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

• Published in: Zeitschrift für Kristallographie. Crystalline materials Vol. 217; no. 2; pp. 63 - 72
• Main Authors: Redhammer, G., Roth, G.
• Format: Journal Article
• Language: English
• Published: De Gruyter Oldenbourg 01.02.2002
• ISSN: 2194-4946; 2196-7105
• DOI: 10.1524/zkri.217.2.63.20631

The structural variations within the aegirine (NaFeSi ) – Li–aegirine (LiFeSi ) 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 2/ . Replacing Na by Li in aegirine causes a distinct decrease of the lattice parameter by 1.54% and an increase of the monoclinic angle by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2–fold in aegirine to 6–fold in pure LiFeSi , 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 – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na Li FeSi with Li concentrations > 0.75 crystallize in the space group . For the compounds with < 0.75, no phase transition from 2/ → was observed down to low temperatures (30 K). A – phase diagram for the stability of the low temperature structure is provided. The characteristica of the low temperature form of the (Na,Li)FeSi compounds agree with those of pure LiFeSi , recently described by Redhammer et al. (2001).