The rate of flow in the melts is controlled by the longest lived bonds, whereas there are structural relaxations for each of the bond types in the melt; and the slowest structural relaxation is identified with the rate of flow.
Thermodynamic and rheological properties of magma (viscosity, heat capacity, diffusivity, conductivity, expansivity, compressibility) are required to measure to better interpretation and understanding the flow mechanisms.
Stebbins & Sen (1998) have investigated a microscopic dynamics and viscous flow in a borosilicate glass-forming liquid (44.5 mol% Na2O, 11.0 mol%B2O5, and 44.5 mol%
SiO2). They have proved with NMR measurements that in multi – component oxide liquids some larger structural groups with different relaxation times are created. In borosilicate, the β – relaxation (connected with breaking of Si-O bonds) occurs at frequencies up to 50 times faster than Maxwell relaxation theory assumed; with the lifetime of B-O bonds being identical to τM (Fig. 88). The authors explain this fact with a creation of the polyhedra of
320 360 400 440 480 520 560 600 640 680 320
360 400 440 480 520 560 600 640 680
Ea from Shear Viscosity data (kJ mol -1) NS2 melt
Fe-free melts Fe-bearing melts
E a from Attenuation data (kJ mol-1 )
Ea from Micropenetration data (kJ mol-1)
320 360 400 440 480 520 560 600 640 680
NS2 melt Fe-free melts Fe-bearing melts
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SiO4, BO4 and BO3, which number depends on the temperature. These complex structural units influence the flow mechanism of the melt and lead to separation the measured relaxation time of Si-O bonds and that calculated from Maxwell equation (Eq. 3). Results obtained by Stebbins & Sen (1998) confirm the results for aluminosilicates from this study that Si-O bonds do not always control the flow in silicate melts.
In the case of aluminosilicate melts investigated in this study, it appears that, as suggested by Martens et al. (1987), the Si-rich units are “glued” together by Al-rich units.
The Si-rich units can be called “icebergs” and surrounding them Al-tetrahedra – as sea. The term “iceberg” was discussed by Bockris & Kojonen (1960) for the structure of the alkali silicates and borates (see also references therein).
Stebbins (1995) suggested that Q4 species can create the “icebergs” and Q3 units – mica like “sheets”. They move as relatively big parts of the structure and the lifetimes of the bonds creating the “icebergs” can be different than Maxwell theory assumes, because atoms in them are strongly connected. Through the flow the icebergs break the weakest bonds of network modifiers. The idea of such type of structure can explain our results.
Thus, the simple flow mechanism proposed by Farnan & Stebbins (1994) and the relationship between viscosity and Si-O lifetime as determined by Liu et al. (1988) apply only to simple melt.
In the case where large structural units appear in the melt, a more complex relationship between flow mechanism, viscosity and bond lifetime occurs.
Figure 83 shows a difference between Maxwell relaxation time and relaxation times obtained in this study as a function of composition, presented here as (Al3++Fe3+)/Si ratio (in atoms). Because in the melts occur also some local changes of structure, the relaxation times of aluminosilicate melts do not agree with Maxwell relaxation theory.
In Si-rich melts the flow mechanism is controlled by the lifetime of the Si-O bonds.
Si-O bonds have the longest lifetime in the structure and because silicate melt is a simple melt, Maxwell relaxation works (Fig. 92 a).
With addition more and more Al2O3 the structure changes. New network former is introduced into the structure. As long as the ratio between Al and Si does not excess 1:3 (e.g. in albite) the Al-tetrahedra do not influence on the flow mechanism (Fig. 92b). As has been investigate (Dirken et al., 1997) in such melt there is no Al-O-Al bonds.
The increase in the number of Al-tetrahedra causes that these structural units begin to connect with each other, surrounding the separate silica rich clusters (icebergs) (Fig. 92c). The flow mechanism starts to be controlled by the shorter lifetime of Al-O bonds, in spite of presence longer lived Si-O bonds. It means that the flow mechanism
Figure 92d shows the stage when the melt is almost aluminate with some single Si-tetrahedra. The pure aluminate melt (Fig. 92e) is a simple melt again, where Maxwell relaxation time works.
