Magnesium only-phyllosilicates such as serpentines (1:1) and stevensite (2:1) do exist in nature but are not abundant in pure form, whereas synthesized materials can ensure higher purity. Clay mineral hydrothermal synthesis is quite easy to perform, however, the condensation reactions leading to the formation of these phyllosilicates and their precursors are not well understood. Such knowledge would allow to predict the synthesis outcome in case of variation of experimental parameters. This study reports a chemical reaction mechanism explaining the parameters that are determining the type of the formed clay mineral. After a detailed characterization of each obtained phase, it is shown that pH has a lower effect compared to Mg to Si molar ratio. Our results show that above pH 10 the mechanisms of lizardite and stevensite formation are similar with the same elementary units, but that the type of material obtained depends on the availability in excess of magnesium (formation of lizardite) or silicon (formation of stevensite).