INFLUENCE OF CATION SIZE IN THE SEQUENCE NA-K-RB ON THE STRUCTURE OF QUARTZ-FELDSPAR MELTS

Abstract

This paper presents the results of theoretical and experimental investigations of the system rubidium feldspar-quartz-water at a pressure of 1 kbar. It was found that rubidium leucite (RbAlSi2O6) rather than feldspar is stable both in the dry and hydrous systems. Near the eutectic point of the dry system, the liquidas of RbAlSiO4 approaches that of RbAlSi2O6. The latter phase crystallizes owing to the heterogeneity of initial glass obtained at a temperature of 1500°C. The water solubility in RbAlSi3O8 melt is 4.2 ± 0.15 wt % at 1300°C and a water pressure of 1 kbar. The eutectic composition contains 53 ± 2 mol % quartz (recalculated to RbAlSi3O 8 and Si6O12) at a water pressure of 1 kbar. A comparison of the sodium, potassium, and rubidium systems reveals a systematic shift of eutectic composition toward quartz and simultaneous depression of the quartz liquidus. This is related to an increase in the activity of an aluminosilicate phase (leucite in the rubidium system) and a concurrent decrease in SiO2 activity in the melt with increasing cation radius. The IR spectroscopy of dry feldspar glasses with the Na, K, and Rb cations showed a systematic increase in the concentration of three-membered rings. The structural patterns of rubidium-bearing melt can be compared with the structure of the RbAlSiO4 phase. This phase is a chemical analog of nepheline showing a peculiar structure arising from the need to accommodate the large Rb cation. This results in a modification of the nepheline structure, which is made up of stacking six-membered rings. In the rubidium analog, the six-membered rings aligning one above another change their attachment patterns, which results in the loss of hexagonal symmetry, and a pair of rings acquires an "open-jaw" configuration holding Rb+. According to IR spectroscopy, the proportion of six-membered and four-membered rings in the glass is similar to that of the crystal, and three-membered rings close the network with NBO/T = 0.