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In situ ambient and high‐temperature Raman spectroscopic studies of nyerereite (Na,K) 2 Ca(CO 3 ) 2 : can hexagonal zemkorite be stable at earth‐surface conditions?
Author(s) -
Golovin A. V.,
Korsakov A. V.,
Zaitsev A. N.
Publication year - 2015
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/jrs.4756
Subject(s) - raman spectroscopy , hydrothermal circulation , orthorhombic crystal system , metasomatism , hexagonal crystal system , analytical chemistry (journal) , volcano , in situ , chemistry , chemical composition , mineralogy , materials science , crystallography , crystal structure , geology , geochemistry , environmental chemistry , physics , mantle (geology) , organic chemistry , seismology , optics
In this study we present Raman spectroscopy data obtained from a naturally occurring orthorhombic nyerereite (Na,K) 2 Ca(CO 3 ) 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) 2 Ca(CO 3 ) 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 K 2 O (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.