Raman spectra of nyerereite and gregoryite

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Marco E. Ciriotti
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Raman spectra of nyerereite and gregoryite

Messaggio da Marco E. Ciriotti » gio 13 apr, 2017 17:47

▪ Golovin, A.V., Korsakov, A.V., Gavryushkin, P.N., Zaitsev, A.N., Thomas, V.G., Moine, B.N. (2017): Raman spectra of nyerereite, gregoryite, and synthetic pure Na2Ca(CO3)2: diversity and application for the study micro inclusions. Journal of Raman Spectroscopy, 48, (in press).

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(CO3)2 and hexagonal gregoryite (Na,K,Cax)2-x(CO3) from Oldoinyo Lengai natrocarbonatites (Tanzania) to show the diversity of spectra and permit the use of Raman spectroscopy to identify these minerals 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(CO3)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(CO3)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(CO3)2− vibrations at 1078 cm−1; (2) presence of one broad band ν4(CO3)2− with a peak position at 702–707 cm−1; (3) a stronger band ν1(SO4)2− that occurs at 1003–1006 cm−1 and the presence of additional weaker bands ν2(SO4)2− and ν4(SO4)2− at 460–462 and 630–635 cm−1; and (4) a ν1(PO4)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 (P21ca), 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(CO3)2− and inplane bending ν4(CO3)2− vibration regions or even the complete absence of the Raman bands in antisymmetric stretching ν3(CO3)2− and 2*ν2(CO3)2− vibration regions compared with pure Na2Ca(CO3)2.
Marco E. Ciriotti

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