Frontiers in Earth Science (Oct 2018)
Crystal Chemistry of Three Volcanic K-rich Nepheline Samples From Oldoinyo Lengai, Tanzania and Mount Nyiragongo, Eastern Congo, Africa
Abstract
Natural nepheline (Nph) samples are intermediate members of solid solutions that extend from the pure Na end-member Na[AlSiO4] to the pure K end-member K[AlSiO4] (=kalsilite, Kls). We examined two volcanic nepheline (ideally, K2Na6[Al8Si8O32] or Nph0.75Kls0.25) samples from Oldoinyo Lengai, Tanzania (samples 1 and 2) and a third sample 3 from Mount Nyiragongo, Eastern Congo. The crystal structure of these three samples was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements in space group P63. Satellite reflections were not observed in these samples. The EPMA composition of sample 1 is {K1.50Na0.19Ca0.07☐0.23}Σ2(Na)6[Si8.35Al7.48Fe0.18]Σ16.00O32. The A site contains 0.23 vacancies (☐) with some Na and Ca atoms, whereas the B site is completely filled with only Na atoms. From the EPMA formula, K(sof) = 0.844 and Na(sof) = 1.0. The refinement unit-cell parameters are a = 10.00247(1), c = 8.38396(1) Å, V = 726.431(1) Å3; K(sof) = 0.779(2) and Na(sof) = 0.988(2). The composition of sample 2 is {K1.67☐0.33}Σ2(Na5.76K0.174Ca0.06)Σ6[Si8.44Al7.37Fe0.20]Σ16.01O32, so K(sof) = 0.835 and Na(sof) = 1.030. Sample 2 has a = 10.01769(2), c = 8.39586(2) Å, V = 729.678(1) Å3, K(sof) = 0.812(2), and Na(sof) = 0.995(2). Sample 3 is a three-phase (3a, 3b, 3c) intergrowth with an average formula of {K1.77☐0.23}2(Na4.30K1.51Ca0.19)Σ6[Si8.13Al7.76Fe0.11]Σ16.00O32, K(sof) = 0.887, and Na(sof) = 1.208. A large amount of K and Ca atoms occur in the B site. Based on the weight percentages and sofs of the three phases, the Rietveld refinement gives an average K(sof) = 0.859 and Na(sof) = 1.145. The average <T-O> distances indicate a high degree of Al-Si disorder in samples 1, 2, and phase 3b. These African samples extend natural nepheline solid solutions to about Nph0.55Kls0.45. The average <Na-O> distance increases linearly with Na(sof) and V across nepheline solid solutions because K atoms enter the Na site. K-rich nepheline seems to always contain vacancies in the A site.
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