Control of alkali-metal oxide activity in molten silicates

Na2O solubility in CaO–MgO–SiO2 melts

“…This method (Khedim et al, , 2009Mathieu et al, 2008;Mathieu, 2009) consists of imposing a specific Nametal vapor pressure in a closed system (sealed silica tube, see below) by controlled evaporation of a Na 2 O-xSiO 2 melt reservoir (source), where x refers to mol% of SiO 2 :…”

“…At equilibrium, and for the same temperature, the sodiummetal oxide activity (O'Neill, 2005;Mathieu et al, 2008;Mathieu, 2009) in molten samples is fixed by the source, i.e. :…”

“…They encompass a large range of compositions (0 < CaO and MgO < 40; 40 < SiO 2 < 100; in wt%) with the degree of polymerization as expressed by the number of non-bridging to tetrahedral oxygens (NBO/T), ranging from 0 to 3. Because large portions of the CMS diagram have liquidus temperatures exceeding 1400°C, Na 2 O-enriched CMS compositions (Mathieu et al, 2008;Mathieu, 2009) were also used as starting compositions (Table 1).…”

“…To determine the Na 2 O equilibrium values, the reactor has been tested at 1400°C using a CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) glass composition and a Na 2 O-2SiO 2 melt for the Na source (see also Mathieu et al, 2008). The selected CMAS composition (Table 1) is the 1 atm anorthite-diopside eutectic composition (EDiAn), frequently used in the literature as a proxy of basaltic magmas (see also Libourel et al, 1989;O'Neill, 2005;Borisov, 2008Borisov, , 2009.…”

“…In order to document the behavior of alkali elements in more complex liquids, we developed a new device to control sodium-metal oxide activity (aNa 2 O) by equilibrating the melts with gaseous environment of known Na partial pressure (PNa) at high temperature and fixed oxygen fugacity (fO 2 ) (Khedim et al, , 2009Mathieu et al, 2008;Mathieu, 2009). In this paper, we establish an internally consistent experimental database of Na 2 O solubility and sodium-metal oxide activity coefficient (cNa 2 O) for a large range of CaO-MgO-SiO 2 (CMS) + Na 2 O silicate melts at a magmatic temperature of 1400°C, and show that these new results help to understand how melt composition and melt structure control the solubility of sodium in molten silicates.…”