Feldspars are the most common minerals in the Earth’s crust, and 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 KMeGe3O8 (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 transformed irreversibly upon heating (930 ± 30 and 960 ± 30°C for KGaGe3O8 and KAlGe3O8, respectively) into feldspar-like 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 the crystallization process, thermal transformation and thermal expansion of feldspar- and paracelsian-like compounds is discussed.