Peter M. Bell

Specific volume measurements of Cu, Mo, Pd, and Ag and calibration of the ruby R1 fluorescence pressure gauge from 0.06 to 1 Mbar

The wavelength shift with pressure of the ruby R1 fluorescence line (Δλ) has been calibrated in the diamond‐window pressure cell from 0.06 to 1 Mbar. This was done by simultaneously making specific volume measurements of four metals (Cu, Mo, Ag, and Pd) and referring these results to isothermal equations of state derived from shock‐wave experiments. The result is P (Mbar) = (19.04/5) {[(λ0+Δλ)/λ0]5−1}, where λ0 is the wavelength measured at 1 bar.

An interlaboratory comparison of piston-cylinder pressure calibration using the albite-breakdown reaction.

The pressure of the reaction albite=jadeite and quartz was measured at 600° C by workers in six geophysical laboratories for the purpose of comparing pressure calibration procedures for the solid-pressure piston-cylinder apparatus. All groups used the same starting mix of crystalline reactant and products and all obtained hydrothermal reversals of the equilibrium. Solid pressure media used included talc, NaCl, boron nitride, pyrophyllite, pyrex glass and crushable ceramic. Various means of calibration were used, including internal standardization by transitions in indicator substances and the piston-in, piston-out bracketing method.There was agreement among all groups—the average preferred value of 16.3 kilobars at 600° C is enclosed by all of the error brackets assigned by the various investigators. This average preferred value is lower by nearly two kilobars than the often-quoted extrapolation of Birch and LeComtes data (1960). It will be important for both petrology and high-pressure technology to test this result in a very high gas pressure apparatus.

Structure and crystal chemistry of perovskite-type MgSiO3

Synthetic clinoenstatite (MgSiO3) has been converted to a single phase with the perovskite structure in complete reactions at approx. 300 kbar in experiments that utilize the laser-heated diamond-anvil pressure apparatus. The structure of this phase after quenching was determined by powder X-ray diffraction intensity measurement to be similar to that of the distorted rare-earth, orthoferrite-type, orthorhombic perovskites, but it is suggested that such distortion from ideal cubic perovskite would diminish at high pressure.The unit cell dimensions and density of perovskite-type MgSiO3 at ambient conditions (1 bar, 25°C) are a=4.780(1) Å, b=4.933(1) Å, c=6.902(1) Å, V=162.75 Å3, and ρ=4.098(1) g/cm3. This phase is 3.1% denser than the isochemical oxide mixture [periclase (MgO)+stishovite (SiO2)]. The small crystal-field stabilization energy of the cation site in the perovskite structure may play an important role in limiting the high-pressure stability field of perovskites that contain transition metal cations. Approximate calculations of the crystal-field effects indicate that perovskite of pure FeSiO3 composition may become stable at 400–600 kbar; pressures greater than 800 kbar would be required to stabilize CoSiO3 or NiSiO3 perovskite.

Absolute pressure measurements and analysis of diamonds subjected to maximum static pressures of 1.3–1.7 Mbar

Force per unit area measurements made in the megabar pressure cell, independently of other pressure calibration systems, are consistent with the ruby R1 scale of Mao, Bell, Shaner, and Steinberg and its extrapolation to 1.4 Mbar. Physical analysis of diamond anvils removed after experiments to maximum pressures of 1.3-1.7 Mbar suggests that the nitrogen platelet concentration may be related to the strength of the diamonds. The pressure face of one of the diamonds from the 1.7-Mbar experiment was deformed plastically by a macroscopic amount.

Melting relations in the Fe-rich portion of the system FezFeS at 30 kb pressure

Abstract The melting relations of FezFeS mixtures covering the composition range from Fe to Fe 67 S 33 have been determined at 30 kb pressure. The phase relations are similar to those at low pressure. The eutectic has a composition of Fe 72.9 S 27.1 and a temperature of 990°C. Solubility of S in Fe at elevated temperatures at 30 kb is of the same order of magnitude as at low pressure. Sulfur may have significantly lowered the melting point of iron in the upper mantle during the period of coalescence of metal prior to core formation in the primitive earth.

B1-b2 transition in calcium oxide from shock-wave and diamond-cell experiments.

Volume and structural data obtained by shock-wave and diamond-cell techniques demonstrate that calcium oxide transforms from the B1 (sodium chloride type) to the B2 (cesium chloride type) structure at 60 to 70 gigapascals (0.6 to 0.7 megabar) with a volume decrease of 11 percent. The agreement between the shockwave and diamond-cell results independently confirms the ruby-fluorescence pressure scale to about 65 gigapascals. The shock-wave data agree closely with ultrasonic measurements on the B1 phase and also agree satisfactorily with equations of state derived from ab initio calculations. The discovery of this B1-B2 transition is significant in that it allows considerable enrichment of calcium components in the earths lower mantle, which is consistent with inhomogeneous accretion theories.

High-Pressure Ruby and Diamond Fluorescence: Observations at 0.21 to 0.55 Terapascal

A diamond-anvil, high-pressure apparatus was used to extend the upper pressure limit of static laboratory experiments. Shifts of the R1 strong fluorescent line of ruby were observed that correspond to static pressures of 0.21 to 0.55 terapascal (2.1 to 5.5 megabars) at 25�C. Sensitive spectroscopic techniques in the pressure range 0.15 to 0.28 terapascal were used to observe ruby and diamond fluorescence separately; these two fluorescent emissions overlap strongly at high pressures. At pressures greater than approximately 0.28 terapascal, the diamond fluorescence diminished and the ruby fluorescence reappeared strongly. Pressure was determined by extrapolation of the calibrated shift of the ruby R1 line.

The equation of state of dense argon: A comparison of shock and static studies

New static high pressure measurements for room temperature solid argon to 800 kbar are in excellent agreement with one predicted from a theoretical reduction of shock wave data. The results demonstrate the agreement between shock and static techniques even in cases where shock temperatures are extremely high and a large thermal correction is required to reduce the Hugoniot to an isotherm. The results suggest that solid argon may provide a useful static pressure standard up to about 3–4 Mbar.

High-Pressure Physics: The 1-Megabar Mark on the Ruby R1 Static Pressure Scale

Ruby crystals were subjected to a static pressure greater than 1 megabar in a diamond-windowed pressure cell. The pressure was monitored continuously by observing the spectral shift of the sharp fluorescent R1 ruby line excited with a cadmium-helium gas-diffusion laser beam. One megabar appears to be the highest pressure ever reported for a static experiment in which an internal calibration was employed.

Electrical conductivity and the red shift of absorption in olivine and spinel at high pressure.

Above 100 kilobars the apparent absorption edges (approximately 3 electron volts) of single-crystal and polycrystalline samples of the metastable olivine and stable spinel forms of Fe2SiO4 shift rapidly with pressure from the near-ultraviolet into the lower-energy infrared region. Simultaneously, an exponential increase in electrical conductivity occurs. These effects are reversible as pressure is reduced or reapplied and are not accompanied by a first-order phase change in olivine or spinel. These observations relate to fundamental concepts of electrical conductivity and photon absorption in complex transition-metal silicates in that they cannot be readily interpreted in terms of an intrinsic band-gap model. The intensity and energy changes are too great and the effect occurs at too low a pressure to be explained by processes such as spin-pairing and other crystal-field effects. The results suggest that a new mechanism of conduction, perhaps symbiotic and employing an efficient charge-transfer process, is induced at high pressure.