrating lawsonite and become H2O carriers in a very cold subduction zone (≤800 °C, 9 GPa). Howe...

High-pressure cells for in situ multi-anvil experiments

A new series of high-pressure cells for in situ multi-anvil experiments is described. The cells are based on the conventional COMPRES cells, but modifications are made to improve the passage of X-rays. The modifications include cutting slits in parts of the assemblies that have very high X-ray absorption, such as lanthanum chromite and rhenium, the use of low-Z thermal insulation, such as forsterite, in place of zirconia, and the partial replacement of zirconia by MgO equatorial windows combined with a mullite octahedron. Details of the designs, thermal characterizations, and examples of the application of these cells are described.

Hardness, elasticity, and fracture toughness of polycrystalline spinel germanium nitride and tin nitride

The hardness, elastic moduli, and fracture toughness of the spinel phases, γ–Ge 3 N 4 and γ–Sn 3 N 4 , were determined using indentation data and theoretical calculations. Measurements were performed on polycrystalline specimens using the technique of nanoindentation to determine the reduced moduli and hardnesses from the unloading portion of the indent curves. Reduced moduli of γ–Ge 3 N 4 and γ–Sn 3 N 4 were found to be 295 and 167 GPa, respectively. The nanohardnesses of γ–Ge 3 N 4 and γ–Sn 3 N 4 were found to be 31 and 13 GPa, respectively. The shear moduli G 0 and Poisson’s ratios ν 0 were derived using theoretical bulk moduli B 0 obtained from density-functional theory calculations. The calculated values were B 0 = 260 GPa, G 0 = 146 GPa, ν 0 = 0.26 for γ–Ge 3 N 4 , and B 0 = 186 GPa, G 0 = 64 GPa, ν 0 = 0.34 for γ–Sn 3 N 4 . Fracture toughness was estimated by direct measurement of radial cracks emanating from Vickers microindents. It was determined that for γ–Ge 3 N 4 , K I C = 2.3 MPa(m) 1/2 , while for γ–Sn 3 N 4 , K I C = 1.4 MPa(m). 1/2