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Structure and Rheology of High Silica Melts 

William C. Wilson 

Introduction 

High silica melts are those with silica melts with silica in excess of approximately 58% silica in the bulk composition. They are those melts that contain significant free quartz as a modal phase or that would show significant quartz in norm calculations. In general the represent the granodiorites and granites and the aphanitic equivalents of these rocks as well as many granitiod migmatites and migmatitic gneisses. High silica melts are rich in network forming chemical species and in volatiles that tend to reduce the viscosities of the melt. In many cases they are rich in crystallites and larger xstals either as retained xenocrysts or as retained portions of higher temperature less silica rich source melts. 

These multiple interacting components have significant impacts on the structures of the melts and on the rheological properties. The primary problems deal with the extent to which the network forming species polymerize acting to bind the melt itself into a quasi-crystalline liquid/glass structure, the impact of crystal fractions on the general structure and rheology of the melt, the interactions between the crystal fractions and the liquid networks, the effects of temperature, pressure and volatiles on the melt as conditions change during both melting and crystallization. This paper will attempt to summarize what is known of these different interactions and effects. Depending on volatile content high silica melts and magmas can vary in temperature and pressure conditions from approximately 600 degrees C up to over 1200 degrees C and pressures from 1 bar to over 10 kilobars. With crystal fractions from 0% to effectively 100% 

Network Formers 

the primary constituents of these melts are network forming species, crystals and crystallites (<1mm) , depolymerizing volatiles and cations, and minor phases and trace elements . 

The main network forming species is silica which forms linkable (polymerizable) SiO4 tetrahedra. The primary species involved in the formation of these units are silica (SiO4-4) and charge balanced Alumina (XalO4-4) units. Additional minor network formers are Fe+3 and P+5 based units . These units polymerize to form the rings, chains, sheets, and networks of the crystals and polymerized fluid units. Withing the melt fluids these units are believed to be undergoing a continuous polymerization-depolymerization process as the melt moves and crystalizes. Based on a variety of spectral studies these tetrahedra are believed to form units up to 200 angstroms across consisting of one or more three dimensional structures resembling the 4 and 6 silica rings or the Crystabolite or tridymite structures (Mysen, 1990). These fine scale structures can be grouped into 2 general but not mutually exclusive models. 

Diagram 1: general tetrahedral components 

for silicate melts from Mysen, 1990 


 
 
 
 
 
 
 
 
 
 
 

Random network Model 

This model described the melt as a series of essentially random polymerized units that form and break apart over short periods. These units tend to form structures that resemble the Crystabolite and or tridymite structures. My impression is that this model ( the first proposed and based on mostly high temperature studies) represents the sort of structure to be found in high temperature crystal free melts especially. Some of the spectroscopic work suggest that rather than a single species of Crystabolite/tridymite structure 2 units may be present - a 4 tetrahedral ring and a 6 tetrahedral ring. 

Crystallite Model 

This model represents the other end of the model spectrum. In the crystallite model the melt is believed to be formed of small incipient crystallites up to 200 angstroms across that interact in the melt and can form incipient true small crystals given proper conditions . In fact some combination of the two models is probably most correct with the melt perhaps tending more towards a random network at high temperatures and a crystallite form at lower near solidus temperatures. 

Diagram 2: Silicate Melt Structure Models, from Brown et al, 1995 


 
 
 
 
 
 
 
 
 
 
 

Crystals and Crystallites: 

