Composition, stability, and structure of a new member of the aenigmatite group, Na₂Mg(4+x)Fe(2-2x)³⁺Si(6+x)O₂₀, synthesized at 13-14 GPa

A new phase isostructural with the minerals of the aenigmatite group Na₂Mg(4+x)Fe(2-2x)³⁺Si(6+x)O₂₀ 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, P1, 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) ų, Z = 2, calculated density = 3.335 g/cm³. 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 [110](*). Due to the coupled substitution, 2Fe³⁺ = 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.