Composition, stability, and structure of a new member of the aenigmatite group, Na2Mg4+xFe2-2x3+Si6+xO20, synthesized at 13-14 GPa

A new phase isostructural with the minerals of the aenigmatite group Na2Mg4 +xFe2-2x3+Si6+xO20 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 d etermined from twinned-crystal X-ray diffraction data. The unit cell is tri clinic, P (1) over bar, a = 10.328(1), b = 10.724(1), c = 8.805(1) Angstrom , alpha = 105.15(1), beta = 96.85(1), gamma = 125.47(1)degrees, V = 719.67( 3) Angstrom(3), Z = 2, calculated density = 3.335 g/cm(3). The twin law, in dependently determined from electron diffraction and transmission electron microscopy and by inspection of the X-ray data collected with an area detec tor, relates the twin components by a 180 degrees rotation about [110]*. Du e to the coupled substitution, 2Fe(3+) = MgSi, which introduces octahedral Si, the stability of the phases with the aenigmatite structure apparently e xpands with increasing pressure. Hence, these phases could play a major rol e in the transition zone (410-660 km), where the more common minerals they are replacing, olivine and clinopyroxene, reach the limit of their stabilit y. The new evidence for the stability of aenigmatite-like minerals in the d eep mantle could have important implications for the origin of the parental magmas producing aenigmatite-bearing and other agpaitic rocks.