An analysis was made of 85 compounds with a huntite type structure RM3(BO3)4, where R = rare-earth element (Y, La - Lu), M = Al, Sc, Cr, Fe, Ga. Crystal structure analysis revealed the critical displacements of atoms at phase transition R32 ↔ P3121 and their dependence from ionic radii. As a result, the tolerance factor was derived and its threshold, below which the structure is stable in the R32 phase, and above which the structure is stable in the distorted phase P3121. The formula has been tested on over 30 compounds with the huntite type structure and gave good agreement. Therefore, it can be used with confidence to predict new compounds. At this work, the tolerance factor revealed consistent patterns in huntites that were previously unknown.