The dissolution mechanism of CO2 in aluminosilicate melts - infrared spectroscopic constraints on the cationic environment of dissolved [CO3]2-
Taylor, Wayne R.
European Journal of Mineralogy Volume 2 Number 5 (1990), p. 547 - 564
published: Oct 4, 1990
manuscript accepted: Apr 9, 1990
manuscript received: Aug 10, 1989
ArtNo. ESP147050205002, Price: 29.00 €
Abstract To clarify the cationic environment of dissolved carbonate in high pressure aluminosilicate melts, mid-IR and far-IR spectra were recorded on [CO3]-bearing nepheline (Ne) and sodamelilite (Sm) glasses. Far-IR difference spectra reveal a number of distinct absorption bands below 380 cm-1. These bands can be assigned to essentially non-lattice-like vibrational modes of CaOn and NaOn polyhedra associated with dissolved carbonate anions. In the mid-IR region, the v3 C-O antisymmetric stretch band of dissolved carbonate is split by 211 cm- 1 in [CO3]-bearing Ne and 83 cm- 1 in [CO3]-bearing Sm. Comparable splittings in the chemically analogous carbonate minerals shortite and dawsonite, indicate that this effect is due to departure of the carbonate anion from D3h molecular and site symmetry as a result of the local cation coordination geometry about [CO3]. In both [CO3]-bearing Sm and Ne glasses there is evidence for an additional, though less abundant, carbonate site. The mid-IR difference spectra of Ne and Sm [CO3]-bearing glasses contain a strong band at≈560 cm- 1 assignable to Al in six-fold coordination. For [CO3]-bearing Sm glass the far-IR and mid-IR spectra are consistent with both Na and Ca coordination to carbonate while for [CO3]-bearing Ne, Na and probably AlVI are associated with the carbonate site. The cationic environment of dissolved carbonate cannot, therefore, be interpreted in terms of the presence of discrete complex-ions or ion-pairs. On this basis it is proposed that the role of cation complexation is less important than the nature of the aluminosilicate network in the mechanism of CO2 dissolution. Relative solubilities of CO2 in aluminosilicate and silicate melts can be rationalized if CO2 is reactive towards Si-O-Al linkages and favourable Si-O-Si linkages (i.e. those in which the Si atoms have one, two or three attached bridging oxygens) rather than non-bridging oxygen atoms. In aluminosilicate melts, generation of AlVI and [CO3]2- is interpreted to be more a consequence of abstraction of bridging oxygen from Si-O-Al linkages by CO2 than removal, by carbonate complexation, of cations that normally charge compensate AlIV in the network.