Phase Diagram and Thermal Expansion of orthopyroxene-, clinopyroxene- and ilmenite-structured MgGeO3

Simon A Hunt,David P Dobson,J R Santangeli ,Ian G Wood

doi:10.2138/am-2021-7685ccby

Abstract

Abstract The MgGeO3 system is a low-pressure analog for the Earth-forming (Mg,Fe)SiO3 system and exhibits recoverable orthopyroxene, clinopyroxene, and ilmenite structures below 6 GPa. The pressure-temperature conditions of the clinopyroxene to ilmenite phase transition are reasonably consistent between studies, having a positive Clapeyron slope and occurring between 4 and 7 GPa in the temperature range 900–1600 K. There are, though, significant discrepancies in the Clapeyron slope of the orthopyroxene to clinopyroxene phase transition in existing works that also disagree on the stable phase at ambient conditions. The most significant factor in these differences is the method used; high-pressure experiments and thermophysical property measurements yield apparently contradicting results. Here, we perform both high pressure and temperature experiments as well as thermal expansion measurements to reconcile the measurements. High-pressure and -temperature experiments yield a Clapeyron slope of −1.0−0.7+1.0 MPa/K for the MgGeO3 orthopyroxene-clinopyroxene phase transition, consistent with previous high-pressure and -temperature experiments. The MgGeO3 orthopyroxene-clinopyroxeneilmenite triple point is determined to be at 0.98 GPa and 752 K, with the ilmenite phase stable at ambient conditions. The high-temperature (&gt;600 K) thermal expansion of the clinopyroxene phase is greater than that of the other phases. Debye-Grüneisen relationships fitted to the volume-temperature data give Debye temperatures for the orthopyroxene, clinopyroxene, and ilmenite phases of 602(7), 693(10), and 758(13) K and V0 of 897.299(16), 433.192(10), and 289.156(6) Å3, respectively. The Clapeyron slopes calculated directly from the Debye-Grüneisen relationships are consistent with previous thermophysical property measurements. The presence of significant anharmonicity and/or formation of defects in the clinopyroxene phase at high-temperatures, which is not apparent in the other phases, accounts for the previous contradictions between studies. The inferred increased heat capacity of the clinopyroxene corresponds to an increase in entropy and an expanded phase field at high temperatures.

Highlights

The MgO-GeO2 system has long been studied because of its similarity to the MgO-SiO2 system but with structure types stable at lower pressures
About 4 GPa and 1000 K in MgGeO3, the ilmenite structure is stable, and there is no appearance of the Mg2SiO4 + SiO2 field that separates the clinopyroxene and ilmenite phases in the MgSiO3 system
Our1e7x0p0erimental phase diagram is consistent with most of the previou1s6e0x0perimental data (Fig. 1) and like previous studies we find that ilmenite-structured MgGeO3 is the high-pressure phase

Summary

Introduction

The MgO-GeO2 system has long been studied because of its similarity to the MgO-SiO2 system but with structure types stable at lower pressures. 10 GPa, MgGeO3, like MgSiO3, exhibits orthopyroxene, clinopyroxene, and ilmenite structures (Ringwood and Seabrook 1962; Kirfel and Neuhaus 1974; Ozima and Akimoto 1983; Ross and Navrotsky 1988). Both the MgO-GeO2 and MgO-SiO2 systems have ambient-pressure olivine structures that transform to spinel structures at elevated pressure and temperature (Ross and Navrotsky 1987; Akaogi et al 1989). Reported as Mg4GeO6 (Robbins and Levin 1959; McCormick 1964) and subsequently refined to be Mg3.5Ge1.25O6 with space group Pbam (von Dreele et al 1970; Kostiner and Bless 1971), at ambient pressure, it is stable below 1768 K, whereupon it breaks down to Mg2GeO4 + MgO (Robbins and Levin 1959)

Methods

Results

Conclusion