972 research outputs found
The combined effects of chlorine and fluorine on the viscosity of aluminosilicate melts
No full textThe effect of fluorine and fluorine + chlorine on melt viscosities in the system Na2O-Fe2O3-Al2O3-SiO2 has been investigated. Shear viscosities of melts ranging in composition from peraluminous [(Na2O + FeO) (Al2O3 + Fe2O3)] were deter-mined over a temperature range 560-890 degrees C at room pressure in a nitrogen atmosphere. Viscosities were determined using the micropenetration technique in the range of 10(8.8) to 10(12.0) Pa s. The compositions are based on addition of FeF3 and FeCl3 to alummosilicate melts with a fixed amount Of SiO2 (67 mol%). Although there was a significant loss of F and Cl during glass syntheses, none occurred during the viscometry experiments. The presence of fluorine causes a decrease in the viscosity of all melts investigated. This is in agreement with the structural model that two fluorines replace one oxygen; resulting in a depolymerisation of the melt and thus a decrease in viscosity. The presence of both chlorine and fluorine results in a slight increase in the viscosity of peraluminous melts and a decrease in viscosity of peralkaline melts. The variation in viscosity produced by the addition of both fluorine and chlorine is the opposite to that observed in the same composition melts, with the addition of chlorine alone (Zimova M. and Webb S.L. (2006) The effect of chlorine on the viscosity of Na2O-Fe2O3-Al2O3-SiO2 melts. Am. Mineral. 91, 344-352). This suggests that the structural interaction of chlorine and fluorine is not linear and the rheology of magmas containing both volatiles is more complex than previously assumed. (c) 2007 Elsevier Inc. All rights reserved
Configurational heat capacity and viscosity of (Mg, Ca, Sr, Ba)O-Al2O3-SiO2 melts
No full textThe configurational heat capacity, viscosity and density of a series of Mg-, Sr- and Ba-aluminosilicate melts and glasses were determined across the metaluminous/peraluminous compositional and structural join. With increasing amounts of Al2O3 in the melts, the C-p(conf) of the metaluminous Sr-, Ca- and Mg melts increases. This indicates that the range of structures within the melt have, on average, higher energy states as the number of non-bridging oxygens decreases. The C-p(conf) of the Ba-aluminosilicate melts does not change with changing Al2O3 content. In contrast to the behaviour of the alkaline-earth aluminosilicates, the C-p(conf) of peralkaline Na2O-Al2O3-SiO2 melts decreases with increasing Al2O3 content. The C-p(conf) for Na-, Ba-, Sr-, Ca- and Mg-aluminosilicate melts was found to increase with increasing field strength of the cations in the melt - for both the peraluminous and the metaluminous(peralkaline) compositions. In contrast, the viscosity of the peraluminous alkaline earth melts decreases with increasing field strength; but viscosity increases with increasing field strength in the metaluminous field
Influence of Fe2+/Fetot on the viscosity of melts from the Colli Albani Volcanic District (Italy) – foidite to phonolite
No full textViscosity of a melt strongly depends on melt composition and the ratio of network modifiers to network formers. In silicate melts, Fe exists as Fe2+ and Fe3+, both affecting the viscosity in different manners: Fe2+ acts as a network modifier and lowers the viscosity whereas Fe3+ behaves as a network former and increases the viscosity. The Fe speciation strongly depends on the oxygen fugacity fO2.
In this study, the viscosities of Fe-rich synthetic tephri-phonolitic, foiditic and tephritic melts from the Colli Albani Volcanic District (Italy) are investigated as a function of Fe2+/Fetot in the glass transition range. Base glasses made in air at 1600 °C at log10 fO2 = −0.68 are re-equilibrated at log10 fO2 = −4 and − 7 in a gas mixing furnace at 1250 °C. Melts made in air and at log10 fO2 = −4 have nearly identical Fe2+/Fetot due to the different experimental temperatures. At 700 °C, the viscosity of these melts drops by ~2, ~1.5 and ~ 0.4 log10 Pa s with increasing Fe2+/Fetot from ~0.45, ~0.39 and ~ 0.44 to ~0.76 for the tephri-phonolite, the foidite and the tephrite, respectively. These melts have H2O concentrations up to 0.03 wt%. It is shown that the addition of ~0.23 wt% H2O reduces the viscosity by the same amount as the increase in Fe2+/Fetot.
