Shear modulus, heat capacity, viscosity and structural relaxation time of Na2O–Al2O3–SiO2 and Na2O–Fe2O3–Al2O3–SiO2 melts

Abstract

The configurational heat capacity, shear modulus and shear viscosity of a series of Na 2 O-Fe 2 O 3 -Al 2 O 3 -SiO 2 melts have been determined as a function of composition. A change in composition dependence of each of the physical properties is observed as Na 2 O/(Na 2 O ? Al 2 O 3 ) is decreased, and the peralkaline melts become peraluminous and a new charge-balanced Al-structure appears in the melts. Of special interest are the frequency dependent (1 mHz-1 Hz) measurements of the shear modulus. These forced oscillation measurements determine the lifetimes of Si-O bonds and Na-O bonds in the melt. The lifetime of the Al-O bonds could not, however, be resolved from the mechanical spectrum. Therefore, it appears that the lifetime of Al-O bonds in these melts is similar to that of Si-O bonds with the Al-O relaxation peak being subsumed by the Si-O relaxation peak. The appearance of a new Al-structure in the peraluminous melts also cannot be resolved from the mechanical spectra, although a change in elastic shear modulus is determined as a function of composition. The structural shear-relaxation time of some of these melts is not that which is predicted by the Maxwell equation, but up to 1.5 orders of magnitude faster. Although the configurational heat capacity, density and shear modulus of the melts show a change in trend as a function of composition at the boundary between peralkaline and peraluminous, the deviation in relaxation time from the Maxwell equation occurs in the peralkaline regime. The measured relaxation times for both the very peralkaline melts and the peraluminous melts are identical with the calculated Maxwell relaxation time. As the Maxwell equation was created to describe the timescale of flow of a mono-structure material, a deviation from the prediction would indicate that the structure of the melt is too complex to be described by this simple flow equation. One possibility is that Al-rich channels form and then disappear with decreasing Si/Al, and that the flow is dominated by the lifetime of Si-O bonds in the Al-poor peralkaline melts, and by the lifetime of Al-O bonds in the relatively Si-poor peralkaline and peraluminous melts with a complex flow mechanism occurring in the mid-compositions. This anomalous deviation from the calculated relaxation time appears to be independent of the change in structure expected to occur at the peralkaline/peraluminous boundary due to the lack of charge-balancing cations for the Al-tetrahedra.

Rheology of melts from the colli albani volcanic district (Italy): a case study

“…The same conclusion is reached by Russell and Giordano (2017) who concluded that an increasing SiO 2 accompanied with a rising degree of polymerisation would result in a less re-ordering of the melt structure at the glass transition and therefore a lower C p conf . Combining the viscosity data and the calculated C p conf enables the evaluation of the configurational entropy (S conf ) using the Equation (Richet 1984;Webb 2008;Falenty and Webb 2010) with A e and B e as constants, T-temperature in Kelvin and T g 12 -glass transition temperature in Kelvin, conventionally assumed as the temperature at which the viscosity is 10 12 Pa s. This Equation presumes that there is no change in S conf for temperatures lower than T g 12 . The value for the parameter A e is a highly debated topic.…”

Modeling of viscoelastic properties of nonpermeable porous rocks saturated with highly viscous fluid at seismic frequencies at the core scale

“…Second, we have assumed here that Maxwell's phenomenological model provides a valid basis for describing the viscoelastic behavior of the saturant. It is not clear that this is the case for the very complex behavior that such “viscous” fluids exhibit during shearing in silicate melts [e.g., Falenty and Webb , ; Green and Cooper , ] or heavy hydrocarbons [e.g., Mortazavi‐Manesh and Shaw , ]. This lack of knowledge with regard to the detailed time‐dependent rheological character of such viscous fluids will continue to be a limiting factor in the applicability of any modeling.…”

“…At one extreme in igneous systems, silicate melts deep in the Earth inhabit void spaces between the minerals. Shear viscosities of the silicate melts depend critically on composition, deformation rate, and temperature ranging with typical values broadly on the order of 10 10 Pa s [e.g., Webb , ; Falenty and Webb , ]. At the other, highly heavy hydrocarbons saturate both siliclastic and carbonate formations in many sedimentary basins globally.…”

Eshelby description of highly viscous flow—Half model, half theory

“…For (SiO 2 ) 67 (Na 2 O) 33 , later measurements [35] showed that the original measurement in the melt [26] had a smaller modulus than the real one, indicating some gliding at the clamps holding the sample. FIG.…”