Here, the original goal of the study – to determine the different behaviour of peralkaline and peraluminous melts – is decoupled from the observations of fast flow mechanism in these Al-rich melts. The existence of triclusters should have an effect on relaxation time and flow mechanism but this is overshadowed by the large effect of a large number of Al-tetrahedra surrounding icebergs of Si-tetrahedra.
__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Fig. 92. Fast flow mechanism in aluminosilicate melts. Description in the text. Red: Si-tetrahedra; blue: a)
c)
d)
e)
b)
6. CONCLUSIONS
The topology of our planet is dominated by the effect of silicate melts – in the form of mid-oceanic ridges, plumes, subduction zones, volcanoes and the differentiation into mantle and core at the creation of our planet. It therefore behoves us to understand the structure and physical and thermodynamic properties of silicate melts.
The structure of sodium aluminosilicate melts is taken to be built of network formers in tetrahedral coordination; and octahedrally coordinated network modifiers and charge balancers. In peralkaline structure, Si and Al form tetrahedra and Na is a network modifier and charge balancer. In peralkaline Fe-bearing melts Fe3+ is placed in tetrahedra and Fe2+ is octahedrally coordinated. The structural rearrangement occurs when composition changes into peraluminous (γ~0.5) - where there is no longer enough Na+ to charge balance the Al3+ in tetrahedral coordination. There is no clear model of the peraluminous structure in aluminosilicate melts and for the purposes of this study here it is assumed that peraluminous structure consists Si, Al (and Fe3+ in Fe-bearing melts) in tetrahedra which form triclusters, because of the lack in charge balancers (Na and Fe2+).
Negligible part of Fe3+ changes coordination from tetrahedral to octahedral, but this amount does not affect the change of structure.
Pure silicate or pure aluminate melt’s and glass’s networks show high similarity.
However, when we start to change composition, one can observe strong variation in physical and thermodynamic properties. Into consideration should be taken not only the kind of the elements playing a network former and network modifiers role, but also the ratio between different ions.
The first observation is that viscosity, heat capacity and shear modulus data show a change in trend at γ~0.5 indicating different structure in the melts as the composition changes from peralkaline to peraluminous. That is caused by the presence of triclusters.
Here, the melt viscosity and glass density data confirm the information from previous studies, that there is a structural and flow mechanism change at γ~0.5, as expected. The heat capacity data show simple trends as a function of composition.
For the first time the trends in configurational heat capacity as a function of composition indicate the presence of a change in structure. Literature data do not cover a controlled chemical composition range and thus have not observed this. With the addition of Fe2O3/Al2O3 this effect is additionally enlarged. The configurational entropy indicates that the range of structures available to the melt increases with increasing Al2O3 content.
The shear modulus and shear viscosity of Na2O-Al2O3-SiO2 melts and the density of the glasses indicate that there is a change in melt structure at Al~Na. Such a change in
__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
structure with composition also requires a change in flow mechanism with composition, and thus a change in the rates at which parts of the melt structure move. The rate of motion of structural units in silicate melts can be determined via forced oscillation methods.
A low-frequency forced oscillation technique has been used to measure the frequency and temperature dependence (to 1000°C) of the shear modulus and viscosity of a range of Na2O-Al2O3-SiO2 melts. The frequency range is between 1 and 0.001 Hz, and thus the viscosity range is from 108-1015 Pa s. The frequency-dependence of the shear modulus can be described by simple structural relaxation theory. The measured relaxation times (τ) for the simple peralkaline compositions agree with the Maxwell relaxation time (τM). With increasing Al2O3 content, the structural relaxation time deviates from the
calculated Maxwell relaxation time (gradually become shorter than τM) and after (Al3++Fe3+)/Si(atoms) ~ 0.55 goes back and became longer again. The pure aluminate melts
seem to be also simple and their relaxation times agree with Maxwell relaxation time (τM).
This is not effect of triclusters.
