Pascal Richet

Water and the viscosity of andesite melts

The viscosity of a synthetic andesite-like melt has been measured between 1010 and 1014 P for water contents in the range 0–3.5 wt%. The very slow kinetics of water exsolution over this viscosity range allowed the measurements to be made at 1 bar with a high precision. After a steep viscosity decrease of > 5 orders of magnitude for 1 wt% H2O, an additional 2.5 wt% H2O causes a further viscosity decrease of only 2 orders of magnitude. These viscosity decreases are qualitatively similar to those observed previously for more silica-rich compositions. The new data join smoothly with available high-temperature measurements made at high pressures on water-bearing andesite melts. Because the intrinsic effects of pressure are as small for water-bearing as for water-free samples, the depressing effect of water on the viscosity of natural andesite melts can be estimated. Changes in water speciation as a function of either temperature or pressure do not seem to have marked effects on the viscosity. Although quantitative applications are not yet possible, the configurational entropy theory accounts qualitatively for these features.

Thermal expansion of forsterite up to the melting point

As determined from powder X-ray diffraction experiments with synchrotron radiation, the thermal expansion coefficient of forsterite increases smoothly from 2.8 to 4.5 K−1 from 400 K to 2160 K. No anomalous increases of the cell parameters are observed near the melting point. The consistency between the observed and calculated value of the initial slope of the melting curve of forsterite suggests that defects do not make a large contribution to thermal expansion near the melting point. Along with previous results, the new data confirm the influence of anharmonicity on the high-temperature heat capacity of forsterite and indicate that both the Gruneisen parameter and αKT (α = thermal expansion coefficient, KT = bulk modulus) have nearly constant values at high temperatures.

Rheology of bubble-bearing magmas

Abstract The physical effects of air or argon bubbles on the rheology of a calcium aluminosilicate melt have been measured at temperatures ranging from 830° to 960°C, at 1 bar pressure. The melt composition is SiO 2 :64, Al 2 O 3 :23, and CaO:13 (wt%), while bubble volume fractions are: 0, 0.06, 0.13, 0.32, 0.41 and 0.47. Measured Newtonian viscosities range from 10 10 to 10 14 dPa s. Melts with bubble fractions of 0.06 and 0.13 show with increasing temperature ( T ) an increasing relative viscosity for T T > 850°C, for all bubble fractions the viscosity decreases markedly with temperature. The observed maximum decrease of the relative viscosity is 75% for a bubble fraction of 0.47 at 907°C. At all bubble fractions the viscosity is independent of the applied stress, which ranged from 11 to 677 bars. No clear indications were observed of non-Newtonian rheological behavior. Under our experimental conditions the relative viscosity of the two phase liquid depends primarily on the bubble fraction. Physical and volcanological implications of these measurements are discussed.

Silicate melts: The “anomalous” pressure dependence of the viscosity

Abstract The decrease of the specific volume, when the extent of polymerization diminishes, is a cause of the pressure sensitivity of the viscosity of silicate melts. This effect can be explained by means of the Adam and Gibbs (1965) theory, taking into account the pressure dependence of the degree of polymerization of the melt and its influence on the configurational entropy. At temperatures close to their glass transitions, liquid silica and SiO2 Na2O melts have configurational entropies that are probably due to the mixing of their bridging and nonbridging oxygen atoms.

New high viscosity data for 3D network liquids and new correlations between old parameters

Abstract The Adam–Gibbs and the Vogel–Tamman–Fulcher equations are very often used to express the temperature dependence of the viscosity of liquids. Both equations reproduce very well the same sets of viscosity measurements, but the adjustable constants in these equations, evaluated by fitting the equations to the same data, are always incompatible. The cause of this incompatibility is a lack of good high viscosity measurements. We report new measurements on a series of stabilized liquid silicates which allow us to fit both equations to sets of temperature–viscosity data for 10 different compositions in the temperature interval 700–2700 K, with viscosities ranging from 1016 to 10 Pa s. The precision of the fits of the equations to the data sets is comparable to the experimental precision of the data. The values of the adjustable constants in both equations are in mutual agreement. Moreover the adjustable constants in the Adam–Gibbs equation are physically acceptable and in perfect harmony with already published experimentally obtained thermodynamic information. The nature of the glass transition and the significance of the Vogel temperature have been discussed.

