The influence of bulk composition on the diffusivity of carbon dioxide in Na aluminosilicate melts

Abstract

The bulk diffusivity of dissolved carbon dioxide (CO 2 and CO 3 2-) in NaAlSi 3 O 8 + nNa 2 O (n = 0-6.87 wt%) and in NaAlSi 3 O 8 + nH 2 O (n = 0-2 wt%) melts was investigated at 1523 K and 0.5 GPa using the diffusion couple technique. CO 2 contents of the starting glass pairs varied between 0 and 0.2 wt%. Symmetrical concentration-distance profiles of bulk CO 2 were determined by infrared spectroscopy. An error function was fitted to the profiles to obtain apparent chemical diffusion coefficients of bulk CO 2 . In the investigated compositional range, the diffusivity of bulk CO 2 increases exponentially with Na 2 O and H 2 O content and thus exponentially with the ratio of nonbridging oxygen atoms per tetrahedral cations (NBO/ T). The bulk CO 2 diffusivity increases from logD CO 2 = -11.38 (D CO 2 in m 2 /s) in NaAlSi 3 O 8 melt to logD CO 2 = -10.92 in NaAlSi 3 O 8 melts containing 6.87 wt% Na 2 O excess, and to logD CO 2 = -10.91 in NaAlSi 3 O 8 melts containing 2 wt% H 2 O. These data imply that either: (1) the diffusivities of the CO 2 species (molecular CO 2 and CO 3 2-) are very similar, or (2) the speciation of CO 2 in the quenched glasses is very different from the speciation in the melt.

Argon and CO2 on the race track in silicate melts: A tool for the development of a CO2 speciation and diffusion model

“…Sierralta et al (2002) have recently demonstrated that in contrast to Pt capsule material, CO 2 -bearing glass cylinders synthesized in Au80Pd20 capsules show no CO 2 loss and zoning of CO 2 concentration towards the capsule walls. Silver oxalate (Ag 2 C 2 O 4 ) equivalent to 0.2 wt% CO 2 was used as the CO 2 source.…”

“…Silver oxalate (Ag 2 C 2 O 4 ) equivalent to 0.2 wt% CO 2 was used as the CO 2 source. Details for capsule preparation are given in Sierralta et al (2002) and Nowak et al (2003). The syntheses were run at 1623 K and 500 MPa in the IHPV at intrinsic oxidising conditions equivalent to the MnOMn 3 O 4 solid oxygen buffer (logfO 2 ϭ QFM ϩ 4) (Berndt et al, 2002).…”

“…The diffusion couple technique using a cylindrical geometry described by Nowak and Behrens (1997) and Sierralta et al (2002) was used to obtain one-dimensional semi-infinite medium diffusion profiles of Ar and total CO 2 . Noble metal jacketed volatile bearing glass cylinders were brought into contact with volatile free glass cylinders of the same glass composition and welded shut in 4 mm Pt capsules for Ar diffusion and in 4 mm Au80Pd20 capsules for CO 2 diffusion.…”

“…Recently, Sierralta et al (2002) have suggested that in the system NaAlSi 3 O 8 ϩ nNa 2 O (n ϭ 0 -6.9 wt%) at 1523 K and 500 MPa the diffusivity of bulk CO 2 increases exponentially with Na 2 O and H 2 O content and thus exponentially with NBO/T. The bulk CO 2 diffusivity increases from logD CO 2,total ϭ Ϫ11.38 (D in m 2 s Ϫ1 ) in NaAlSi 3 O 8 melt with NBO/T close to zero to logD CO 2,total ϭ Ϫ10.92 in NaAlSi 3 O 8 melt containing 6.9 wt% Na 2 O excess with NBO/T of 0.156.…”

“…Spectroscopic measurements of CO 2 -bearing aluminosilicate glasses have shown that CO 2 is present as molecular CO 2 and CO 3 2Ϫ groups linked to the silicate network (Mysen and Virgo, 1980;Fine and Stolper, 1985;Kohn et al, 1991;Brooker et al, 1999Brooker et al, , 2001aSierralta et al, 2002;Nowak et al, 2003), although more independent and mobile 'ionic carbonate pairs' may also exist in highly peralkaline compositions (Brooker, 2001b). The ratio of the CO 2 species concentrations depends strongly on bulk composition of the glass.…”