AUTHOR=Antao Sytle M. , Nicholls James W. 

TITLE=Crystal Chemistry of Three Volcanic K-rich Nepheline Samples From Oldoinyo Lengai, Tanzania and Mount Nyiragongo, Eastern Congo, Africa

JOURNAL=Frontiers in Earth Science

VOLUME=6

YEAR=2018

URL=https://www.frontiersin.org/journals/earth-science/articles/10.3389/feart.2018.00155

DOI=10.3389/feart.2018.00155

ISSN=2296-6463

ABSTRACT=<p>Natural nepheline (Nph) samples are intermediate members of solid solutions that extend from the pure Na end-member Na[AlSiO<sub>4</sub>] to the pure K end-member K[AlSiO<sub>4</sub>] (=kalsilite, Kls). We examined two volcanic nepheline (ideally, K<sub>2</sub>Na<sub>6</sub>[Al<sub>8</sub>Si<sub>8</sub>O<sub>32</sub>] or Nph<sub>0.75</sub>Kls<sub>0.25</sub>) 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 <italic>P</italic>6<sub>3</sub>. Satellite reflections were not observed in these samples. The EPMA composition of sample 1 is {K<sub>1.50</sub>Na<sub>0.19</sub>Ca<sub>0.07</sub>☐<sub>0.23</sub>}<sub>Σ2</sub>(Na)<sub>6</sub>[Si<sub>8.35</sub>Al<sub>7.48</sub>Fe<sub>0.18</sub>]<sub>Σ16.00</sub>O<sub>32</sub>. 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(<italic>sof</italic>) = 0.844 and Na(<italic>sof</italic>) = 1.0. The refinement unit-cell parameters are <italic>a</italic> = 10.00247(1), <italic>c</italic> = 8.38396(1) Å, <italic>V</italic> = 726.431(1) Å<sup>3</sup>; K(<italic>sof</italic>) = 0.779(2) and Na(<italic>sof</italic>) = 0.988(2). The composition of sample 2 is {K<sub>1.67</sub>☐<sub>0.33</sub>}<sub>Σ2</sub>(Na<sub>5.76</sub>K<sub>0.174</sub>Ca<sub>0.06</sub>)<sub>Σ6</sub>[Si<sub>8.44</sub>Al<sub>7.37</sub>Fe<sub>0.20</sub>]<sub>Σ16.01</sub>O<sub>32</sub>, so K(<italic>sof</italic>) = 0.835 and Na(<italic>sof</italic>) = 1.030. Sample 2 has <italic>a</italic> = 10.01769(2), <italic>c</italic> = 8.39586(2) Å, <italic>V</italic> = 729.678(1) Å<sup>3</sup>, K(<italic>sof</italic>) = 0.812(2), and Na(<italic>sof</italic>) = 0.995(2). Sample 3 is a three-phase (3a, 3b, 3c) intergrowth with an average formula of {K<sub>1.77</sub>☐<sub>0.23</sub>}<sub>2</sub>(Na<sub>4.30</sub>K<sub>1.51</sub>Ca<sub>0.19</sub>)<sub>Σ6</sub>[Si<sub>8.13</sub>Al<sub>7.76</sub>Fe<sub>0.11</sub>]<sub>Σ16.00</sub>O<sub>32</sub>, K(<italic>sof</italic>) = 0.887, and Na(<italic>sof</italic>) = 1.208. A large amount of K and Ca atoms occur in the B site. Based on the weight percentages and <italic>sofs</italic> of the three phases, the Rietveld refinement gives an average K(<italic>sof</italic>) = 0.859 and Na(<italic>sof</italic>) = 1.145. The average &lt;<italic>T</italic>-O&gt; 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 Nph<sub>0.55</sub>Kls<sub>0.45</sub>. The average &lt;Na-O&gt; distance increases linearly with Na(<italic>sof</italic>) and <italic>V</italic> across nepheline solid solutions because K atoms enter the Na site. K-rich nepheline seems to always contain vacancies in the A site.</p>