Hydrogen and carbon in sodalite group-minerals: an FTIR study

The sodalite-group include: sodalite s.s. Na₈[Al₆Si₆O₂₄]Cl₂, noseane Na₈[Al₆Si₆O₂₄]SO₄•H₂O, haüyne, Na₆Ca₂[Al₆Si₆O₂₄](SO₄)₂, and lazurite Na₆Ca₂[[Al₆Si₆O₂₄]S₂. These minerals occur almost exclusively in alkali-rich, silica-deficient igneous rocks, and occasionally in metasomatized limestones. The structure of the sodalite-group minerals is cubic P 3n and is characterized by an ordered framework of AlO₄ and SiO₄ tetrahedra, with an Al:Si ratio = 1:1. These tetrahedra are linked such as to form six-membered rings stacked following an ABCABC... sequence, and are arranged such as to form continuous channels parallel to [111] that offer diffusion paths for intra-framework ions (Barrer and Vaughan, 1971). The overall linkage between the rings gives rise to cubo-octahedral “sodalite” cages, also known as $IMM_COM_0005 cages (Pauling, 1930); these may host a large variety of cations (e.g. Na⁺, Ca²⁺, K⁺), anions (e.g. OH^-, Cl^-, SO₄^2-, S^2-) and neutral molecules (H₂O), but also noble gases (e.g. Xe, Ar). The cage has a diameter of  4 Å hence sodalite-group minerals can be considered as microporous materials (Rouquerol et al. 1994). We relate here the recent developments of our micro-FTIR studies on a series of sodalite-group minerals from different localities and geological occurrences. Spectra were collected on doubly-polished slabs using a NicPlan microscope with a nitrogen-cooled MCT detector attached to a Nicolet Magna 760 spectrometer with a KBr beamsplitter and a Globar as IR source. Microspectrometric mappings were acquired with a Hyperion 3000 Bruker microscope equipped with a computer-controlled motorized stage. HT spectra in situ were collected using a Linkam FTIR600 heating stage (single-crystals); annealing experiment were done using an HT device. Single-crystal FTIR spectra show that the different species in the group, and different samples of the same species, have different degrees of hydration; H is present in various forms, including OH, H₂O, H₃O⁺ and H₃O₂^- groups. Carbon occurs as CO₂, CO₃^2- and possibly as HCO₃^-. Carbonate and hydrogencarbonate arrangements are detected in several haüyne and sodalites s.s. specimens, while CO₂ is observed, in decreasing amount, in noesane, lazurites and haüynes; CO₂ has been so far observed only in one sample of sodalites s.s. HT-FTIR measurements show, in all samples, a continuous and linear release of H₂O up to the structure collapse at ~ 900° C. On heating, the CO₂ absorption increasingly broadens, as already observed for beryl and codierite (Aines and Rossman 1984), and recovers its initial shape for decreasing T. However, release of CO₂ from the sample occurs only for T > 900°C. Microspectrometry mappings shows typically a non-homogeneous distributions of hydrogen and carbon across the samples. Using the molar absorption coefficients available in the literature for glasses with similar compositions, H contents in the range 400 to 800 ppm and CO₂ contents in the range 200 to 800 ppm are calculated. References Aines R.D., Rossman G.R. (1984) Am. Mineral., 69, 319-327. Barrer, R.M., Vaughan D.E.W. (1971) J. Phys. Chem. Solids 32, 731-743. Pauling, L. (1930) Z. Kristallogr., 74, 213-225. Rouquerolt T., Avnir D., Fairbridge C.W., Everett D.H., Haynes J.H., Pernicone N., Ramsay J.D.F., Sing K.S.W., Unger K.K. (1994) Pure and Appl. Chem., 66, 1739-1758.