Crystals and crystallites frequently form a major part of high silica melts and are present in almost all melts to some extent. In high silica melts the primary crystals are feldspars with mafic phases (primarily amphiboles and micas) being the second most common. As fully polymerized structures these crystals can act as links in the polymer structure of the melt and as well as functioning as suspended solids in a fluid . The percentage of crystals in a melt has a significant effect splitting the rheology of the melts into three major fields. One from 0 to the critical melt fraction where the crystals begin to oppose the shear deformations, a second from the critical melt fraction to the point at which the crystals form a supporting network that opposes significant mass flow with in the melt, and a third from this point to full crystallization during which the remaining melt flows in interstitial pores or in dikes and cracks in the developing rock (Lejeune and Richet 1995). In most studies this critical melt fraction is taken as the percentage of macroscopic crystals in the rock. Based on this the critical melt fraction is estimated to l ranger from 25% to 50% of the rock volume as crystals (Barboza and Bergantz, 1996). However this frequently leaves out the the crystallites in the ground mass, when these are included they can increase the percentage of crystals in the rock by as much as 18% (Geschwind and Rutherford, 1995). Additionally little is know of the effects of multiple crystal sizes or crystal shape and composition on the structure and rheology of melts. In regime 3 (grain supported mushes) the interfacial angles of the crystals can have a significant effect on melt mobility. If the interfacial angles remain below 60 degrees then the structure remains porous and it is possible for melt fractions to move and for minor phase to diffuse thru the porous structure. If the interfacial angles Become greater than 60 degrees the pore spaces begin to occlude and melt fractions can rapidly become isolated. (Bryon et al, 1996). This may also happens in the very latest stages of crystallization or the earliest stages of melting. 

Diagram 3: from Bryon et al, 1996 


 
 
 
 
 
 
 
 
 
 
 
 

Network Modifiers 

The presence of volatiles and non network forming species in the melt act to modify the silicate networks within the melt. Their presence acts to reduce the numbers of Si-O-Si (or related) bridges with in the networks by bonding with the oxygens and ending the polymer chains . The primary network modifiers are the volatiles ( OH-, Cl-,F-, SO42-,CO2,H2O). As the volumes of these increase the polymerization of the melt decreases and the viscosity of the melt decrease. In peralkaline rocks (CaO+Na2O+K2O>AL2O3) the excess Alkalis also act to modify and depolymerizer the silicate networks reducing the viscosities. In non peralkaline rocks the alkalis act only to charge balance the Alumina and show no effect on the network structure (Wolf and McMillan, 1995). The degree of polymerization can be measured by determining the average number of Non-bridging Oxygens (NBOs) in each Tetrahedra. A Non-bridging Bridging oxygen is one not involved in a T-O-T bridge between tetrahedra (T= Si, AL, Fe3+). This number varies from 0 (tectosilicates) to 4 (Orthosilicates) and normally ranges from 0 to 1 in high silica melts with the number increasing in more mafic melts (Mysen, 1990). 

The effects of volatiles can be extreme reducing the viscosity of the melts by several orders of magnitude. 

Diagram 4: from Holtz et al, 1996 Effects of water on melt viscosity 


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Diagram 5: from Hess et al, 1995 Effects of excess alkali on melt viscosity 


 
 
 
 
 
 
 
 
 
 
 
 
 

As can be seen in diagram 4 at constant temperature increases in the water/OH- percentage have significant effects on melt viscosity with decreases of as much as 4 orders of magnitude as water concentrations change from less than 1% to over 4% water in the melt. In diagram 4 we see the effects of excess alkali in the melt. As can be seen even small excesses can have large effects on the viscosity representing significant changes in polymerization.. 

Fluorine and Chlorine to a lesser extent, act in a manner similar to H2O/OH- reducing viscosity and polymerization even more strongly. 

CO2 is believed to have a similar effect but little work has been done on its effects on melt rheology. 

Diagram 6 : From Hess et al, 1995; effects of alkalies and volatiles 


 
 
 
 
 
 
 
 
 
 
 
 
 

Effects of Pressure and Temperature 

Pressure affects the solubilities of volatiles with a consequent reduction in polymerization and viscosity. This is partially counteracted by pressure created increases in density. While Low silica melts have normal behaviors with pressure increasing in density with pressure in some cases High silica melts may actually decrease in density as pressure increases. (Wolf and McMillian, 1995)at lower crust - mantle pressures evidence from spectroscopic studies suggests that some of the tetrahedrally coordinated Silica and Alumina are converted to 5 and 6 coordinated forms which acts to reduce the polymerization of the melt. 

