The structure of silicate melts: Implications for chemical and physical properties of natural magma

Abstract

The structure of melts in binary metal oxide‐silica systems may be described in terms of monomers, dimers, chains, sheets, and three‐dimensional network structures. For the bulk compositions between orthosilicate and tectosilicate, three well‐defined ranges may be distinguished. For bulk nonbridging oxygen per silicon (NBO/Si) of about 2 or less, monomers, dimers, and chains coexist. In the range between metasilicate and disilicate there is usually a combination of monomers, chains, and sheets. Sheets are, however, uncommon or absent in systems where the field strength of the metal cation exceeds that of Mg2+. In those cases, monomers, chains, and three‐dimensional network units coexist. In the bulk compositional range of NBO/Si ≳1, sheets, chains, and three‐dimensional network structures coexist. In all systems the cations of high field strength show a preference for the most depolymerized structural units. Aluminum and probably ferric iron are tetrahedrally coordinated when charge balanced by a monovalent or divalent cation. Aluminate complexes thus formed show a preference for the most polymerized structural units in the melt. The degree of preference increases with decreasing field strength of the charge‐balancing cation. Ferrite complexes may form separate (MFe)4+‐O or (M0.5Fe)4+‐O clusters in the melts. Titanium and phosphorus are always tetrahedrally coordinated. These cations do not substitute significantly for Si4+ in tetrahedral coordination, but form separate clusters. The anionic structural model described above is consistent with viscosity and expansivity data for melts on binary metal oxide‐silica joins. The phase equilibrium data, such as the position of liquidus boundaries between mineral phases of different degress of polymerization on binary metal oxide‐silica joins, may be explained with the melt structure model. The observed expansion of immiscible liquid volumes on MO‐SiO2 joins with increasing field strength of the M cation is in accord with the enhanced stability of three‐dimensional network units in the melts as a function of increased field strength of the metal cation. Most volatile‐free natural magmatic liquids will contain chain, sheet, and three‐dimensional structural units. The proportion of sheet units in magmas with the same ratio of nonbridging oxygens to tetrahedral cations will decrease with increasing M2+/M+. The proportion of three‐dimensional structural units increases at the expense of chain and sheet units as the magma becomes more acidic. On the basis of the observed relationships between melt structure and physical properties the decreased compressibility and viscosity of basic magma compared with acidic magma may be explained. Application of this structural model to natural magma also explains why the pressure dependence of the viscosity of basic magmas is smaller than that of andesitic magmas.

Network structure and thermal stability study of high temperature seal glass

“…18 For data analysis, the Raman spectra were fitted to Gaussian bands without any restriction to deconvolute the superimposed Raman peaks. 19,20 The curve fitting was done with a GRAMS/AI ͑7.02͒ software ͑Themo Fisher Scientific, Inc. Waltham, MA͒.…”

Heat capacity, configurational heat capacity and fragility of hydrous magmas

“…Several studies performed on silicate glasses and liquids have demonstrated that anhydrous chemical composition and water, the most abundant volatile in magmatic systems, strongly affect the structure (e.g., Mysen et al, 1982;Stolper, 1982a,b;Mysen, 1997;Mercier et al, 2009;Xue, 2009), the physical properties (viscosity, diffusivity, density, heat capacities; Persikov et al, 1990;Ochs and Lange, 1999;Whittington et al, 2000Whittington et al, , 2009Romano et al, 2001;Bouhifd et al, 2006;Vetere et al, 2007;Behrens and Zhang, 2009;Giordano et al, 2009;Fanara et al, 2012;Di Genova et al, 2013, 2014, the phase relationships and crystallization behavior of magmas (Fenn, 1977;Muncill and Lasaga, 1988;Davis et al, 1997;Vona and Romano, 2013). The isobaric heat capacity (Cp) is a thermodynamic property related to variations in the internal energy in a system and to structural changes of the silicate framework in response to temperature variations.…”

Temperature-dependent structural heterogeneity in calcium silicate liquids

“…[4][5][6][7] Silica is classified as a strong liquid with an Arrhenius viscosity-temperature relation and only subtle differences occur between the liquid and glass structures. 8,9 In contrast CaO-SiO 2 liquids can be considered fragile based on viscosity measurements 10,11 and consequently larger structural rearrangements are likely to be necessary to form a glass. In this study we combine containerless aerodynamic levitation, high-energy x-ray diffraction and molecular-dynamics ͑MD͒ simulations to interpret and follow the structural evolution of calcium silicate liquids from high temperatures to the glassy state.…”