Effects of Fe2+ and Fe3+ contents on cation ordering in omphacite

Omphacites and aegirine-augites with a mole fraction of aegirine (X-Ae) in the range 0-33 % (XAe+Hd = 15-47 %) have been studied by electron-microprobe analysis, single-crystal X-ray structure-refinement and transmission electron-microscopy. The degree of order, as calculated by standard methods either from geometries and occupancies of the M1,M11 and M2,M21 couples of sites or from the I-060/I-050 intensity ratio, is lower than expected on the basis of both the Na contents and the T estimates from inter-crystalline exchange geothermometry. The difference is inversely proportional to XAe+Hd (X-Ae being the most important component) and a semi-quantitative relationship between XAe+Hd and the degree of order is given. TEM-AEM analysis on thin sections shows that compositional zoning with decreasing X-Ae corresponds to increasing degree of order (i.e., stronger h + k = odd reflections and size of anti-phase domains increasing from 380 to 1400 Angstrom). Anti-phase domains cannot be observed at X-Ae higher than 20 %. Defects and other microstructures are rare and precede formation and growth of the anti-phase domains; exsolution processes have not been observed. Structure refinements after annealing at different temperatures on crystals with similar Na contents but increasing XAe+Hd indicate that the latter affects the kinetics of the process but not the critical T of order. The Fe3+Al-1 and Fe2+Mg-1 substitutions affect the degree of order in omphacite because, with increasing iron contents, thr two independent octahedra in the P2/n space-group become more and more similar due to the decreasing difference in the ionic radii involved; ordering is therefore less and less favoured from structural and energetic points of view. Thus, the space group is C2/c for X-Ae > 20 % and XAe+Hd = 47 % (as in this work), but it is P2/n in omphacite with the same X-Ae but higher X-Hd. A Delta G* value for anti-phase domain coarsening in Fe-rich omphacite has been derived from TEM data and T-T-t diagrams; it is about 20 kcal/mole higher than that proposed for Fe-free omphacite.