Temperature affects melts by controlling the average energy levels of the species in the melts and minimizing the activation energies for the reactions between tetrahedra and other species. Melt flow is believed to occur through the continuous forming and breaking of T-O-T bonds between polymerized units and crystals in the melt. This action occurs with greater frequency as temperatures rise. At the same time the sizes of the silicate "liquid crystals" is thought to shrink with increasing temperature. At the low end of the temperature regime of the melts the melt may cross the glass transition so that the structures are "frozen" into the material as the relaxation times and reaction rates become equal to or greater than the measurement times. During cooling the changes in polymerization and viscosity that occur can amount to as many as 10 orders of magnitude change in the viscosity of a melt. During melting reactions similar but reverse changes occur . These are often offset during the early stages of melting at low temperatures by the presence of depolymerizing volatiles which act to provide the viscosity reductions needed to allow melt segregations via porous flow along grain boundaries and in interstitial pores and fractures or micro dikes. Dehydration melting of muscovites and biotites also act to maintain a volatiles phase that helps to keep low temperature partial melts sufficiently non viscous enough to melt and flow. 

Conclusions

High silica melts show a range of structures and viscosities that can cover over 10 orders of magnitude from super-liquidus conditions through the glass transition and crystalized states. The primary controls on the structures of these melts are the bulk compositions, temperatures, pressures during or at crystallization and extent of the crystal fractions during movement. High temperature low silica melts behave in generally well described ways while with increasing silica the melt becomes more viscous as more and more of the melt becomes polymerized. Volatiles and excesses of alkalis over alumina act to depolymerize high silica melts reducing their viscosities and enabling more rapid flow and emplacement. Extreme pressures can act to depolymerize melts by converting some of the tetrahedrally coordinated silica and alumina into 5 or 6 coordinated forms. Crystal fractions affect the properties by resisting shear deformation and providing bases for polymerized links to attach in the melt rich fluids and by providing a porous network for melt and chemical species within the melt to flow or diffuse from one location to another. However large interfacial angles (>60 degrees) represent a near collapse of the grain supported structure that can act to either press out or lock melt fractions from one another within a crystal - melt mush at lower temperatures it may also be impossible to initiate the types of grain boundary movements needed to bath minerals in new solutions continuously. This may have an impact on grain compositions resulting in Zoning of the crystals that form. 

References

Computer simulations of silicate melts. Poole-P-H; McMillan-P-F; Wolf-G-H In: Structure, dynamics and properties of silicate melts. 

Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; Pages 563-616. 1995. Mineralogical Society of America. Washington, DC, United States. 

Pressure effects on silicate melt structure and properties. Wolf-G-H; McMillan-P-F In: Structure, dynamics and properties of silicate melts. 

Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; Pages 505-561. 1995. Mineralogical Society of America. Washington, DC, United States. 

Diffusion in silicate melts. Chakraborty-S In: Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; Pages 411-503. 1995. Mineralogical Society of America. Washington, DC, United States. 

Rheology and configurational entropy of silicate melts. Richet-P; Bottinga-Y In: Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; Pages 67-93. 1995. Mineralogical Society of America. Washington, DC, United States. 

Viscoelasticity Webb, S.L. and Dingwell, D.B. In: Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; 1995. pp. 95-120 Mineralogical Society of America. Washington, DC, United States. 

Energetics of Silicate melts Navrotsky,A. In: Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; 1995. pp. 121-145 Mineralogical Society of America. Washington, DC, United States. 

Dynamics and Structure of silicate and oxide melts: Nuclear Magnetic Resonance Studies Stebbins, J. F. In: Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; 1995. pp. 191-246 Mineralogical Society of America. Washington, DC, United States. 

Vibrational Spectroscopy of Silicate liquids McMillan, P. F. and Wolf, G. H. In: Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; 1995. pp.247-316 Mineralogical Society of America. Washington, DC, United States. 

Structure, dynamics and properties of silicate melts. Stebbins-Jonathan-F (ed.); McMillan-Paul-F (ed.); Dingwell-Donald-B (ed.) Reviews in Mineralogy. 32; 1995. Mineralogical Society of America. Washington, DC, United States. Pages: 616. 

