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

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 aegitine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle beta by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2-fold in aegirine to 6-fold in pure LiFeSi2O6, the (M2-O) bond length decreases b 7.1% with increasingy 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-aegirinc (O3-O3-O3 bridging L angle = 174.2(l)degrees and 180.8(l)degrees 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 - MI octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Nal-xLixFeSi2O6 with Li+ concentrations x > 0.75 crystallize in the space group P2(1)/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 P2(1)/c low temperature form of the (Na,Li)FeSi2O6 compounds agree with those of pure LiFeSi2O6 recently described by Red-hammer et al. (2001).