In addition, the viscosities of a phonolite from Mt. Vesuvius are studied over a range of Fe2+/Fetot values (~0.41 to 0.87). In contrast to the other melts of this study, viscosity appears to increase with increasing Fe2+/Fetot. However, this change in viscosity results from a depletion of ~14 mol% in network modifying alkali and alkaline earths elements and a loss of approx. 50 atom% Fe both affecting the viscosity of this melt more effectively than the increase in Fe2+/Fetot.http://dx.doi.org/10.13039/501100001659 Deutsche Forschungsgemeinschaf
Shear modulus, heat capacity, viscosity and structural relaxation time of Na2O–Al2O3–SiO2 and Na2O–Fe2O3–Al2O3–SiO2 melts
No full textThe configurational heat capacity, shear modulus and shear viscosity of a series of Na2O–Fe2O3–Al2O3–SiO2 melts have been determined as a function of composition. A change in composition dependence of each of the physical properties is observed as Na2O/(Na2O + Al2O3) 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
Anelasticity and microcreep in polycrystalline MgO at high temperature: an exploratory study
No full textThe frequency dependence of the shear modulus and dissipation in polycrystalline MgO has been determined at high temperature using both microcreep (epsilon = 10(-4)) and seismic frequency forced-oscillation (epsilon = 10(-5)) measurements. The frequency-dependent and time-dependent data have been described in terms of the elastic, anelastic and viscous components of deformation using the Andrade model. The forced-oscillation measurements show that for temperatures above 700 degreesC the shear modulus begins to decrease dramatically and the modulus becomes frequency-dependent with increasing temperature. This is accompanied by an increase in dissipation, which also becomes frequency-dependent. The microcreep measurements resolve this frequency-dependent behaviour into an anelastic regime from 700-1050 degreesC, and a viscoelastic regime from 1100-1300 degreesC. At 1300 degreesC, the seismic frequency shear wave speed is similar to60% of the extrapolated low-temperature frequency-independent value, and the dissipation has risen to Q(-1) = 10(-1) from 10(-3) at temperatures below 600 degreesC. The mechanism by which this frequency-dependent rheology occurs appears to be diffusional creep, which produces viscous slip on the grain boundaries. It is proposed that the anelastic behaviour is due to viscous slip occurring on segments of grain boundaries, with the viscous deformation being accommodated by elastic distortion of adjacent unslipped regions of the grain boundary. At higher temperatures, slippage occurs across the entire grain boundary and viscoelastic behaviour begins to occur
Viscosity of evolving magmas: a case study of the Glass House Mountains, Australia
No full textAbstract
The viscosity of the remelted rock compositions of the Glass House Mountains, SE Queensland, Australia, has been determined via micro-penetration in the high-viscosity regime (108–1013 Pa s). The heat capacity of these melts has also been determined from room temperature to above the glass transition. The combination of these two data sets allows the fitting of the viscosity data by the Adam-Gibbs equation using the configurational heat capacity Cpconf(Tg12) and configurational entropy Sconf(Tg12). The resulting fit parameters allow the robust extrapolation of the viscosity data to higher temperature and viscosities of 10–4 Pa s. This data can now be used in the discussion of the emplacement of the magmas of the plugs, laccoliths, sills and dykes that form the Glass House Mountains complex and the plate motion and the plume responsible for the volcano plugs. The large increase in viscosity of the evolving magma and the resulting decrease in discharge rate of the volcanic vents suggest that very little magma appeared as extrusive lavas or pyroclastic material and that the Glass House Mountains are mainly remnants of intrusive bodies exposed by erosion.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Georg-August-Universität Göttingen (1018
Thermal stress, cooling-rate and fictive temperature of silicate melts
No full textAbstract
The unknown cooling-rate history of natural silicate melts can be investigated using differential scanning heat capacity measurements together with the limiting fictive temperature analysis calculation. There are a range of processes occurring during cooling and re-heating of natural samples which influence the calculation of the limiting fictive temperature and, therefore, the calculated cooling-rate of the sample. These processes occur at the extremes of slow cooling and fast quenching. The annealing of a sample at a temperature below the glass transition temperature upon cooling results in the subsequent determination of cooling-rates which are up to orders of magnitude too low. In contrast, the internal stresses associated with the faster cooling of obsidian in air result in an added exothermic signal in the heat capacity trace which results in an overestimation of cooling-rate. To calculate cooling-rate of glass using the fictive temperature method, it is necessary to create a calibration curve determined using known cooling- and heating-rates. The calculated unknown cooling-rate of the sample is affected by the magnitude of mismatch between the original cooling-rate and the laboratory heating-rate when using the matched cooling-/heating-rate method to derive a fictive temperature/cooling-rate calibration curve. Cooling-rates slower than the laboratory heating-rate will be overestimated, while cooling-rates faster than the laboratory heating-rate are underestimated. Each of these sources of error in the calculation of cooling-rate of glass materials—annealing, stress release and matched cooling/heating-rate calibration—can affect the calculated cooling-rate by factor of 10 or more.Georg-August-Universität Göttingen (1018
Viscosity of soda-lime glass and potash-based glass of the Carolingian period (8th – 9th century CE)
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