Viscosity Of Silicate Melts Research Articles

Influence of Solid Inclusions on the Rheology of Silicate Melts

The viscosity of magma is an important parameter for understanding magmatic processes and volcanic eruptions. A great deal of work has been done to understand how the viscosity of silicate melts depends on temperature, on chemical composition, and even on pressure. But natural magmas are not made of only a melt phase; they carry solid and gas inclusions whose influence on viscosity just begin to be investigated. For solid phase inclusions, reliable and accurate data have previously been gathered for melts with spherical crystalline inclusions of constant size. The viscosity of these melts increases abruptly and becomes non-newtonian when the crystal fraction is about the threshold value of 40% vol. The rheology of real magmas is more complicated because of the diversity of form and size of the crystalline species and the possibility of orientation effects. This study aims at filling some important gaps which still exist in understanding the rheology of magmas. This work concerns high viscosities where the kinetic of growth of crystals is slow, so the crystal fraction is well known during the experiments. The influence of non-spherical crystals with different sizes has been determined for CaA12Si2Os melts, which crystallize congruently. Thus we avoid strong variations of viscosity due to a change of the chemical composition of the melt by crystallization, and we just consider the physical effect of the crystal fraction. For a better understanding of the dynamics of volcanic eruptions, the rheological laws obtained with simple systems will be compared with measurements on lavas, and especially on some material from the pelrean eruption of 1929.

Adam-Gibbs Theory and the Prediction of Glass Transition Temperatures: The Join Na2Si2O5-NaAlSi2O6

The viscosity of silicate melts is a parameter of extreme importance, not only in the Earth sciences, but also in a number of applied disciplines such as glass, and steel making. Empirical predictive models of melt viscosity proposed in the geological literature (e.g. Bottinga and Weill 1972), have been shown to be reliable at temperatures well above the glass transition, but these models do not take good account of non-Arrhenain behaviour, and extrapolation to temperatures close to the glass transition can lead to significant errors in estimated viscosity. In contrast to these empirical approaches, various attempts have made to quantitatively link viscosity to some structural property of the melt. Of these, the AdamGibbs theory (Adam and Gibbs, 1965) which considers viscosity to be related to the configurational entropy of the melt, has been shown to be particularly successful in accounting for nonArrhenian variations of viscosity of molten silicates with temperature (Richet, 1984; Bottinga et al., 1995). Despite this fact, the Adam-Gibbs (A-G) theory has not yet been used in a predictive sense, due to highly non-linear variations of the A-G parameters (Ae, Be, temperature independent constants, Sr the configurational entropy at the glass transition) as a function of composition, even along simple pseudo-binary joins, (e.g. CaSiO3MgSiO3, Neuville and Richet, 1991). Compositional dependence, and parameterisation of the A-G parameters As discussed by Toplis (1998), consideration of the A-G theory shows that non-linearities in the A-G parameters are to be expected due to the presence of terms involving the configurational entropy. However, the ratio of parameters Be and Sc(Tg) should show simpler variations as a function of composition, because terms involving the configurational entropy cancel out. The expression for the ratio BJSc(Tg ) is:

The glass-transition, structural relaxation and shear viscosity of silicate melts

The enthalpy, volume and shear relaxation for a wide range of silicate melt compositions is discussed; especially the relationship between the glass transition temperature T g, viscosity and the structural parameters NBO/T and fragility. For a number of silicate melt compositions ranging from 50 to 85 mole% SiO 2 and from 0 to 2 NBO/T and from “strong” to “fragile”; it is shown for each melt composition that the glass transition temperatures obtained from enthalpy, volume and shear relaxation are the same (within error) for a fixed observation rate, and therefore for each composition, the viscosities at the calorimetric and volumetric glass transition temperatures are the same; and for each melt composition the energy terms required to describe shear, volume and enthalpy relaxation are the same. The average shear viscosity of the present silicate melts at T g (defined to be the temperature at the peak of the heat capacity and thermal expansion curves) for a thermal history of 5°C min −1 cooling- and heating-rates is 10 11.22(σ = 0.33) Pa s. The glass transition temperatures discussed here are obtained by scanning calorimetry and dilatometry, and shear viscosity measurements. The relaxation of the physical properties volume and enthalpy is therefore being observed as a function of temperature in melts whose structure is not in equilibrium with the ambient temperature. With variations of more than 30°C in determinations of the calorimetric T g from different laboratories, it has been difficult to resolve a possible variation in viscosity at T g as a function of melt composition (at high viscosities, a ∼ 20°C change in temperature is generally equivalent to ∼ 0.5 log 10 unit in viscosity for silicate melt compositions). For the present very accurate measurements in the same laboratory with melts of the same thermal history, and a consistent definition of T g, one order of magnitude range in viscosity as a function of melt composition has been observed at T g. This range in viscosity at T g is due to a combination of the composition dependence of the common energy terms for enthalpy, volume and shear relaxation; the distribution of relaxation times for each of these processes; and the differencesin elastic shear modulus as a function of melt composition.

