• A. V. Golovin
  • A.~V. Korsakov
  • P. N. Gavryushkin
  • A. N. Zaitsev
  • V. G. Thomas
  • B. N. Moine
Raman spectra of nyerereite, gregoryite, and synthetic pure Na2 Ca(CO3)2 : diversity and application for the study micro inclusions
In this study, we present Raman spectra of pure synthetic hydrothermal orthorhombic Na2Ca(CO3)2 carbonate together with naturally occurring orthorhombic nyerereite $(Na,K)_2Ca(CO_3)_2$ and hexagonal gregoryite $(Na,K,Ca_x)_{2-x}(CO_3)$ from Oldoinyo Lengai natrocarbonatites (Tanzania) to showthe diversity of spectra and permit the use of Raman spectroscopy to identify theseminerals in small inclusions. Synthetic orthorhombic Na2Ca(CO3)2 carbonate has only one type of Raman spectrum, which is independent of the crystallographic orientation. The Raman spectra of nyerereite can be divided into three types, depending on intensity of symmetric stretching ν1$(CO_3)^{2-}$ vibrations in region 1070–1090 $cm^{-1}$. All these nyerereite types have identical chemical compositions; thus, the differences in Raman spectra are likely to be related to crystallographic orientations. Some nyerereite Raman spectra can be erroneously interpreted as gregoryite because of similarity in peak position of strong ν1$(CO_3)^{2-}$ band. However, the diagnostic features of the gregoryite Raman spectrum can be summarized as follows: (1) presence of a symmetric peak attributed to ν1$(CO_3)^{2-}$ vibrations at 1078 $cm^{-1}$; (2) presence of one broad band ν4$(CO_3)^{2-}$ with a peak position at 702– 707 $cm^{-1}$; (3) a stronger band ν1$(SO_4)^{2-}$ that occurs at 1003–1006 $cm^{-1}$ and the presence of additional weaker bands ν2$(SO_4)^{2-}$ and ν4$(SO_4)^{2-}$ at 460–462 and 630–635 $cm^{-1}$; and (4) a ν1$(PO_4)^{3-}$ band that appears at 952–954 $cm^{-1}$. Incorporation of significant amounts of additional cations (up to 0.5 apfu) and particularly K (up to 0.4 apfu) in natural nyerereites leads to a change in the commensurately modulated structure of pure Na2Ca(CO3)2 $\it (P2$_1$ca)$, which becomes an incommensurately modulated structure (less ordered)with space group Cmcm; and these changes are reflected in a reduction of the number of Raman bands in symmetric stretching ν1$(CO_3)^{2-}$ and inplane bending ν4$(CO_3)^{2-}$ vibration regions or even the complete absence of the Raman bands in antisymmetric stretching ν3$(CO_3)^{2-}$ and 2*ν2$(CO_3)^{2-}$ vibration regions compared with pure $Na_2Ca(CO_3)_2$.
Raman spectroscopy, MIR, band position, minerals, carbonates, Nyerereite, Gregoryite, Na2Ca(CO3)2, (Na,K,Ca)CO3, inclusions, Earth Science
band list data, spectral data, earth sciences
Journal of Raman Spectroscopy
1559 - 1565
Document type
Publication state