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Composition, stability, and structure of a new member of the aenigmatite group, Na~2~Mg (sub 4+x) Fe (super 3+) (sub 2-2x) Si (sub 6+x) O~20~, synthesized at 13-14 GPa by Tibor Gasparik; John B. Parise; Richard J. Reeder; Victor G. Young; Wendy S. Wilford is a Earth and Planetary Sciences article available to read on EtoBox.

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A new phase isostructural with the minerals of the aenigmatite group Na 2 Mg 4ϩx Fe Si 6ϩx O 20 with x ϭ 0.25 Ϫ 0.5 was synthesized at 13-14 GPa with a split-3ϩ 2Ϫ2x sphere anvil apparatus (USSA-2000). The structure (for x ϭ 0.4) was determined from twinned-crystal X-ray diffraction data. The unit cell is triclinic, P1, a ϭ 10.328(1), b ϭ 10.724(1), c ϭ 8.805(1) A ̊, ␣ ϭ 105.15(1), ␤ ϭ 96.85(1), ␥ ϭ125.47(1)Њ, V ϭ 719.67(3) A ̊3, Z ϭ 2, calculated density ϭ 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 180Њ rotation about [110]\*. Due to the coupled substitution, 2Fe 3ϩ ϭ 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 impli

Who reads Composition, stability, and structure of a new member of the aenigmatite group, Na~2~Mg (sub 4+x) Fe (super 3+) (sub 2-2x) Si (sub 6+x) O~20~, synthesized at 13-14 GPa?

Author

Tibor Gasparik; John B. Parise; Richard J. Reeder; Victor G. Young; Wendy S. Wilford

Publisher

Mineralogical Society of America

Published

1999

Language

EN

Field

Earth and Planetary Sciences (Physical Sciences)