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The structural variations within the aegirine (NaFeSi 2 O 6 ) – Li–aegirine (LiFeSi 2 O 6 ) 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  C 2/ 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 LiFeSi 2 O 6 , 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 Fe 3+ O 6  – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na 1-   x  Li  x  FeSi 2 O 6  with Li +  concentrations  x  &gt; 0.75 crystallize in the space group  P 2 1 / c . For the compounds with  x  &lt; 0.75, no phase transition from  C 2/ c  →  P 2 1 / c  was observed down to low temperatures (30 K). A  x  –  T  phase diagram for the stability of the low temperature  P 2 1 / c  structure is provided. The characteristica of the  P 2 1 / c  low temperature form of the (Na,Li)FeSi 2 O 6  compounds agree with those of pure LiFeSi 2 O 6 , recently described by Redhammer et al. (2001).

Structural variations in the aegirine solid-solution series (Na,Li)FeSi 2 O 6 at 298 K and 80 K