Volatiles in cancrinite-sodalite group minerals

The minerals of the cancrinite-sodalite group are characterized by layers of six-membered rings of tetrahedra: each ring is linked to three rings in a preceding layer and to three rings in the succeeding one, such as to form a three dimensional framework (Bonaccorsi and Merlino 2005). Different stacking sequences give rise to different structures leading to cages, channels and cavities filled by extra-framework anions and cations. Common anions within the structural channels are Cl-, F-, SO42- and CO32-. Recent studies have however shown that carbon dioxide is also a common constituents of these minerals (Della Ventura et al. 2005, 2007a, 2007b). Other possible molecules are H3O+ or HCO3- groups (Gesing and Buhl 2000, Galitskii et al. 1978). IR spectroscopy allows the detection and possibly quantitative analysis of structural H-C-O species, and is thus particular suitable for characterising these minerals. We relate here the recent developments of our micro-FTIR and crystal-structure studies on a series of cancrinite-sodalite group minerals. Spectra were collected on well-characterized samples, mostly on oriented, doubly-polished slabs, with polarized radiation, using a NicPlan microscope equipped with a nitrogen-cooled MCT detector, a KBr beamsplitter and a ZnSe wire-grid IR polarizer. Microspectrometric mappings were acquired with a Hyperion 3000 Bruker microscope equipped with a computer-controlled motorized stage. HT spectra were collected using a Linkam FTIR600 heating stage (single-crystals) or a Specac HT/HP cell (powders). Single-crystal FTIR spectra show the common presence of CO2 in most samples, from a wide variety of geological provenance. In particular, systematically high amounts of CO2 are detected in franzinite, nosean and hauyine, while minor but significant amounts are found in vishnevite, marinellite, giuseppettite, vishnevite, davyne and sodalite. Polarized-light spectra collected on [001] sections of hexagonal cancrinite-group minerals show in all cases maximum absorption with E c, suggesting that the linear CO2 molecules are oriented perpendicular the crystallographic c axis of the mineral, like in beryl or cordierite (Aines and Rossman 1984). Combination of in situ and annealing high-T experiments shows that in the different species the carbon dioxide molecules are bound in different ways within the structure. In addition, release of CO2 occurs at significantly different temperatures due to the different connectivity of the structural pores. Detailed microspectrometry mappings shows non-homogeneous distributions of hydrogen and carbon across the samples, and suggest a possible use of these minerals as a tool for geothermometric modelling. The finding that most cancrinite-sodalite group minerals are able to trap carbon dioxide opens a new frontier in the design of materials having potential for carbon sequestration from the atmosphere. References Aines, R.D., Rossman, G.R. (1984) Am. Mineral., 69 319-327. Bonaccorsi E., Merlino S. (2005) In G. Ferraris and S. Merlino, eds., Micro- and Mesoporous Mineral Phases, p. 241-290. Reviews in Mineralogy and Geochemistry. Della Ventura G., Bellatreccia F., Bonaccorsi E. (2005) Eur. J. Mineral., 17, 847-851. Della Ventura G., Bellatreccia F., Parodi G.C., Cámara F., Piccinini M. (2007a) Am. Mineral. (in press). Della Ventura G., Bellatreccia F., Piccinini M. (2007b) Rend. Fis. Acc. Lincei (submitted). Galitskii, V.Yu., Grechushnikov, B.N., Sokolov, Yu.A. (1978) Russian J. Inorg. Chem., 23, 1749-1750. Gesing, M., Buhl, J.-Ch. (2000) Z. Kristallog., 215, 413-418.