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

¹ Dipartimento di Scienze della Terra, Università degli Studi di Firenze,Via la Pira, 4, I-50121 Firenze, Italy
² Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK

^* e-mail: lbindi{at}steno.geo.unifi.it

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: (Ca_0.84Sr_0.45Na_0.71)(Mg_0.19Fe²⁺_0.06Mn_0.03Zn_0.02Al_0.46Fe³⁺_0.23Ti_0.01)(Si_1.99Al_0.01)O_7.00and (Ca_0.83Sr_0.45Na_0.72) (Mg_0.20Fe²⁺_0.06Mn_0.01Zn_0.01Al_0.45Fe³⁺_0.24Ti_0.02)(Si_1.99Al_0.01)O_7.00, 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 R_all = 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.

Key-words: strontian soda melilite, crystal chemistry, single-crystal X-ray diffraction, Etinde, Cameroon.

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