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1 results

  • Mineral chemistry of a peralkaline combeite-lamprophyllite nephelinite from Oldoinyo Lengai, Tanzania

  • J. B. Dawson, P. G. Hill
  • Journal:
    Mineralogical Magazine / Volume 62 / Issue 2 / April 1998
    Published online by Cambridge University Press:
    05 July 2018, pp. 179-196

  • A peralkaline nephelinite lava ([Na+K]/Al 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to alumina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.