Silicate melts, Viscosity

Viscosity is one of the most frequently measured properties of silicate melts. Viscosity data vary in the range 0.1-10 Pa s. To obtain precise and correct viscosity data is experimentally very difficult, especially at high temperatures. Contradictory data frequently found in the literature demonstrate this fact. This can be illustrated by the CaSiOs melt, for which at 1873 K, values of 0.15 to 0.25 Pa s are given. 

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

For kinetic disequilibrium partitioning of trace elements, equation (9.6.6) after Burton et al. (1953) is commonly presented as an alternative to equation (9.6.5) due to Tiller et al. (1953) (e.g., Magaritz and Hofmann, 1978 Lasaga, 1981 Walker and Agee, 1989 Shimizu, 1981). However, the relative values of viscosity and chemical diffusivity in common liquids and silicate melts make the momentum boundary-layer (i.e., the liquid film which sticks to the solid) orders of magnitude thicker than the chemical boundary layer. It is therefore quite unlikely that, except for rare cases of transient state, liquid from outside the momentum boundary-layer may encroach on the chemical boundary-layer, i.e., <5 may actually be taken as infinite. As a simple description of steady-state disequilibrium fractionation, the model of Tiller et al. (1953) has a much better physical rationale. A more elaborate discussion of these processes may be found in Tiller (1991a, b). 

For instance, a quite marked effect on the structure of the silicate melt is produced by the basic oxide H2O. Addition of water causes a drastic decrease in the viscosity of silicate melts adding 6.4 weight % of H2O to a granitic melt at T = 1000 °C causes a decrease in viscosity of about six orders of magnitude (i.e., from about 10 to about 10 poise cf Burnham, 1975). The dissolution mechanism of H2O is important in magma rheology and will be discussed more extensively in section 9.6.1. 

Mysen B. O., Virgo D., and Scarfe C. M. (1980). Relation between the anionic structure and viscosity of silicate melts A Raman spectroscopic study. Amer. Mineral, 65 690-710. Nafziger R. H. and Muan A. (1967). Equilibrium phase compositions and thermodynamic properties of olivines and pyroxenes in the system MgO-FeO-SiOj. Amer. Mineral, 52 1364-1384. 

The dependence of diffusivity in silicate melts on composition is related to how melt structure (including degree of polymerization and ionic porosity) depends on composition. One the one hand, as Si02 concentration increases, the melt becomes more polymerized and the viscosity increases. Hence, diffusivity of most structural components, such as Si02 and AI2O3, decreases from basalt to rhyolite. On the other hand, as Si02 content increases, the ionic porosity increases. The increasing He diffusivity from basalt to rhyolite to silica, opposite to the viscosity... 

Hui H. and Zhang Y. (2007) Toward a general viscosity equation for natural anhydrous and hydrous silicate melts. Geochim. Cosmochim. Acta 71, 403-416. 

In addition, modem fabrication techniques demand good molding characteristic, i.e., low melt viscosity, in order to enhance the shaping cycle and increase the productivity. When a layered silicate, e.g., montmorillonite is added as an inorganic filler, the fluidity and the surface properties can be improved (24). 

Lejeune A.-M. and Richet P. (1995) Rheology of crystal bearing silicate melts an experimental study at higji viscosities. J. Geophys. Res. 100, 4215—4229. 

Nevertheless, this hypothesis has been unpopular since the 1980s, because geodynamic models, incorporating either constant mantle viscosity or a rigid upper plate of prescribed thickness, predicted that solidus temperatures could not be reached in basalt or sediment at the top of the subducting plate, except under unusual circumstances (see reviews in Kelemen et al., 2003a Peacock, 1996, 2003 Peacock et al., 1994). For example (Filer et al., 2000, p. 247), discounted their successful model involving H20-rich silicate melt because [the] successful melt-fluxed [model]. .. require[ s]... temperatures... that are... not obviously compatible with...thermal models... (Peacock, 1996). ... 

Textile glass fibers are produced from silicate melts with sufficient fiber drawing potential. The viscosity increase upon cooling ensures that the molten filament cools to a fiber... 

Silicate melts suitable for fiber production have to fulfill many requirements first a sufficient fiber drawing potential, which is dependent upon the ratio of surface tension to viscosity. Fixing of the molten filament in the form of a fiber is a consequence of the increase in vi.scosity with decreasing temperature. Got)d fiber formation requires a viscosity-temperature dependence which is relatively flat. The glass must also not exhibit a tendency to crystallization. 

Mineral fibers are manufactured from silicate melts of appropriate composition. These melts are converted into fibers with considerably more efficient use of time and space than in the manufacture of textile glass fibers, since the melts are spun at much lower melt viscosities. After solidification the fibers consist of amorphous glasses (according to X-ray diffraction measurements) with... 

Viscosity (r ). Several models have been reported for the estimation of viscosities (q) of silicate melts to cover the compositional ranges of glasses steelmaking slags magmas (15-lJ)... 

Additional work on liquid metals, simple chloride salts and some small molecule organic liquids ( ) indicates that the advantage of the Kirchoff-Rankine equation over the Andrade-Arrhenius equation improves as the size of the melt species increases. The improvement in the description of viscosity vs. temperature for metals and simple salts (e.g., NaCl and BiCl2) is not great, but for materials with larger melt species (e.g., silicate melts and organic liquids), there is a distinct improvement. 

Weed, H. C. Piwinskii, A. J. Dibley, L. L. "Experimental Study of the Dynamic Viscosity of Some Silicate Melts to 1953k at 150 kPa", UCRL-52757, Lawrence Livermore National Laboratory, Livermore, CA, 94550, 1979, p. 2. 

Taniguchi, H. (1995) Universal viscosity-equation for silicate melts over wide temperature range, J. Vole, and Geotherm. Res. 66, 1-8. 

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