High‐Pressure Investigation in the System SiO₂–GeO₂: Mutual Solubility of Si and Ge in Quartz, Coesite and Rutile Phases

Abstract

Mutual solubilities in crystalline phases in the system SiO2–GeO2 have been investigated up to 10 GPa pressure and 1500°C temperature, using a bulk composition of 50 mol% GeO2. Solid solution of up to 40 mol% GeO2 into the mineral quartz has been confirmed as well as solubility of Si into GeO2 rutile (argutite) and Ge into SiO2 rutile (stishovite) phases and limited Ge into coesite. Solubility of Ge in quartz is very high, and decreases with pressure, with the univariant quartz‐out reaction occurring near 3.4 GPa at 1200°C. The solubility of GeO2 in coesite is highest at 3.4 GPa (about 8 mol%) and decreases with increasing pressure. Significantly more extensive solubility than previously reported for the rutile phases has been found and measured in detail as a function of pressure and temperature. Extensive solubility of SiO2 in GeO2 is found in argutite at 1200°C, increasing strongly with pressure and reaching a maximum of 25.2 mol% SiO2 in GeO2 at 9 GPa. At this point coesite (ss) plus argutite (ss) react to form a stishovite phase with 18 mol% GeO2, and the mutual solubility in both phases decreases above this pressure. At 1500°C, similar solubilities are observed but the maximum SiO2 solubility in argutite of just over 25 mol% occurs near 10 GPa. All these solid solutions can be recovered to ambient temperature and pressure. Phase diagrams and unit cell information of the phases are presented here. Based on these results, a useful and industrially relevant, application for accurately measuring high pressure is suggested.

Saturation curve of SiO2 component in rutile-type GeO2: A recoverable high-temperature pressure standard from 3 GPa to 10 GPa

“…The reference activity coefficient of 3.439 corresponds to a positive deviation from ideality, indicating that the solution is destabilized compared to an ideal solution of the same composition. This is consistent with the observation of a solvus between stishovite and argutite at high pressures, as observed by Gullikson et al [14].…”

“…The parameters are obtained using the following procedure: the volume contribution ΔV ef f and the reference state activity coefficient γ n are first obtained from the fit to the data at 1200 1C (corresponding to the case T ¼T n ); their values are shown in Table 3. The ΔV ef f of À 7.614E À 6 cm 3 /mol can be compared to existing data -the volume change for the coesite to stishovite transition along that isotherm calculated from Akaogi et al [18] is about À 5.8E À 6 cm 3 /mol, and the excess volume of solution at ambient pressure measured by Gullikson et al [14] contributes an additional 20% to the effective volume, giving an expected value of about À6.9E À 6 cm 3 /mol. The additional magnitude could be due to the effect of the up to 8 mol% germania in coesite that diminishes with pressure, which was not explicitly modeled.…”

“…So far the equation is exact and the coesite can contain some GeO 2 . We then introduce an approximation that the effect of GeO 2 solubility in coesite is small, which is supported by the EPMA data which shows maximum solubility of GeO 2 in coesite of 8 mol% at 3 GPa and decreasing with pressure to nearly pure SiO 2 at 9 GPa [14]. Likewise, the stishovite end-member reference state is defined as pure SiO 2 .…”

“…Another extension of the system could be made below 3 GPa. In this range, according to the existing data which is sparse, argutite (ss) coexists with a quartz solid solution, and the lattice parameters and structure of the quartz (ss) phase vary strongly with pressure from 1 to 3 GPa, due to the decreasing solubility of GeO 2 in SiO 2 [14,19], causing a significant change in lattice parameters that could be used as a pressure gauge for piston-cylinder experiments. Calibrations of the SiO 2 -GeO 2 system below 3 GPa and above 10 GPa would be useful goals for future studies.…”

“…The possible use of this system as a pressure gauge was evaluated by Furshenko et al [13] but at the time it was not adopted because significant solubility was not found using the techniques available at the time. However, because of the significant pressure-dependent solubility and the measureable volume change of the solid solution reported recently [14] we are reintroducing this system as a pressure gauge.…”