Crystal chemistry of strontian soda melilite from nephelinite lava of Mt. Etinde, Cameroon

The crystal structure and chemical composition of two crystals (labelled ET 1 and ET2) of strontian soda melilite from Mt. Etinde, Cameroon , are inves tigated. Quantitative analysis leads to the following formulae: (Ca0.84Sr0. 45Na0.71)(Mg0.19Fe0.062+Mn0.03Zn0.02Al0.46Fe0.233+Ti0.01)(Si1.99Al0.01)O-7. 00 and (Ca0.83Sr0.45Ni0.72) (Mg0.20Fe0.062+Mn0.03Zn0.01Al0.45Fe0.243+Ti0.02 )(Si1.99Al0.01)O-7.00 for ET1 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) Angstr om for ET2. The structure is refined up to R-all = 2.64 and 2.78%, respecti vely, using 430 independent reflections. Substitution of Sr for Ca causes s ufficient modification of the tetrahedral laver to destabilize the incommen surate modulated structure.