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 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 ((VI)Si + Fe)-O bond lengths are 2.075 and 1.850 Å, respectively.