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A new phase isostnictural with the minerals of the aenigmatite group Na 2 Mg 4+x Fe 3+  2-2x Si 6+x O 20  with x = 0.25 - 0.5 was synthesized at 13-14 GPa with a split- sphere anvil apparatus (USSA-2000). The structure (for x = 0.4)_was determined from twinned-crystal X-ray diffraction data. The unit cell is triclinic. P1̅, a = 10.328(1), b = 10.724(1), c = 8.805(1) Å, α = 105.15(1), β = 96.85(1), γ =125.47(1)°, V = 719.67(3) A 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° rotation about [HO]*. Due to the coupled substitution. 2Fe 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 then 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.

Composition, stability, and structure of a new member of the aenigmatite group, Na 2 Mg 4+x Fe 3+ 2-2x Si 6+x O 20 , synthesized at 13–14 GPa