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

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 SiO2Na2O melts have configurational entropies that are probably due to the mixing of their bridging and nonbridging oxygen atoms.

Viscosity regimes of homogeneous silicate melts

Other| April 01, 1995 Viscosity regimes of homogeneous silicate melts Yan Bottinga; Yan Bottinga Institut de Physique du Globe, Department des Geomateriaux, Paris, France Search for other works by this author on: GSW Google Scholar Pascal Richet; Pascal Richet Search for other works by this author on: GSW Google Scholar Anne Sipp Anne Sipp Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (3-4): 305–318. https://doi.org/10.2138/am-1995-3-412 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Yan Bottinga, Pascal Richet, Anne Sipp; Viscosity regimes of homogeneous silicate melts. American Mineralogist 1995;; 80 (3-4): 305–318. doi: https://doi.org/10.2138/am-1995-3-412 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Extrapolation of laboratory measurements of the viscosity (η) of silicate melts is frequently needed in order to analyze petrological and volcanological processes. Therefore a general understanding of silicate melt viscosities is required. In this paper we survey the present state of our knowledge and distinguish three flow regimes for homogeneous silicate liquids: (1) a low-viscosity regime (η <1 Pa·s), where the viscosity obeys a temperature-dependence power law in accordance with mode coupling theory (these low viscosities are typical for depolymerized melts); (2) an intermediate regime (1 <η <1012 Pa·s), where silicate melt viscosity is determined by the availability of configurational states (the dependence of the viscosity on the temperature is described aptly by the configurational entropy theory of Adam-Gibbs); and (3) a high-viscosity regime, where the liquid has been transformed into a glass (η >1012 Pa·s) (this regime is not well known, but available measurements indicate an Adam-Gibbs or an Arrhenian temperature dependence of the glass viscosity). Examples are given of igneous rocks whose geneses were affected by these flow regimes. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Non-Newtonian rheology of homogeneous silicate melts

Several observations of non-Newtonian viscosity of silicate liquids at high stress or strain rates have been published in recent years. However, this phenomenon is not well understood yet. In this paper attention is drawn to the fact that steady state logarithmic values of reduced viscosities of silicate liquids under high stress show a linear dependence on the squared value of the applied stress. This relationship suggests that the elastic work done by the stress on the liquid is related to the observed viscosity decreases. It is shown that the development of non-Newtonian viscosity in silicate melts under high stress can be explained with the Adam and Gibbs (1965) theory, if one accepts that this elastic work generates configurational entropy.

Molecular dynamics study on the shear viscosity of molten Na 2O·2SiO 2

This paper describes the applicability of molecular dynamics simulation to the shear viscosity of silicate melts. Simulations of molten Na 2O·2SiO 2 have been performed on 756 and 189 particle systems to evaluate the viscosity by using three different methods: the Green-Kubo integrand, the Stokes-Einstein equation, and the non-equilibrium molecular dynamics (NEMD) simulation. Of these methods, the Green-Kubo treatment and the Stokes-Einstein equation, adopting the SiO 4 4− anion for the diffusing unit, gave the most reasonable results, and they qualitatively reproduced the temperature dependence of viscosity. The simulated values of viscosity were consistent with experimental values within an order of magnitude at temperatures, higher than 1500 K, but were smaller by more than one order of magnitude at lower temperatures. The NEMD simulation of shear flow showed a strong shear-thinning features and failed to reproduce the viscosity value at the experimental shear rate region. The discrepancy between the simulated and experimental values is attributed to the short sampling time and the small system size in the MD simulation.

