Titre du document / Document title

CHEMICAL COMPOSITION OF NYEREREITE AND GREGORYITE FROM NATROCARBONATITES OF OLDOINYO LENGAI VOLCANO, TANZANIA

Auteur(s) / Author(s)

ZAITSEV A. N. ; KELLER J. ; SPRATT J. ; JEFFRIES T. E. ; SHARYGIN V. V. ;

Résumé / Abstract

Alkali carbonates nycrereite, 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 microphenocrystals in groundmass consisting of fluorite and sylvite; nyerereite typically forms prismatic crystals and gregoryite occurs as rounded, oval crystals. Both minerals are characterized by relatively high contents of various minor elements. Raman spectroscopy data indicate the presence of sulphur 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 amount 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 both in nyerereite and gregoryite. Laser ablation ICP-MS analyses show that rare earth elements, Mn, Mg, Rb and Li are typical trace elements in these minerals. Nyerereite is enriched in rare carth elements (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), compared with nycrercitc.

Revue / Journal Title

Zapiski Vserossijskogo mineralogičeskogo obŝestva   ISSN 0869-6055 

Source / Source

2008, vol. 137, n^o4, pp. 101-111 [11 page(s) (article)]

Langue / Language

Russe

Editeur / Publisher

Nauka, Sankt-Peterburg, RUSSIE, FEDERATION DE  (1992) (Revue)

Localisation / Location

INIST-CNRS, Cote INIST : 4673, 35400018684663.0070