The improper ferroelastic phase transition of synthetic letovicite has been studied using optical birefringence, hard mode Raman spectroscopy and X-ray diffraction measurements. The critical point is the L-point on the surface of the Brillouin zone of the rhombohedral high-temperature phase. The domain pattern of the ferroelastic phase is related to the arms of the L-star leading to orientational domains of the species 3mF2/m. The integrated intensity of the Ag-Raman-mode near 483 cm−1, the thermal evolution of the spontaneous strain and coupled quantities like the morphic optical birefringence show a rapid saturation on approaching the transition point near 413 K. The saturation has been fitted using Landau-type free energy expansions with high power terms of the order parameter. The non-classical thermal variation of the order parameter is compatible with a Slater instability of infinite order.