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

Crystal chemistry of synthetic amphiboles along the richterite - ferro-richterite join

In this work we relate the synthesis and crystal-chemical study of amphiboles along the join richterite - ferrorichterite [nominally Na(NaCa)(Mg5-xFe2+ x)Si8O22(OH)2], at 700°C, 1 kbar, under redox conditions imposed by a MW solid buffer. XRPD shows essentially single-phase run products at both endmember compositions, with minor (10-15%) pyroxene and quartz for intermediate compositions. Interestingly, there is a significant evolution of the amphibole morphology across the solid-solution series: Mg end-member richterite is extremely acicular (length up to 15 μm, diameter up to 1 μm) whereas for increasing Fe in the system the amphibole becomes increasingly stubby, with a length:diameter ratio≈ 3:1 for the Fe -rich endmember. Notably, the refined cell parameters show linear variations across the series, suggesting a complete Mg-Fe2+ solid-solution along the join. The OH-stretching FTIR spectra show complex patterns for intermediate compositions with up to eight bands which can be assigned to the various configurations involving Mg/Fe2+ at M(1,3), locally associated with both full and empty A-sites. Combination of Mössbauer and FTIR spectroscopy however shows that the composition of the amphibole is much more complex than expected just from XRPD data: Fe2+ is in fact disordered among all M-sites, and, despite the strongly reducing conditions during syntheses, there is significant Fe3+ in the amphibole; it is fully ordered at M(2). For increasing Fe in the system, the amount of Fetot in the amphibole is systematically lower than expected as is Fe2+ at the M(1-3) octahedra, while M(2)Fe3+ increases toward more ironrich compositions (Fig. 1). These results definitively stress the need for a proper characterization of the experimental run products because these may depart from the expected stoichiometry in a significant way. Direct EMP or single-crystal X-ray refinement is highly desirable, however spectroscopic methods may provide a valuable alternative, particularly when the Fe3+ vs. Fe2+ composition is needed

Mössbauer research at University Roma Tre

The principal goal of the Rome group is studying crystalline materials, especially minerals: MS information (Fe oxidation state, Fe3+/Fetot ratios, Fe ions ordination number, Fe atoms aggregation state) is used together with stmchlral and chemical data collected on the same single-phase material to retrieve the topochemical model and to explain the physical properties. Outstanding materials shldied so far are: nahlral and synthetic spinels, natural borosilicates (tourmaline and axinite), natural amphiboles (especially fibres), natural and synthetic zeolites, but also obsidian, pumice, day and other heterogeneous materials

Oxidation or cation re-arrangement? Distinct behavior of riebeckite at high temperature

In this work we address the stability of riebeckite at high temperatures and compare the different behaviors observed under various oxidation conditions. For this purpose, we annealed powders of a sample from Mt. Malosa (Malawi), which is compositionally close to the end-member; the run products obtained after annealing in air vs. in vacuum were studied by Mossbauer spectroscopy and powder X-ray difraction. The results show that riebeckite follows two distinct paths depending on the external environment. Under oxidizing conditions, it is stable in the hydrous form up to relatively low temperatures (400-450 degrees C), then it undergoes a rapid (within similar to 50 degrees C) dehydrogenation, forming oxo-riebeckite, which is stable up to similar to 900 degrees C. The final breakdown products of the oxo-amphibole include aegirine + cristobalite + hematite. Based on the relative intensity of the (310) Bragg reflection, the activation energy (E-a) for the riebeckite to oxo-riebeckite transition is 166 +/- 6 kJ/mol.Under vacuum conditions, no Fe oxidation is observed, and riebeckite is stable up to much higher temperatures (750-800 degrees C); however, in the 550 < T < 700 degrees C range, it undergoes a significant rearrangement of the C cations (those hosted in the strip of octahedra). Indeed, the amphibole stable in the 700-800 degrees C range has the same chemical formula as riebeckite but has a disordered and non-standard cation distribution at the octahedra, i.e., (M(1))(Fe3+Fe2+)(M(2))(Fe3+Fe2+)Fe-M(3)(2+); we call this phase "R-C(3+) disordered riebeckite". For T >= 800 degrees C, it decomposes to aegirine + fayalite + cristobalite + H2O.External oxygen is required for the release of water into the surrounding system, being a prerequisite for the Fe-amphiboles to be a carrier of H2O in the lower crust and upper mantle. One important implication of our results is that characterization of the overall oxidation state of iron does not necessarily provide the redox conditions of the environment of formation because a crystal-chemical re-arrangement under reducing conditions allows riebeckite to maintain its Fe3+/Fe2+ composition up to higher temperatures