SiO2 solubility in rutile at high temperature and high pressure

Abstract

Silicon-bearing rutile has been found in chromitite from the Luobusa (罗布莎) ophiolite, Tibet. However, the extent of SiO 2 solubility in rutile and the nature of its origin are still unclear. At high pressure, SiO 2 takes a rutile structure with Si in 6-fold coordination. Thus, high pressures may enhance its solubility in rutile because of possible isovalent exchange in the octahedral site. In this study, we report new experimental results on SiO 2 solubility in rutile up to 23 GPa and 2 000 ℃. Starting materials were mixtures of powdered pure rutile and pure quartz, with compositions of (Ti 0.5 Si 0.5 )O 2 , (Ti 0.93 Si 0.07 )O 2 , and (Ti 0.75 Si 0.25 )O 2 . The mixtures were loaded into either platinum capsules (for a 10/5 assembly) or rhenium capsules (for an 8/3 assembly). The experiments were carried out using multi-anvil high-pressure apparatus with a rhenium resistance heater. Sample temperatures were measured with a W5%Re-W26%Re thermocouple and were controlled within ±1 ℃ of the set temperature. TiO 2 -rich and SiO 2 -rich phases were produced in all the quenched samples. Microprobe analyses of the phases show that the solubility of SiO 2 in rutile increases with increasing pressure, from 1.5 wt.% SiO 2 at 10 GPa to 3.8 wt.% SiO 2 at 23 GPa at a temperature of 1 800 ℃. The solubility also increases with increasing temperature from 0.5 wt.% SiO 2 at 1 500 ℃ to 4.5 wt.% SiO 2 at 2 000 ℃ at a pressure of 18 GPa. On the other hand, the solubility of TiO 2 in coesite or stishovite is very limited, with an average of 0.6 wt.% TiO 2 over the experimental P-T ranges. Temperature has a much larger effect on the solubility of SiO 2 in rutile than pressure. At high pressure, the melting point of SiO 2 is definitely higher than that of TiO 2 and the eutectic point moves towards SiO 2 in the TiO 2 -SiO 2 system. Lower oxygen fugacity decreases the solubility of SiO 2 in rutile, whereas water has little effect on the solubility. Our experimental data are extremely useful for determining the depth of origin of the SiO 2 -bearing rutile found in nature.

Silicon coordination in rutile and TiO2-II at ambient and high pressures: Si-29 NMR

“…Rather, we purposely optimized sample volume at the expense of careful control of temperature gradients, to recover sufficient material for highprecision NMR study. However, the estimated silica content of the 6 GPa rutile in the presence of excess coesite, about 0.6 wt%, is roughly consistent with previous experimental studies (Ren et al 2009) and theoretical calculations (Vinograd et al 2008). For the 12 GPa, 1200 °C sample of TiO 2 -II, the run time and temperature were insufficient for silica to diffuse through the grains, as indicated by the highly heterogeneous EPMA data.…”

“…This was the expected result, given that the nominal pressure of the experiment was 2.9 GPa higher than the rutile-TiO 2 -II boundary (Withers et al 2003) and that a strong partitioning of Si between the two TiO 2 polymorphs, which could extend the stability field of rutile, is not expected based on available solubility data (Ren et al 2009). The grain size of this sample was significantly smaller than in the 6 GPa run, about 10 to 30 µm.…”

“…This last report motivated an experimental study of Si solubility in TiO 2 as a function of pressure and temperature (Ren et al 2009). The experiments were conducted in the stability fields of rutile and its high-pressure polymorph, α-PbO 2 -structured TiO 2 (or TiO 2 -II), and possibly even into the stability field of the still higher-pressure baddeleyite-structured polymorph.…”

“…The exact nature of IV Si substitution in titania remains incompletely characterized: tetrahedral interstitial vacancies are indeed present in the structure of rutile, but whether Si substitution is accompanied by the formation of oxygen interstitials as well, by octahedral Ti 4+ vacancies (more likely), or by another mechanism, is not known. Future application of Si in TiO 2 as a geothermobarometer will not only require consideration of IV Si solubility but a more extensive set of experiments than those conducted by Ren et al (2009), particularly at lower pressures and temperatures (and thus lower Si contents).…”

“…The fact that stishovite (stable above about 9 GPa at mantle temperatures) is isostructural with rutile suggests that there should be significant solid solution at high pressures, which could in principle be used as a geothermobarometer (Stebbins 1992;Vinograd et al 2008;Ren et al 2009). At least at lower pressures, where the stable silica polymorph is coesite or quartz, there could be a strong pressure effect on solubility, given the expected large negative molar volume change on transition from tetrahedral ( IV Si) to octahedral ( VI Si) silicon coordination.…”