{H-1-Si-29} CP/MAS NMR allowed detection of an amorphous silicate formed during contact of solvated Mg2+ cations with silica at room temperature and moderately basic pH. At constant contact time between the two spin systems the sample without magnesium showed three CP/MAS NMR resonances at -90, -100, and -110 ppm, while two new features at -84 and -92.5 ppm appeared and increased concurrently with the magnesium loading. Fitting of the polarization growth provided for spin dynamics information (TSI-H) as well as for a mean to relatively quantify each silicon species (Mo). The resonances could be clearly separated into two groups according to their cross-polarization dynamics : the resonances at -84, -90, and -100 ppm with T-Si-H in the range of milliseconds, on the one hand, and the resonances at -92.5 and -110 ppm, on the other hand, with much longer cross-polarization time in the order of tens of milliseconds. The resonances at -90, -100, and -110 ppm are attributed to geminal silanols, simple silanols, and siloxane silicons. The dynamic parameters were consistent with this attribution and in line with what has been reported by other investigators on silica. The resonance at -92.5 ppm was attributed to a Q(3)(Mg) environment for the following reasons. First, the chemical shift indicated a Q(3) coordination; second, the spin dynamics (T-Si-H) was not compatible with a Q(3)(OH) environment; and, third, the magnetization limit (M(0)) was proportional to the Mg(II) loading. The -84 ppm resonance was attributed to terminal. Q(2)(Mg, OH). FT-IR analysis confirmed the quantitative formation of a magnesium silicate. Therefore, a high-surface area proto-phyllosilicate gel phase on the surface of the silica was responsible for the fixation of Mg(II). More generally, careful quantitative analysis of CP-MAS NMR was established here as a powerful technique for the quantitative detection of surface amorphous hydrous silicates of ill-defined structures. The constitution of a Si-O-Mg bond during impregnation at room temperature was clearly evidenced, demonstrating the formation of a secondary phase including both the cation constitutive of the support (Si) and the dispersed species (Mg), and thereby recusing a surface-assisted magnesium hydroxide precipitation model.