Crystal Chemistry of Synthetic Mg(Si1−xGex)O3 Pyroxenes: A Single-Crystal X-ray Diffraction Study

Abstract

Germanate-pyroxenes often are used as model systems to study the stability and phase relationships of analog silicate systems. Based on such analyses, it is assumed that silicates and germanates behave ideally in terms of mixing. A systematic study was performed to monitor in detail the changes introduced by a Si4+ through Ge4+ replacement in the important rock-forming pyroxene enstatite MgSiO3. Well-shaped, idiomorphic singe crystals of a MgSi1−xGexO3 pyroxene solid solution were grown at ambient pressure from a high-temperature flux-assisted synthesis. Structural analysis using single-crystal X-ray diffraction methods revealed orthorhombic symmetry, Pbca, Z = 8, for the complete solid-solution series. Long-term storage over a period of 8 years at ambient conditions or annealing at 525 °C over a period of 10 weeks did not change the symmetry of the proposed thermodynamically stable monoclinic polymorph. Within the solid-solution series, lattice parameters increased almost linearly with increasing Si4+ by Ge4+ substitution. The main changes occurred on the tetrahedral sites, which showed an almost linear increase in individual and average bond lengths but also in distortion parameters. The refined site occupancy of Si4+ and Ge4+ showed a distinct preference of Ge4+ for the TB site. The altered topology and kinking state in the tetrahedral chains also imposed significant changes to the bonding topology and geometry of the neighboring M1 and M2 sites.