Determination of amorphous and nanocrystalline calcium carbonate polymorphs in pearls by Raman spectroscopy

WEHRMEISTER, URSULA; JACOB, DORRIT E.; SOLDATI, ANALIA L.; LOGES, NICOLAS; HÄGER, TOBIAS; HOFMEISTER, WOLFGANG

Budapest, Hungry

Congreso; IMA2010, 20th General Meeting of the International Mineralogical Association; 2010

International Mineralogical Association

The nacre of pearls and mussels is well known to display a“brick and mortar” – structure which is a challenging compositeorganic-inorganic model system for research in nanotechnology.FIB/TEM and AFM methods show that nacre presents ananostructure with the smallest structures in the range of 50 –100 nm [1]. The CaCO3 vesicles, extruded by the mantleepithelial tissue cells of the bivalve are primarily composed ofamorphous calcium carbonate (ACC) before being transformedinto the crystalline phase [2].Micro Raman spectroscopy is a powerful tool in identifyingthe different calcium carbonate polymorphs [3, 4]. Analyseswere carried out on cultured pearls from freshwater (genus Hyriopsis) and marine bivalve species (Pinctada maxima).Raman spectra for vaterite and ACC are discussed. Results forACC are compared to synthetically produced ACC and toRaman spectroscopic features of stable biogenic ACC from thecrustacean Porcellio scaber.ACC in the pearls is confined to areas close to the centre ofthe pearls [1]. Decomposition of the most intense signal of thecalcium carbonate polymorphs – the  1 symmetric stretchingmode of the carbonate ion, leads to the identification of twopolymorphs within the ACC areas: a mix of an amorphous and ananocrystalline fraction.Fig. 1: Raw Raman spectrum of ACC in a South Sea cultured pearl(Pinctada maxima is clearly to decompose into anamorphous (Gaussian line profile, centred at 1078cm-1) and ananocrystalline fraction (Lorentzian line profile, centred at 1085 cm-1).As a rule, the degree of mineral crystallinity is mirrored by theFWHM, the broader the spectral bandwidth, the lower thedegree of the mineral crystallinity. The amorphous phase in thesamples is characterised by a broad peak (FWHM of ca. 120cm-1) in the region of the lattice modes. While formonocrystalline and biogenic aragonite typically FWHMs of1.3 cm-1 and 2.2 cm-1 are measured, nanocrystalline aragonite inparagenesis with ACC shows an FWHM between 5 cm-1 and 10cm-1 of the  1 – Raman band. A shift to lower wavenumbers isclearly related to the peak broadening.[1] Jacob, D.E. et al. (2008) Geochim. Cosmochim. Ac., 72(22),5401-5415. [2] Addadi, L. et al. (2003) Adv. Mater., 15(12),959-969. [3] Urmos, J. et al. (1991) Am. Mineral., 76, 641-646.[3] Wehrmeister, U. et al. (2010) J. Raman Spectrosc., 41, 193-201.