Determination of water solubilities in high-viscosity melts: An experimental study on NaAlSi3O8 and KAlSi3O8 melts
European Journal of Mineralogy Volume 7 Number 4 (1995), p. 905 - 920
published: Aug 1, 1995
manuscript accepted: Mar 8, 1995
manuscript received: May 28, 1994
ArtNo. ESP147050704017, Price: 29.00 €
Abstract The solubility of water in melts of albitic (Ab: NaAlSi3O8) and orthoclasic (Or: KAlSi3O8) compositions was re-investigated at T = 1100°C and at pressures between 0.5 and 5.0 kbar. The influence of starting materials, sample preparation, run procedure and analytical methods on the solubility data was investigated systematically. Different analytical techniques (Karl-Fischer titration (KFT), near-infrared spectroscopy (NIR), 105°C weight loss and weight loss on ignition) give consistent results under certain conditions, whereas water-specific methods (KFT and NIR) have a higher precision. The lowest solubilities of water were found for samples of exact feldspathic composition. An excess of several wt% of SiO2 relative to a feldspar composition has a negligible effect on water solubility, but an excess of Na2O or AI2O3 relative to NaAlSi3O8 increases the solubility by ca. 3 percent relative per wt% of excess oxide. At P = 5 kbar, significantly higher solubilities than reported in previous studies were obtained (11.60 wt% H2O for Ab and 10.32 wt% H2O for Or). This is attributed to drying of run products at 105°C in previous studies. At P = 2 kbar, experiments using single glass pieces, crystal plates or glass pieces embedded in glass powder as starting materials yielded a water solubility of 5.95 wt% for Ab melts, which is less than solubilities determined in previous studies. The reliability of the new solubility data could be confirmed by reversals. The water solubility in Ab melts at P = 5 kbar increases apparently with the ratio of fluid to melt. This effect is probably caused by formation of strongly hydrated quench products on the melt body and diffusional exchange between both during cooling.