Such a decrease in τ/τM is observed for the first time. This shortening of relaxation times indicates that the large amount of Al3+ in these melts changes not only the structure, but also the flow mechanism of the melt. The shorter Al-O bond lifetimes appear to control the flow mechanism, in spite of the presence of the longer lived Si-O bonds. The structure flows faster than whole structure is relaxed. This has never been seen before. But also melts in this study have a much higher (Al3++Fe3+)/Si(atoms) ratio (0.62) than used in previous studies (0.2).
The structure of these melts is more complex than first thought. Not only is there the structural change with Na/Al composition, but the large amount of Al3+ has created melts in which the motion of Al tetrahedra is as important as the motion of the Si tetrahedra. Thus, the theories created to describe the flow and structure of silicate melts need to be expanded to include the large effect due to the presence of almost as many Al tetrahedra as Si tetrahedra in the melt.
The torsion data show the lifetimes of motion of various structures in the melt. It was not possible to separate the lifetime of Al-O bonds from Si-O bonds, nor was the lifetime of AlSi2O5 triclusters determined, but the lifetime of Na-O bonds was measured. A second relaxation peak is seen at timescales ~5.5-7.5 orders of magnitude faster than the slowest relaxation time. This very fast relaxation is associated with the movement of Na+ in the melt.
What was determined here for the first time was the effect of a large number of fast
to triclusters but simply to the lifetime of the different bonds. This effect is seen in the failure of the Maxwell relationship to calculate the structural relaxation times of melts with Siatom ~ Alatom.
Figure 93 shows predicted relaxation trend for aluminosilicate melts as a function of the Al/Si ratio. Relaxation times vary depending on the melt composition.
Fig. 93. Predicted relaxation trend for aluminosilicate melts as a function of composition.
Thus studies in which η is related to τM and to diffusion of Si or O will be in error in the composition range Si~Al: this applies to phonolite melts, e.g. Laacher See (Eifel, Germany), Tenerife (Canary Islands, Spain), Sardinia (Italy), Dunedin Volcano (East Otago, New Zealand), Mont Dore (Auvergne, France), Bohemian Massif (Czech Republic) or Mount Saint-Hilaire (Québec, Canada).
The error could also occur in the modelling of the dynamic of the phonolitic volcano: wrong cooling rate gives wrong eruption temperature and viscosity, what influence on wrong calculation of the eruption rate and then on magma heat capacity, volume and temperature.
log η
composition
SiO2 Al2O3
Na2O Na2O
log τ
aluminosilicate melt Al melt
Si melt
FASTSLOW
LOW HIGH
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7. OUTLOOK
The work described in this thesis concentrates on the structure of aluminosilicate melts as a function of composition and their physical and thermodynamic properties. In light of presented results the structural rearrangement has been shown to occur close to the subaluminous point. The failure of the Maxwell relaxation theory has been found and faster relaxation time of the complex melts has been measured.
Yet, more detailed studies are required in order to fully understand, what really occurs in these melts, e.g. NMR measurements of the bond lifetimes, diffusion measurements, and other torsion investigations.
To test of the present conclusions, the other series of the melts need to be investigated (Fig. 94), e.g. Ca-aluminosilicates, aluminosilicates with Al- and Si-end or K-Mg or Na-K compositions. Choosing the end members of the series, the influence of triclusters can be omitted and then their effect on viscosity will not be observed, but the effect of changing bond lifetimes as a function of composition and structure will be revealed.
Fig. 94. Past, present and future of the studies of the relationship between structure and flow mechanism in silicate and aluminosilicate melts in the Earth.
structure ↔ flow Na2O-SiO2
melt
structure ↔ flow Na2O-Al2O3-SiO2
melt
structure ↔ flow Na2O-Al2O3-SiO2
melt +CaO +TiO2 +P2O5
+H2O +F +Cl
all of which are known to produce anomalous viscosity and structure
Solved Hypothesis
presented here
Unsolved
Vesuvius
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Dietzel AZ, Die Kationenfeldstirken und ihre Beziehungen zu Entglasungsvorgangen, zur
Dietzel AZ, Die Kationenfeldstirken und ihre Beziehungen zu Entglasungsvorgangen, zur