Minerals and Parageneses of Minerals

Geology of Ore Deposits

, Volume 51, Issue 7, pp 608-616

Chemical composition of nyerereite and gregoryite from natrocarbonatites of Oldoinyo Lengai volcano, Tanzania

  • A. N. ZaitsevAffiliated withDepartment of Mineralogy, St. Petersburg State University Email author 
  • , J. KellerAffiliated withMineralogisch-Geochemisches Institut, Universitat Freiburg
  • , J. SprattAffiliated withDepartment of Mineralogy, The Natural History Museum
  • , T. E. JeffriesAffiliated withDepartment of Mineralogy, The Natural History Museum
  • , V. V. SharyginAffiliated withInstitute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences

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Abstract

Alkali carbonates nyerereite, ideally Na2Ca(CO3)2 and gregoryite, ideally Na2CO3, are the major minerals in natrocarbonatite lavas from Oldoinyo Lengai volcano, northern Tanzania. They occur as pheno- and microphenocrysts in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite occurs as round, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman spectroscopy data indicate the presence of sulfur and phosphorous as (SO4)2− and (PO4)3− groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt % K2O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO3 (0.8–1.5 wt %), P2O5 (0.2–0.8 wt %) and Cl (0.1–0.35 wt %). Gregoryite contains 5.0–11.9 wt % CaO, 3.4–5.8 wt % SO3, 1.3–4.6 wt % P2O5, 0.6–1.0 wt % SrO, 0.1–0.6 wt % BaO and 0.3–0.7 wt % Cl. The content of F is below detection limits in nyerereite and gregoryite. Laser ablation ICP-MS analyses show that REE, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is enriched in REE (up to 1080 ppm) and Rb (up to 140 ppm), while gregoryite contains more Mg (up to 367 ppm) and Li (up to 241 ppm) as compared with nyerereite.