P2/n omphacite

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na0.97Mg0.03)(Mg0.43Fe0.17 Si0.40)Si2O6, is isotypic with ordered P2/n omphacite. Its structure is characterized by single chains of corner-sharing SiO4 tetrahedra, extending along the c axis, which are crosslinked by bands of edge-sharing octahedra (site symmetry 2), statistically occupied by (Mg + Fe + Si). 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 and Si result in complete, rather than partial, ordering of Mg and Si into two distinct octahedral sites, whereas Fe 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 (Si + Fe)—O bond lengths are 2.075 and 1.850 Å, respectively.