Structural relaxation in silicate melts and non-Newtonian melt rheology in geologic processes

The timescale of structural relaxation in a silicate melt defines the transition from liquid (relaxed) to glassy (unrelaxed) behavior. Structural relaxation in silicate melts can be described by a relaxation time, τ, consistent with the observation that the timescales of both volume and shear relaxation are of the same order of magnitude. The onset of significantly unrelaxed behavior occurs 2 log10 units of time above τ. In the case of shear relaxation, the relaxation time can be quantified using the Maxwell relationship for a viscoelastic material; τS = ηS/G∞ (where τS is the shear relaxation time, G∞ is the shear modulus at infinite frequency and ηS is the zero frequency shear viscosity). The value of G∞ known for SiO2 and several other silicate glasses. The shear modulus, G∞, and the bulk modulus, K∞, are similar in magnitude for every glass, with both moduli being relatively insensitive to changes in temperature and composition. In contrast, the shear viscosity of silicate melts ranges over at least ten orders of magnitude, with composition at fixed temperature, and with temperature at fixed composition. Therefore, relative to ηS, G∞ may be considered a constant (independent of composition and temperature) and the value of ηS, the relaxation time, may be estimated directly for the large number of silicate melts for which the shear viscosity is known.

The Viscoscity of Synthetic and Natural Silicate Melts and Glasses at High Temperatures and 1 Bar (105 Pascals) Pressure and at Higher Pressures, U.S. Geol. Surv. Bull. 1764

Over the past decade, I have routinely collected papers dealing with the physical properties of rocks and other materials. Their dog‐eared and coffee‐stained appearance is just one indication of their continuing value to me. Focusing on viscosity of silicate melts, Michael Ryan and James Blevins have considerably extended and formalized this collection process, resulting in the publication of a massive report containing viscosity data on an extensive variety of melt compositions. According to the authors, this report represents an initial step in establishing a comprehensive U.S. Geological Survey (USGS) data base for the properties of multicomponent silicate melts.

Viscosity estimation of slags

The viscosity of silicate melts and slags is a property difficult to measure accurately using sophisticated apparatus, it is both time consuming and expensive. The use of models to estimate the viscosity is the first step to be carried out, mainly with metallurgical slags involving numerous components. The proposed model is an experimental one based on accurate data for pure binary and ternary melts. From this data, using a simple hypothesis on the behaviour of unusual compounds, it is possible to estimate the viscosity of a complex melt. This estimation is used as a first approximation and can be amended, if necessary, by only a few direct measurements.

Viscosity and configurational entropy of silicate melts

With the configurational entropy theory of relaxation processes of Adam and Gibbs (1965), one predicts that the viscosity depends on temperature according to log η = Ae + BeTSconf, where Sconf is the configurational entropy of the liquid. Thermochemical calculations of Sconf performed for some mineral compositions show the importance of non-configurational contributions to the entropy differences between amorphous and crystalline phases. Except for the case of SiO2, the available thermodynamic data indicate that the above equation for viscosity accounts quantitatively for the experimentally determined temperature dependence of the viscosity of silicate melts. The Adam and Gibbs theory also provides a simple rationale for the non linear variation of the logarithmic viscosity with composition in mixed alkali silicate liquids at low temperatures, the minimum of viscosity resulting from the contribution of the entropy of mixing to Sconf.

Field measurements of the rheology of lava

KNOWLEDGE of magma rheology is required to understand many igneous and volcanic processes. Viscosities of silicate melts can be estimated from their chemical composition and temperature using empirical methods1–3. However, the validity of these estimates has been established only at supraliquidus temperatures where common igneous melts behave as newtonian fluids. Recent experimental evidence from field and laboratory studies4–7, has demonstrated that, at subliquidus temperatures, some magmas behave as non-newtonian fluids due to the presence of dispersed crystals and gas bubbles and possibly as a consequence of structuring in the silicate melt. We present data here on the rheological properties of small lava flows which were erupted on Mount Etna in 1975. These data were obtained by field measurements using a new field viscometer8, a shear vane attached to a torque wrench, a conventional penetrometer and various field techniques for estimating rheological properties from the dimensions of lava flows.

珪酸塩の粘度におよぼすAl&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt;の影響

珪酸塩の粘度におよぼすAl&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt;の影響

Comments on viscosity, crystal settling, and convection in granitic magmas

The effect of H 2 O on viscosity of silicate melts is now known. Most granitic melts are expected to have viscosities between 10 8 and 10 5 poises at temperatures at or above the liquidus. At temperatures down to 100 degrees C or more below the liquidus, melts are expected to behave virtually as Newtonian fluids. Hydrostatic pressure to at least 7 kb has little effect on fluidity. Suspended crystals or gas bubbles increase viscosity by less than an order of magnitude. Forced convection can result in virtually instantaneous formation of dikes. Natural convection must be reckoned with in any large granitic pluton; it is possible in a stock less than 100 m in diameter connected to an underlying magma reservoir.

An instrument for measuring and recording the viscosity of silicate melts

An instrument for measuring and recording the viscosity of silicate melts