Anomalous rheology of peraluminous melts

Webb, Sharon L.; Müller, Evi; Büttner, Heinz

doi:10.2138/am-2004-5-616

Cited by 24 publications

(18 citation statements)

References 15 publications

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“…The glass transition temperatures T g 12 (temperature at which g = 10 12 Pa s) are shown as a 12 values for the iron-bearing melts are too large to determine the composition at which the trend-change occurs. This change in trend as a function of Na/Al has been discussed previously by Toplis et al (1997a, b) and Webb et al (2004) in terms of the structural changes and the presence of tri-clusters in the melts with decreasing Na/Al ratio, and also in terms of the changes in flow mechanism accompanying such changes in melt structure.…”

Section: Viscositysupporting

confidence: 82%

“…A number of studies of the rheology and density of aluminosilicate melts have indeed shown that there is a change in the trend as a function of composition as the melts investigated change from peralkaline to peraluminous (e.g. Day and Rindone 1962a;Riebling 1966;Taylor and Rindone 1970;Hunold and Brückner 1980;Toplis et al 1997a, b;Webb et al 2004Webb et al , 2007Giordano et al 2006). Therefore, it is clear that the extrapolation of physical and thermodynamic data across the composition divide will lead to increasing errors in predictions of the behaviour of multicomponent melts and magmas of differing compositions.…”

Section: Introductionmentioning

confidence: 99%

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

Falenty

Webb

2010

Phys Chem Minerals

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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.

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Section: Viscositysupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

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

Falenty

Webb

2010

Phys Chem Minerals

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“…Over the $100°C temperature range for each set of viscosity measurements the data are best fit by an Arrhenian equation: are listed in Table 3, together with data for the fluorine-and chlorine-free Na 2 O-Fe 2 O 3 -Al 2 O 3 -SiO 2 melts (from Zimova and Webb, 2006). As previously observed by Webb et al (2004) and Webb (2004), the activation energies from Arrhenian fits to the viscosity data show a general trend that peraluminous melts tend to have activation energy 50-100 kJ mol À1 higher than peralkaline melts. As shown in Fig.…”

Section: Resultsmentioning

confidence: 71%

“…The micropenetration technique gives an absolute determination of viscosity in the range the 10 8.5 to 10 13.5 Pa s (Richet et al, 1996;Webb et al, 2004). The sample is placed in an alumina sample holder in a nitrogen atmosphere at room pressure.…”

Section: Viscosity Measurementsmentioning

confidence: 99%