Nyerereite from Calcite Carbonatite at the Kerimasi Volcano, Northern Tanzania

作者：Zaitsev A N^*

来源：Geology of Ore Deposits, 2010, 52(7): 630-640.

DOI：10.1134/S1075701510070159

The extinct Quaternary Kerimasi volcano located in the southern part of the Gregory Rift, northern Tanzania, contains both intrusive and extrusive calciocarbonatites. One carbonate mineral with a high content of Na and Ca has been found in a sample of volcanic carbonatite, which is probably a cumulate rock. On the basis of Raman spectroscopy and SEM/EDS, this mineral was identified as nyerereite, ideally Na(2)Ca(CO(3))(2). It occurs as solid inclusions up to 300 x 200 mu m in size in magnetite and contains (wt. %) 25.4-27.4 Na(2)O, 26.0-26.8 CaO, 1.6-1.9 K(2)O, 0.6-1.8 FeO, 0.3-0.6 SrO, <0.4 BaO, 1.4-2.3 SO(3), and 0.6-0.9 P(2)O(5). The average mineral formula is (Na(1.84)K(0.08))(Sigma 1.92)(Ca(1.00)Fe(0.03)Sr(0.01))(Sigma 1.04)[(CO(3))(1.91) (SO(4))(0.05)(PO(4))(0.02)](Sigma 1.98). A few inclusions in magnetite also contain calcite, which is considered here to be a late-stage, subsolidus mineral. The occurrence of nyerereite in carbonatite supports Hay's (1983) idea that some of the extrusive carbonatites at the Kerimasi volcano were originally alkaline rich and contained both calcite and nyerereite as primary minerals. DOI: 10.1134/S1075701510070159

• 出版日期2010-12