In situ ambient and high‐temperature Raman spectroscopic studies of nyerereite (Na,K)₂Ca(CO₃)₂: can hexagonal zemkorite be stable at earth‐surface conditions?

Abstract

In this study we present Raman spectroscopy data obtained from a naturally occurring orthorhombic nyerereite (Na,K)2Ca(CO3)2 from the Oldoinyo Lengai volcano (Tanzania) at ambient and high temperature. Nyerereite was heated up to 300 °C, 400 °C and 500 °C, and is believed to have been transformed at high temperatures to hexagonal β‐ and γ‐phases similar to zemkorite, (Na,K)2Ca(CO3)2. There are no major differences in the chemical composition of natural nyerereite and zemkorite. However, Raman spectra reveal significant differences between the nyerereite at ambient temperature and the phases obtained in the course of all our high‐temperature experiments. In situ measurements show that none of the high‐temperature hexagonal phases are quenchable, even at very high cooling rates (up to 50–80 °C/min), and they transform to nyerereite at ambient temperature. Contrary to previous studies even the presence of high K2O (up to 7.5 wt.%) did not stabilize the hexagonal structure during cooling. The cooling rates in our experiments were much higher than those at natural magmatic or hydrothermal systems; therefore hexagonal zemkorite is not likely to survive within natural systems as it transforms to nyerereite at ambient conditions. Thus, we do not expect to find zemkorite in any magmatic, metasomatic or hydrothermal rocks exposed at the Earth's surface. Copyright © 2015 John Wiley & Sons, Ltd.

Raman spectra of nyerereite, gregoryite, and synthetic pure Na2Ca(CO3)2: diversity and application for the study micro inclusions

“…The compositions of the nyerereite studied are almost identical to those published previously for the mineral from the natrocarbonatites of the Oldoinyo Lengai volcano . The nyerereite contains elevated amounts of K, Sr, and Ba (up to 0.5 apfu) compared with the synthetic Na 2 Ca(CO 3 ) 2 phase (Tables and S2).…”

“…The Raman spectrum of nyerereite type I (Figs (a) and (a)) was initially found in natrocarbonatites from Oldoinyo Lengai volcano . Later also identified in melt inclusions in minerals .…”

“…Nyerereite (Na,K) 2 Ca(CO 3 ) 2 and gregoryite (Na,K,Ca x ) 2‐x (CO 3 ) are rare, water‐soluble, alkali‐bearing carbonates. They only occur as rock‐forming minerals in the unaltered natrocarbonatite of the Oldoinyo Lengai volcano in northern Tanzania . There is no other active volcano in the world that erupts natrocarbonate lava.…”

“…It is stable at temperatures up to 290 °C at ambient pressure. Natural nyerereite always accommodates partial substitutions of Na by K, Sr and Ba for Ca, and (CO 3 ) by (SO 4 ), (PO 4 ), F and Cl . Its empirical formula is (Na,K) 2 Ca(CO 3 ) 2 , but nyerereite from different localities, and even from the Oldoinyo Lengai natrocarbonatites, may differ significantly in chemistry .…”

High‐pressure Raman study of nyerereite from Oldoinyo Lengai

“…Figure shows an ambient‐pressure Raman spectrum of nyerereite, which is typical of the locality . As its main feature, it contains an intense 1077 and 1086 cm −1 ν 1 (CO 3 2− ) doublet (its lower‐wavenumber and higher‐wavenumber components are marked hereafter by L and H subscripts, respectively).…”