Effects of Fe (super 2+) and Fe (super 3+) contents on cation ordering in omphacite

An EPMA, TEM and single-crystal structure-refinement study of omphacites and aegirine-augites with the aegirine mole fraction (X (sub Ae) ) in the range 0-33 % (X (sub Ae + Hd) 15-47 %) is reported. The degree of order 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 X (sub Ae + Hd) (X (sub Ae) being the most important component) and a semi-quantitative relationship between X (sub Ae + Hd) and the degree of order is given. Analyses on thin sections shows that compositional zoning with decreasing X (sub Ae) corresponds with increasing degree of order. Structure refinements after annealing at various T on crystals with similar Na contents but increasing X (sub Ae + Hd) indicate that the latter affects the kinetics of the process but not the critical T of order. The Fe (super 3+) Al (sub -1) and Fe (super 2+) Mg (sub -1) substitutions affect the degree of order in omphacite because, with increasing Fe contents, the two independent octahedra in the P2/n space group becomes more and more similar due to the decreasing difference in the ionic radii involved. Thus the space group is C2/c for X (sub Ae) > 20 % and X (sub Ae + Hd) = 47 %, but is P2/n in omphacite with the same X (sub Ae) but higher X (sub Ae + Hd) . A Delta G (super *) value for anti-phase domain coarsening in Fe-rich omphacite has been derived from TEM data and T-T-t diagrams; it is approximately 20 kcal/mole higher than that proposed for Fe-free omphacite.