A new phase isostructural with the minerals of the aenigmatite group Na 2 Mg (sub 4+x) Fe (super 3+) (sub 2-2x) Si (sub 6+x) O 20 with chi = 0.25-0.5 was synthesized at 13-14 GPa with a splitsphere anvil apparatus (USSA-2000). The structure (for chi = 0.4) was determined from twinned-crystal X-ray diffraction data. The unit cell is triclinic, P1, a = 10.328(1), b = 10.724(1), c = 8.805(1) Aa, alpha = 105.15(1), beta = 96.85(1), gamma = 125.47(1) degrees , V = 719.67(3) Aa 3 , Z = 2, calculated density = 3.335 g/cm 3 . The twin law, independently determined from electron diffraction and transmission electron microscopy and by inspection of the X-ray data collected with an area detector, relates the twin components by a 180 degrees rotation about [110] (super *) . Due to the coupled substitution, 2Fe (super 3+) = MgSi, which introduces octahedral Si, the stability of the phases with the aenigmatite structure apparently expands with increasing pressure. Hence, these phases could play a major role in the transition zone (410-660 km), where the more common minerals they are replacing, olivine and clinopyroxene, reach the limit of their stability. The new evidence for the stability of aenigmatite-like minerals in the deep mantle could have important implications for the origin of the parental magmas producing aenigmatite-bearing and other agpaitic rocks.

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