Redox state, microstructure and viscosity of a partially crystallized basalt melt

Abstract As measured in air above the glass transition range, the viscosity of an alkali basalt increases markedly with time by about two orders of magnitude in 12 h. This effect is essentially physical and due to the presence of microcrystals although partial crystallization of the melt into spinel and an SiO2-poor pyroxene leads to a considerable enrichment in silica of the residual liquid. Partial crystallization depends strongly on the initial redox state of samples in that the presence of ferrous iron is required for spinel crystals to form and for pyroxene to nucleate and grow around them. Other measurements show that the viscosity of the crystal-free liquid decreases slightly with increasing ratios r=Fe2+/∑Fe because the differences between samples with r=0.16 and 0.83 amount to about 1.5 and 0.3 log-units at 950 and 1400 K, respectively. Comparisons of the viscosities of the residual liquid matrix and of the initially crystal-free basalt show that physical effects caused by the presence of microcrystals begin to be observed at a low crystal fraction of 5 vol%. Finally, a model of viscosity calculation is developed for the melts which reproduces all data obtained in this work to better than 10%.

High-temperature heat capacity and phase transitions of CaTiO3 perovskite

Drop-calorimetry measurements performed on CaTiO3 perovskite between 400 and 1800 K have shown the occurrence of two overlapping phase transitions at 1384 and 1520 K. The 1384 K transition shows a λ-type Cp variation with a very sharp Cp decrease after the transition; in contrast, the 1520 K transition exhibits a unusual λ shape with a long high-temperature tail spanning more than 400 K. By comparison with previous structural studies, we suggest that the 1384 K transition may be due to an orthorhombic Pbnm to orthorhombic Cmcm transition and that the peak centered at 1520 K represents the effects of overlapping orthorhombic to tetragonal and tetragonal to cubic phase transitions. The large anomaly of specific heat above 1520 K suggests that the cubic phase produced may be strongly disordered up to the melting point.

Amorphous Materials: Properties, Structure, and Durability: The viscosity of hydrous NaAlSi3O8 and granitic melts: Configurational entropy models

Abstract We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi3O8, haplogranite (SiO2-KAlSi3O8-NaAlSi3O8), and complex (natural) granite composition from available measurements and recently published configurational heat-capacity data. The equation log η = Ae + Be/TSconf(T), where Sconf is configurational entropy, reproduces viscosity data for individual samples as well as or better than the empirical three-parameter TVF equation (defined below), and has the advantage of being based on thermodynamic theory. The variables Ae, Be, and Sconf(Tg), where Tg is glass transition temperature, were parameterized as a function of water content for compilations of viscosity data for hydrous NaAlSi3O8, haplogranite, and peraluminous granite melts. With the simplest assumption of ideal mixing between silicate and water components, configurational entropy models with between 4 and 10 fitting parameters reproduce experimentally determined η-T-XH2O relationships significantly better than previous literature models based on empirical equations. Our preferred configurational entropy models have root-mean-square deviations of 0.26 log units for NaAlSi3O8 (n = 77), 0.16 log units for haplogranite (n = 55), and 0.28 log units for peraluminous granites (n = 79). The best statistical fits to the data sometimes require thermodynamically unlikely variations in Ae, Be, and Sconf(Tg) as a function of water content, however, such that further calorimetry data are needed to extract accurate thermodynamic information from viscosity data sets for hydrous melts.

Elastic properties of silicate melts up to 2350 K from Brillouin scattering

Compressional wave velocities (Vp) have been determined up to 2350 K on CaSiO3, MgSiO3, CaMgSi2O6, CaAl2Si2O8 and Ca3Al2Si3O12 glasses and melts from Brillouin-scattering experiments made with a 180° backscattering geometry. At the glass transition, the decrease of Vp with increasing temperatures becomes much stronger and the width of the Brillouin lines begins to increase markedly. At the highest temperatures investigated, Vp is similar to the relaxed values determined in acoustic measurements for several of these melts. This indicates that the configurational degrees of freedom of these liquids have become accessible within the very short timescale of Brillouin scattering experiments: equilibrium compressibilities of silicate melts can thus be determined with this technique. Our measurements also suggest that the shear modulus at infinite frequency of silicate melts could vary with either temperature or composition more strongly than assumed currently.

Configurational heat capacities : alkali vs. alkaline-earth aluminosilicate liquids

Abstract The heat capacities of three aluminosilicate glasses and liquids have been determined from drop-calorimetry measurements made between 400 and about 1800 K. At the glass transition, the observed Cp changes vary from 16% to 36%, increasing when the SiO2 content decreases or when alkaline-earth are substituted for alkali elements. Whereas empirical models for calculation allow the heat capacities to be calculated accurately for glasses, they give less good results for liquids owing to strong interactions between alkali or alkaline-earth elements with aluminum for which there are no current accounts. Such comparisons nonetheless demonstrate that the configurational properties of complex compositions can be predicted from measurements performed on simple systems. The variations of the configurational heat capacity as a function of temperature and composition are discussed with reference to short-range order around cations.