Abstract: A high-pressure modification of NaAlSiO4, with the calcium ferrite structure was synthesized at pressures higher than 24 GPa using a double-stage split-sphere type high-pressure apparatus. The unit cell dimensions are: a=10.1546(8), b=8.6642(8), c= 2.7385(4) Å, and V = 240.93(3) Å3 with the space group Pbnm. Calculated density is 3.916(1) g/cm3. The crystal structure was determined by the ordinary powder X-ray method. Both M1O6 and M2O6 octahedra run parallel to the c-axis, forming edge-shared double chains. The shared edges exhibit remarkable shortening (2.25–2.39 Å). Sodium atoms are located in the ‘tunnel’ formed by the linked double chains, being in eightfold co-ordination. The structure is very similar to that predicted by Dempsey and Strens (1976) with the aid of the DLS method. The possible instability of the calcium ferrite type of Mg2SiO4 is discussed.