Hiroshi Sawamoto

High‐pressure generation by a multiple anvil system with sintered diamond anvils

High‐pressure experiments using a multiple anvil high‐pressure system with sintered diamond anvils are presented and discussed. Pressures in excess of 41 GPa were obtained, on the basis of the lattice constants of gold determined by the in situ x‐ray diffraction technique using synchrotron radiation.

Single crystal growth of the modified spinel (β) and spinel (γ) phases of (Mg,Fe)2SiO4 and some geophysical implications

Single crystals of ferromagnesian orthosilicates with modified spinel (β) and spinel (γ) structure as large as 500 μm have been grown by solid state crystallization at high temperature and high pressure using an MA8-type apparatus driven in a 2,000-ton uniaxial press. This system is capable of generating pressures of 24.0 (±0.3) GPa at 2,400 (±50)°C for one hour in a sample assembly volume of 0.14 cm3. Crystals larger than 100 μm were observed to grow only at pressures within 5 percent of the phase boundary between the stability fields of the β and γ phases.Experimental determination of the phase boundaries between β or β+γ and γ phases for (Mg,Fe)2SiO4 has been extended to 22 GPa and 2,400°C. The effect of configurational entropy due to disordering is evaluated to be minimal on the basis of the cationic distribution in the synthesized samples; thus, we conclude that the phase boundary between β or β+γ and γ phases remains essentially linear to 2,400°C. In (Mg,Fe)2SiO4 solid solutions, the stability field of the γ phase shifts towards the lower pressures with increasing iron content at a rate of a 1 GPa for each 10 mole percent Fe.Assignment of the β→β+γ→γ transition to the seismic 550 km discontinuity is rejected by the present phase diagram results for (Mg0.9Fe0.1)2SiO4 and measurement of acoustic velocities for β and γ Mg2SiO4, but the discontinuity may be caused by a phase transition of pyroxene to a garnet-like structure.

Precision Lattice-Parameter Determination of (Mg,Fe)SiO3 Tetragonal Garnets

The tetragonal garnet (Mg,Fe)SiO3 is a high-pressure phase of pyroxene that is thought to be a major constituent of the earths upper mantle. Its crystal structure is similar to that of cubic garnet, but it is slightly distorted to tetragonal symmetry so that its x-ray powder diffraction pattern shows a very small line splitting. A suite of tetragonal garnets with different compositions in the MgSiO3-rich portion of the MgSiO3-FeSiO3 system was synthesized at about 20 gigapascals and 2000�C. The lattice parameters a and c of quenched samples were determined by whole-powder-pattern decomposition analysis of Fe Kα x-ray powder diffraction data, which has the capacity to resolve to a high degree heavily overlapping reflections. It was found that the lattice parameters can be obtained from the following equations; a (in angstroms) = 11.516 + 0.088x and c (in angstroms) = 11.428 + 0.157x, where x, teh mole fraction of FeSiO3, is 0.0 ≤ x ≤ 0.2.

Synthesis of Coesite from Ultra Fine Particles

Coesite, a high pressure phase of SiO2, was synthesized without H2O or any other catalizing component at temperatures as low as 250°C from ultra fine particles (80 A in diameter) of silica produced by the gas evaporation technique. The ultra fine particles were worked by a tetrahedral anvil type high pressure apparatus up to a pressure of 50 kbar at 450°C for 30 minutes and almost 100% of the particles were crystallized into coesite. Mechanically ground quartz and silica-gel of a few µm in size were also worked under the same condition. The reaction speed of recrystallization of the ultra fine particles was roughly estimated to be ten times or more that of particles of micron size.

