The natural phyllosilicates are thermally instable due to the presence of OH group in the lattice. On the basis of the mica phlogopite type structure [KMg₃ (AlSi₃) O₁₀ (OH) ₂] and through the anionic substitution OH^- → O^2- and the cationic substitutions (Al³⁺ → Si⁴⁺ and Mg²⁺ → Al³⁺) a new composition has been proposed : K (Mg₂Al) Si₄O₁₂ [1]. In order to get a homogeneous starting material, due to the complexity of the proposed composition [K (Mg₂Al) Si₄O₁₂], the first step was the preparation of the process precursor through a sol-gel. The conventional treatment (high temperature) of such a precursor leading to the formation of a mixture of three-dimensional silicates, a solvothermal reaction was developed, the non-aqueous solvent required for the preparation of the sol being used in supercritical conditions. The resulting material appears to be a bidimensional oxide isostructural of the mica-type phlogopite. All the physico-chemical characterizations confirm such a structure and the absence of OH group in the lattice, in particular the IR spectroscopy. Through such an illustration, solvothermal reactions, appear a powerful way for preparing new materials.