High-pressure synthetic (Na0.97Mg0.03)(Mg0.43Fe0.173+Si0.40)Si2O6, with six-coordinated silicon, isostructural with P2/n omphacite

Abstract

The title compound, (sodium magnesium) [magnesium iron(III) silicon] disilicate, (Na0.97Mg0.03)(Mg0.43Fe0.173+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 (Mg2+ + Fe3+ + Si4+). 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 Mg2+ and Si4+ result in complete, rather than partial, ordering of Mg and Si into two distinct octahedral sites, whereas Fe3+ 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 (VISi + Fe)—O bond lengths are 2.075 and 1.850 Å, respectively.