Abstract

The crystal structure and chemical composition of two crystals (labelled ET1 and ET2) of strontian soda melilite from Mt. Etinde, Cameroon, are investigated. Quantitative analysis leads to the following formulae: (Ca0.84Sr0.45Na0.71)(Mg0.19Fe2+0.06Mn0.03Zn0.02Al0.46Fe3+0.23Ti0.01)(Si1.99Al0.01)O7.00and (Ca0.83Sr0.45Na0.72) (Mg0.20Fe2+0.06Mn0.01Zn0.01Al0.45Fe3+0.24Ti0.02)(Si1.99Al0.01)O7.00, for ETl and ET2 respectively. Lattice parameters are: a = 7.754(1), c = 5.101(1) A for ET1 and a = 7.764(1), c = 5.095(1) Å for ET2. The structure is refined up to Rall = 2.64 and 2.78 %, respectively, using 430 independent reflections. Substitution of Sr for Ca causes sufficient modification of the tetrahedral layer to destabilize the incommensurate modulated structure.

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