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

• Published in: Geology of ore deposits Vol. 51; no. 7; pp. 608 - 616
• Main Authors: Zaitsev, A. N., Keller, J., Spratt, J., Jeffries, T. E., Sharygin, V. V.
• Format: Journal Article
• Language: English
• Published: Dordrecht SP MAIK Nauka/Interperiodica 01.12.2009
• Subjects: Earth and Environmental Science; Earth Sciences; Mineral Resources; Minerals and Parageneses of Minerals
• ISSN: 1075-7015; 1555-6476
• DOI: 10.1134/S1075701509070095

Alkali carbonates nyerereite, ideally Na 2 Ca(CO 3 ) 2 and gregoryite, ideally Na 2 CO 3 , 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 (SO 4 ) 2− and (PO 4 ) 3− groups. Microprobe analyses show variable composition of both nyerereite and gregoryite. Nyerereite contains 6.1–8.7 wt % K 2 O, with subordinate amounts of SrO (1.7–3.3 wt %), BaO (0.3–1.6 wt %), SO 3 (0.8–1.5 wt %), P 2 O 5 (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 % SO 3 , 1.3–4.6 wt % P 2 O 5 , 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.