Rheology of crystal-bearing silicate melts; an experimental study at high viscosities. Lejeune-Anne-Marie; Richet-Pascal Journal of Geophysical Research, B, Solid Earth and Planets. 100; 3, Pages 4215-4229. 1995. American Geophysical Union. Washington, DC, United States. 

Relationships between silicate melt structure and petrologic processes Mysen, Bjorn O. Earth Science Reviews 27 pp 281-365 1990 Elsevier Science Press BV, Amsterdam 

Melt movement and the occlusion of porosity in crystallizing granitic systems Bryon, D.N., Atherton, M.P., Cheadle, M.J., Hunter,R.H. Mineralogical Magazine 60 pp 163-171 1996 Mineralogical Society 

Crystallization of microlites during magma ascent: the fluid mechanics of 1980-1986 eruptions at Mount St. Helens Geschwind, C.H.,; Rutherford, M.J. Bulletin of Vulcanology 57, pp 356-370 1995 Springer Verlag 

The influence of excess alkalis on the viscosity of of a haplogranitic melt Hess, K.U., Dingwell, D.B., Webb, S.L. American Mineralogist 80: pp 297-304 1995 

Viscosity of Himalayan leucogranites: Implications for mechanisms of granitic magma implacement Scaillet, B.; Holtz, F.;Pichavant, M.;Schmidt, M. Journal of Geophysical Research: 101-B12 pp 27,691 - 27,699 American Geophysical Union 

The Second Hutton Symposium on the origin of Granites and Related Rocks Brown, P.E.; Chappell,B.W. (ED) Geological Society of America Special Paper 272 ,1992  Geological Society of America 

The Nature and Origin of Granite Pitcher, Wallace S. Blackie Academic and Professional Publishers, London 1993 

Magmatic Systems Ryan, Michael P. (Ed) Academic Press, San Diego 1994 

The Third Hutton Symposium on the origin of Granites and Related Rocks Brown, M.; Candela, P.A.; Peck, D.L.; Stephens, W..E; Walker, R .J.; Zen, E-an (Ed) Geological Society of America Special Paper 315 ,1996 

Effects of pressure and H20 on the composition of primary crustal melts Douce, A. E .P. IN: The Third Hutton Symposium on the origin of Granites and Related Rocks Brown,M.; Candela, P.A.; Peck, D.L.; Stephens, W .E; Walker, R .J.; Zen, E-an (Ed) Geological Society of America Special Paper 315 ,1996 Geological Society of America 

Dynamic model of dehydration melting motivated by a natural analogue: applications to the Ivrea-Verbeno zone, northern Italy Barboza, Scott A and Bergantz, George W. IN: The Third Hutton Symposium on the origin of Granites and Related Rocks Brown,M.; Candela, P.A.; Peck, D.L.; Stephens, W .E; Walker, R .J.; Zen, E-an (Ed) Geological Society of America Special Paper 315 ,1996 Geological Society of America 

Water contents of felsic melts: applications to the rheological properties of granitic melts Holtz, F.; Scaillet, B.; Behrens, H.; Schulze, F.; Pichavant, M. IN: The Third Hutton Symposium on the origin of Granites and Related Rocks Brown,M.; Candela, P.A.; Peck, D.L.; Stephens, W .E; Walker, R .J.; Zen, E-an (Ed) Geological Society of America Special Paper 315 ,1996 Geological Society of America 

Granite and granitic pegmatite melts: volumes and viscosities Dingwell, D. B.;Hess, K.-U. And Knoche, R. IN: The Third Hutton Symposium on the origin of Granites and Related Rocks Brown,M.; Candela, P.A.; Peck, D.L.; Stephens, W .E; Walker, R .J.; Zen, E-an (Ed) Geological Society of America Special Paper 315 ,1996 Geological Society of America 

Compositional variation within granite suites of the Lachlan Fold Belt: its causes and implications for the physical state of granite magma IN: The Third Hutton Symposium on the origin of Granites and Related Rocks Brown ,M.; Candela, P.A.; Peck, D.L.; Stephens, W .E; Walker, R .J.; Zen, E-an (Ed) Geological Society of America Special Paper 315 ,1996 Geological Society of America 
 



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