Pressure and temperature dependence of cation distribution in Mg-Mn olivine

Synthetic (Mg0.51, Mn0.49)2SiO4 olivine samples are heat-treated at three different pressures; 0, 8 and 12 GPa, all at the same temperature (∼500° C). X-ray structure analyses on these single crystals are made in order to see the pressure effect on cation distribution. The intersite distribution coefficient of Mg and Mn in M1 and M2 sites, KD = (Mn/Mg)M1/(Mn/Mg)M2, of these samples are 0.192 (0 GPa), 0.246 (8 GPa) and 0.281 (12 GPa), indicating cationic disordering with pressure. The small differences of cell dimensions between these samples are determined by powder X-ray diffraction. Cell dimensions b and c decrease, whereas a increases with pressure of equilibration. Cell volume decreases with pressure as a result of a large contraction of the b cell dimension. The effect of pressure on the free energy of the cation exchange reaction is evaluated by the observed relation between the cell volume and the site occupancy numbers. The magnitude of the pressure effect on cation distribution is only a fifth of that predicted from the observed change in volume combined with thermodynamic theory. This phenomenon is attributed to nonideality in this solid solution, and nonideal parameters are required to describe cation distribution determined in the present and previous experiments. We use a five-parameter equation to specify the cationic equilibrium on the basic of thermodynamic theory. It includes one energy parameter of ideal mixing, two parameters for nonideal effects, one volume parameter, and one thermal parameter originated from the lattice vibrational energy. The present data combined with some of the existing data are used to determine the five parameters, and the cation distribution in Mg-Mn olivine is described as a function of temperature, pressure, and composition. The basic framework of describing the cationic behavior in olivine-type mineral is worked out, although the result is preliminary: each of the determined parameters is not accurate enough to enable us to make a reliable prediction.

Antiferromagnetic transition of fayalite under high pressure studied by Mssbauer spectroscopy

Néel temperature (TmNof α-Fe2SiO4 (fayalite) was measured as a function of pressure by means of Mössbauer spectroscopy in the pressure range 0–16 Gpa. High pressure was generated using a clamp-type miniature diamond anvil cell which was inserted into a cryostat. The Néel temperature increased linearly with increasing pressure at a rate of dTN/dp=2.2±0.2 K/GPa. The result is discussed on the basis of the model proposed for the magnetic structure of fayalite by Santoro et al. (1966). The observed dTN/dp suggests that the superexchange interactions vary as the −10/3 power of the volume while the volume dependence of the direct exchange interactions is positive and small.

?(Mg0.9, Fe0.1)2SiO4: Single crystal structure, cation distribution, and properties of coordination polyhedra

The synthesis boundaries of the phase transformation; α+β→β→β+γ→γ in (Mg0.9, Fe0.1)SiO4, have been clarified at temperatures to 2000° C and pressures up to 20 GPa in order to synthesize single crystals of high quality. A single crystal of β (Mg0.9, Fe0.1)2SiO4 was grown successfully to a size of 500 μm. The crystal structure has been refined from single-crystal X-ray intensities. The ferrous ions prefer M1 and M3 sites to over the larger M2 site. The volume change of all the occupied polyhedra does not contribute to the decrease of total volume in the α→β transformation; rather it tends to increase the bulk volume through the expansion of occupied tetrahedra. The volume reduction in the phase transformations is accounted for by unoccupied polyhedra, with the octahedra contributory 60% and the tetrahedra 40% to the ΔV of the α→β transition. The volume change in the β→γ transformation is caused also partly by the volume decrease of MO6 (25%), partly the unoccupied tetrahedra (45%) and octahedra (30%).

High pressure generation by MASS 3I8−90 type apparatus

MASS 3I8−90 type apparatus is developed to generate pressures up to 500 kilobar in a volume of 7.5 mm3. The geometry, operational mechanism, and the result of the pressure−generating experiment are described in detail.

Generation of Large Volume Hydrostatic Pressure to 8 GPa for Ultrasonic Studies

A new solid-liquid hybrid device has been developed wherein a liquid pressure of 8 GPa can be generated with 150 mm3 in an MA8 high-pressure apparatus. This system was demonstrated by measurements of the velocity of elastic waves in fused quartz; the longitudinal wave had a velocity minimum and an attenuation maximum at 3 GPa.

The Use of Sintered Diamond Anvils in the MA8 Type High-pressure Apparatus

A new multi-anvil type high-presure apparatus has been developed using sintered diamond anvils to generate pressures over 30 GPa and temperatures up to about 2000°C. A maximum sample volume of about 1 mm3 is available in this system. The pressure was confirmed by dissociation of forsterite into Mg-perovskite and periclase. The basic techniques and problems in utilizing sintered diamond in the MA8 type high-pressure apparatus are discussed with an emphasis on the future prospect of incorporating simultaneous X-ray diffraction observation.