Hafnium silicate (HfSiO4; hafnon) is under consideration as an environmental barrier coating material for high-temperature applications. However, its rate of formation from mixtures of monoclinic HfO2 and crystalline (beta-cristobalite) SiO2 powders is unknown. Here it has been synthesized and its formation rate measured during their solid-state reaction at temperatures from 1250 degrees C to 1400 degrees C. Rietveld refinement of X-ray diffraction patterns indicates that at 1250 degrees C the hafnon phase fraction increases linearly with time, while at the highest reaction temperature, the hafnon phase fraction exhibited a parabolic dependence upon time. Between these two limiting temperatures, a region of linear behavior preceded a transition to parabolic kinetics, with the transition occurring at an earlier time as the reaction temperature increased. Arrhenius relations fitted the kinetics of hafnium silicate formation in both the linear and parabolic regimes. Scanning electron microscopy indicated that the reaction proceeded by diffusion of SiO2 into HfO2, similar to the mechanism by which zirconium silicate has been formed from vitreous SiO2 and tetragonal ZrO2. The initial linear rate of reaction is consistent with the growth of the contact area between the SiO2 and HfO2 particles combined with rapid permeation of the Si4+ and O2- through the initial, incompletely formed hafnon. After a thin hafnon layer had formed between the reactants, the rate of hafnium silicate growth slowed and further growth was governed by the rate of diffusion of Si4+ and O2- through the reaction product consistent with the observed parabolic dependence of the phase fraction upon time.