Rutile/TiO2II phase equilibria

Abstract

The transition between rutile and a-PbO 2 structured TiO 2 (TiO 2 II) has been investigated at 700-1,200°C and 4.2-9.6 GPa. Hydrothermal phase equilibrium experiments were performed in the multi-anvil apparatus to bracket the phase boundary at 700, 1,000, and 1,200°C. The equilibrium phase boundary is described by the equation: P (GPa)=1.29+0.0065 T (°C). In addition, growth of TiO 2 II was observed in experiments at 500 and 600°C, although growth of rutile was too slow to bracket unambiguously the equilibrium boundary at these temperatures. Water was not detected in either rutile or TiO 2 II, and dry synthesis experiments at 1,200°C were consistent with the phase boundary determined in the fluid-bearing experiments, suggesting that the equilibrium is unaffected by the presence of water. Our bracket of the phase boundary at 700°C is consistent with the reversed, dry experiments of Akaogi et al. (1992) and the reversals of Olsen et al. (1999). The new data suggest that the phase boundary is linear, in agreement with Akaogi et al. (1992), but in striking contrast to the phase diagram inferred by Olsen et al. (1999). The natural occurrence of TiO 2 II requires formation pressures considerably higher than the graphitediamond phase boundary.

High-pressure highly reduced nitrides and oxides from chromitite of a Tibetan ophiolite

“…The difference in chemical composition between the C-rich nitrogen isotope standards and the Ti-rich, C-poor TiN raises a potential concern over the accuracy of the mass-dependent fractionation correction. Although a full evaluation requires a TiN standard with welldetermined 15 N/ 14 N, the absence of significant variations in instrumental mass-dependent fractionation for a variety of N-bearing materials supports the approach followed here. Data were processed as quantitative isotopic ratio images by using custom software (L'image; L. Nittler, Lawrence Livermore National Laboratory) and were corrected for detector dead time and image shift from layer to layer.…”

“…At temperatures appropriate for mantle upwelling at ocean spreading centers, TiO 2 II is stable only at pressures within the stishovite stability field (Ͼ10 GPa) (15). Nevertheless, neither the TiO 2 II nanoinclusions nor the host coesite show any evidence of reaction to their low-pressure polymorphs, rutile and quartz, respectively.…”

“…2E), a high-pressure polymorph of rutile with the ␣PbO 2 structure (Fig. 2F) (15), and boron carbide of unknown stoichiometry. Electron diffraction analysis established the cubic symmetry of both nitrides, with a ϭ 4.23 Å for TiN (Fig.…”

High-pressure polymorph of TiO2-II from the Xiuyan crater of China

“…Rutile transforms to TiO 2 -II at 4-12 GPa and 400-1500°C [3], 10 GPa and 400°C [4], 4.6-8.5 GPa and 500-900°C [5], 4.5-7.5 GPa and 600-1100°C [6], 4.2-9.6 GPa and 700-1200°C [7]. It appears that the phase transition of rutile to TiO 2 -II involve some thermal activation process.…”

“…i  - had been firstly synthesized by static compression experiment [3]. A number of static compression experiments succeed in the transition of rutile to TiO 2 -II at pressure range between 4 and 12 GPa and temperature between 400 and 1500°C [3][4][5][6][7]. Shock-loading experiments revealed that rutile transforms to i  - at peak shock pressure from 20 to 100 GPa [8][9][10].…”