High-pressure synthetic (Na0.97Mg0.03)(Mg0.43Fe0.17 3+Si0.40)Si2O6, with six-coordinated silicon, isostructural with P2/n omphacite

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na 0.97 Mg 0.03 )(Mg 0.43 Fe 0.17 3+ Si 0.40 )Si 2 O 6 , is isotypic with ordered P 2/ n omphacite. Its structure is characterized by single chains of corner-sharing SiO 4 tetra­hedra, extending along the c axis, which are crosslinked by bands of edge-sharing octa­hedra (site symmetry 2), statistically occupied by (Mg 2+ + Fe 3+ + Si 4+ ). Between the bands built up of the octahedra are two non-equivalent highly distorted six-coordinated sites (site symmetry 2), statistically occupied by (Na + Mg). In contrast to omphacites, the great differences in size and charge between Mg 2+ and Si 4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octa­hedral sites, whereas Fe 3+ is disordered between the two sites. The octa­hedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe)—O and ( VI Si + Fe)—O bond lengths are 2.075 and 1.850 Å, respectively.