The influence of mechanical activation of the mixture basic MgCO3-amorphous SiO2 on reaction of forsterite formation was studied with and without the presence of fluorine ion. Beneficial influence of mechanical activation on the reaction in the mixture without fluorine ion is a consequence of the change of the granulometry during the intense milling. Mechanical activation in the presence of fluorine ion affects the mechanism of forsterite formation via different compounds from the humite group. With non-activated mixture dominant transition phase is hondrodite, while with activated mixture, clinohumite is the transition compound in the process of formation of end reaction products. Fluorine ion is released from the system by hydrolysis.