Structural variations in the aegirine solid-solution series (Na,Li)FeSi₂O₆ at 298 K and 80 K

Abstract

The structural variations within the aegirine (NaFeSi₂O₆) – Li–aegirine (LiFeSi₂O₆) solid-solution series have been determined from structure refinements of single–crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K. At 298 K, all compounds have space group C2/c. Replacing Na⁺ by Li⁺ in aegirine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle β by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2–fold in aegirine to 6–fold in pure LiFeSi₂O₆, the bond length decreases by 7.1% with increasing replacement of Na⁺ by Li⁺. The decrease of the M2–oxygen coordination number correlates with a straightening of the tetrahedral chains which are slightly kinked in aegirine and fully extended in Li–aegirine (O3-O3-O3 bridging angle = 174.2(1)° and 180.8(1)° respectively). Small variations of individual and mean Si-O bond lengths and distinct changes in O-Si-O bond angles were also found. Within the Fe³⁺O₆ – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na_1-xLi_xFeSi₂O₆ with Li⁺ concentrations x > 0.75 crystallize in the space group P2₁/c. For the compounds with x < 0.75, no phase transition from C2/c → P2₁/c was observed down to low temperatures (30 K). A x – T phase diagram for the stability of the low temperature P2₁/c structure is provided. The characteristica of the P2₁/c low temperature form of the (Na,Li)FeSi₂O₆ compounds agree with those of pure LiFeSi₂O₆, recently described by Redhammer et al. (2001).