Thermal induced paracelsian-to-feldspar transformation: case study of K <i>Me</i> Ge ₃ O ₈ ( <i>Me</i> = Al, Ga)

Feldspars are the most common minerals in the Earth's crust, commonly contain trace concentrations of gallium and germanium.However, their gallium / germanium synthetic analogues are widely used both in industrial applications and geological reconstructions.Here KMeGe 3 O 8 (Me = Al, Ga) compounds were synthesized using solid-state reactions and hydrothermal synthesis and studied by microprobe analysis and powder X-ray diffraction (under both ambient and high-temperature conditions).In both (Al and Ga) cases, paracelsian-like phases were the first to form, even though feldspar-like phases are predicted to be thermodynamically stable.Paracelsian-like phases are irreversibly transformed upon heating (930 30 and 960 30 C for KGaGe 3 O 8 and KAlGe 3 O 8 , respectively) into feldsparlike modifications.According to the obtained data, the feldspar-like modifications exhibit a higher degree of anisotropy, whereas the volume thermal expansion coefficients are higher for paracelsian-like modifications.The influence of chemical composition and topology on