Effect of the Na/K mixing on the structure and the rheology of tectosilicate silica-rich melts

Viscosity and structure of Na/K tectosilicate glasses containing 75 and 83 mol% of SiO2 have been investigated. Viscosity increases non-linearly when K+ ions substitute Na+ ions in these melts. The viscosity variations depending on chemical composition cannot be reproduced using an ideal mixing model of the configurational entropy. Consequently, it appears that Na and K elements do not mix randomly in the studied aluminosilicate melts. Raman spectra of glasses show that, during the Na/K substitution, evolutions of both the ring distribution and the T-O-T mean angle, or force constant, occur. The proportion of small-membered (three, four) rings, compared to large-membered rings, is higher in K-rich glasses than in Na-rich glasses. Moreover, Raman spectra features suggest that two different TO2 environments exist and cohabit into the glass. They could represent two populations of six-membered rings differing by their force constant or their T-O-T angles. One of these environments evolves when K substitutes for Na, showing an increase of its mean T-O-T angle and force constant. The other environment remains unchanged. From the observations, we propose that Na/K mixed tectosilicate glasses contained two sub-networks: one composed of the Si, Al, O and K atoms, and another of the Si, Al, O and Na. We suggest that the different size of alkali elements combined to the charge balancing needs of Al3 + ions can be the source of the different clustering of alkali cations into different sub-networks. Furthermore, and as previously inferred by older studies, potassic tectosilicate glasses could present silica-like and leucite-rich regions, explaining notably the incongruent crystallization of the orthoclase liquid.

► Na+ and K+ ions do not mix randomly in aluminosilicate melts.
► Melt viscosity and structure change drastically with the Na/K substitution.
► The Al3 + presence constrains alkali's CN and local environment.
► Two sub-networks in alkali aluminosilicate glasses account for observations.
► Na/K variations in magma impact on volcanic eruptive style.