Mg4Ga8Ge2O20: A new synthetic analog of the mineral sapphirine

A new sapphirine-analog, Mg₄Ga₈Ge₂O₂₀, has been identified and characterized in the system MgO-Ga₂O₃-GeO₂ in samples quenched from 1400° C, 1 atm. It crystallizes with a triclinic unit-cell and is isostructural with the mineral sapphirine-1Tc, Mg₄Al₈Si₂O₂₀. It is stable over a broad range of stoichiometry associated with the exchange reaction 2 Ga³⁺ = Mg²⁺ }+Ge⁴⁺. TEM examination shows the common occurrence of polysynthetic twinning and suggests the existence of a high-temperature triclinic→monoclinic transition. The observation of sapphirine/spinel intergrowths is consistent with the structural relationships described previously.