Single-crystal polarized ftir spectroscopy and neutron diffraction refinement of cancrinite

We relate here a combined single-crystal polarized-light FTIR (Fourier transform infrared) and neutron diffraction study of two natural cancrinites from Cameroun and Canada, respectively. Electron microprobe analyses show both samples to be almost end-member carbonate-cancrinites [ideal chemical formula Na6Ca2(Si6Al6O24)(CO3)2•2H2O]. The structural refinements show that the extra-framework content in the large 12-membered rings channels is represented by one independent Na-site (Na2) and two independent, and statistically distributed, CO3 groups. The geometry of the CO3 groups appears to be almost regular, with C1-OC1 ~ 1.298(7) Å and C2-OC2 ~ 1.300(5) Å, in agreement with the previous studies [1,2,3]. The atoms of the carbonate-groups are not perfectly coplanar, being z(C1) ≠ z(OC1) and z(C2) ≠ z(OC2). The H2O molecules and a further Na-site (Na1) lie in the cancrinite-cage; the H2O oxygen site (OW) lies off from the triad axis, giving rise to a statistical configuration with three equivalent and mutually exclusive water molecules, as already suggested by [1,2]. The high-quality neutron data show that the water molecule is almost symmetric, with very similar Ow-H1 and Ow-H2 bond-distances, and is slightly tilted from the (0001) plane. It is involved in bifurcated hydrogen bridges, with two possible bonds for H1 (i.e. OW-H1•••O2 and OW-H1•••O4) and two for H2 (i.e. OW-H2•••O3 and OW-H2•••O2). The Ow•••O donor-acceptor distances are all > 2.7 Å. The polarised-light FTIR spectra show two main absorptions, at 3602 and 3531 cm-1, respectively. The former is polarised for E  c, while the latter is polarized for E  c. On the basis of the neutron diffraction data, the 3602 cm-1 band is assigned to the anti-symmetric stretching mode (ν3), while the 3531 cm-1 band is assigned to the symmetric stretching mode (ν1) of the same water molecule, in agreement with the presence of a single bending mode at 1630 cm-1. One additional weak component at 4108 cm-1 could possibly indicate the presence of low amounts of additional OH groups in the structure of cancrinite. Several overlapping bands in the 1300-1500 cm-1 range are strongly polarized for E  c, and are assigned to the vibrations of the CO3 group. References. [1] H.D. Grundy, I. Hassan, Can. Mineral., 20, 239-251, 1982; [2] P. Ballirano, A. Maras, Eur. J. Mineral., 16, 135-141, 2004; [3] I. Hassan, S.M. Antao, J.B. Parise, Am. Mineral., 91, 1117-1124, 2006.