High-pressure synthetic (Na _0.97Mg _0.03)(Mg _0.43Fe _0.17 ³⁺Si _0.40)Si ₂O ₆, with six-coordinated silicon, isostructural with P2/n omphacite

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, Na _0.97Mg _0.03)(Mg _0.43Fe _0.17 ³⁺Si _0.40)Si ₂O ₆, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO ₄ tetra-hedra, extending along the c axis, which are crosslinked by bands of edge-sharing octa-hedra (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 octa-hedral sites, whereas Fe ³⁺ is disordered between the two sites. The octa-hedron filled by (Mg + Fe) is larger and markedly more distorted than that occupied by (Si + Fe). The average (Mg + Fe) - O and ( ^VISi + Fe) - O bond lengths are 2.075 and 1.850 Å, respectively.