The thermal evolution of Sn nanocrystals (nC) produced by ion implantation into silica films is studied using Rutherford Backscattering Spectrometry, Transmission Electron Microscopy and Glancing Incidence Small Angle X-ray Scattering. Upon a two step annealing process, consisting of a low temperature long time aging treatment followed by high temperature furnace annealings (773 – 1373 K; 1800 s) in high vacuum, we demonstrate the formation of Sn nC with a modal diameter D~0.7 nm and a size dispersion from 0.4 to 1.8 nm presenting abnormally high thermal stability, characterized by a melting temperature of ~1300 K (i.e. 800 K above the melting of the bulk metallic phase). The results are discussed assuming a size dependent phase transition from metallic to covalent bounded crystals and compared with the literature data from free-standing nC. A model based on classical thermodynamics concepts is proposed. The model provides good fit to the experimental data, including the melting temperature depression with decreasing D up to a critical size Dc~2 nm, and the superheating behavior observed for D below Dc.