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.

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A structural model is proposed for the polymeric units in silicate melts quenched at 1 atmosphere. The anionic units that have been identified by the use of Raman spectroscopy are SiO(4)(4-) monomers, Si(2)O(7)(6-) dimers, SiO(3)(2-) chains or rings, Si(2)O(5)(2-) sheets, and SiO(2) three-dimensional units. The coexisting anionic species are related to specific ranges of the ratio of nonbridging oxygens to tetrahedrally coordinated cations (NBO/Si). In melts with 2.0 < NBO/Si < approximately 4.0, the equilibrium is of the type [See equation in the PDF file]. In melts with NBO/Si approximately 1.0 to 2.0, the equilibrium anionic species are given by [See equation in the PDF file]. In alkali-silicate melts with NBO/Si <~ 1.3 and in aluminosilicate melts with NBO/T < 1.0, where T is (Si + Al), the anionic species in equilibrium are given by [See equation in the PDF file]. In multicomponent melts with compositions corresponding to those of the major igneous rocks, the anionic species are TO(2), T(2)O(5), T(2)O(6), and TO(4), and the coexisting polymeric units are determined by the second and third of these disproportionation reactions.

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Abstract-The viscosities of two melts in the system Na2o-Feo_Fe203s~ have been measured as a function of oxidation state. The experiments were conducted by concentric-cylinder viscometry, on melts equilibrated with CO/COr gas mixtures in a vertical tube, gas-mixing furnace. Viscosity determinations were made during stepwise reduction and oxidation of the melts. "Fe Miissbauer spectra were obtained on quenched melt samples recovered during the viscometty experiments. In addition, a series of loop fusion experiments were performed at calibratedfOz values in order to relate viscosity determinations directly tojO*.The viscosities of acmite and NS4F40 (Na-tich and Si-poor relative to acmite) melts decrease with mduction of Fe in the melts, as nonlinear functions of F$+/ZFe, yielding a region of viscosity invariance at moderate to low values of Fes+/ZFe (~0.4). The 57Fe M&batter spectra of quenched melts as a fimction of Fe3"/ZFe ind&ate the presence of one (ne~ork-rn~~ng) ferrous species and two ferric species with ferric iron acting dominantly as a network-former in oxidixed melts and dominantly as a network-modifier in reduced melts.The presence of two ferric iron species produces a minimum in the degree of polymerization of the melt at intermediate values of Fe'+/ZFe: the region of viscosity invariance corresponds to this minimum. If viscosity is positively correlated with polymeriration for all values of Fe3*/2Fe then the viscosity of very reduced melts will increase with reduction, as the melt polymerizes.The effect of oxidation state on viscosity is large and illustrates that ferric iron should be considered as a separate component in calculation schemes for estimating the viscosity of natural magmas. I~ODU~ONA CHANGE OF the chemical potential of oxygen during igneous petrogenesis will, in general, result in a change in the oxidation state of the igneous melt expressed as the ferric-ferrous ratio. Accordingly, geochemists have long recognized the usefulness of ferric-ferrous ratios of minerals and melts for inferring redox trends in igneous rock series.More recently, the intluence of the ferric-ferrous ratio on the physical properties of silicate melts has received attention (e.g. viscosity, CUKIERMAN and UHLMANN, 1974; density, MO et al., 1982). Viscosity is a physical property of silicate melts that is central to the discussion of mass transfer within or between phases. Processes that involve mass transfer are, in turn, the fundamental mechanisms of igneous differentiation.Despite the observation that iron is a major constituent of igneous melts, current info~ation regarding the effect of redox equilibria on the viscosity of ironbearing melts is scarce. This scarcity of information was noted by BOTTINGA and WEILL (1972). These authors were forced to neglect the possible effects of oxidation state on viscosity in the development of their now widely used calculation scheme for estimating the viscosity of igneous melts. limited the precision of some of these data and complicate comparison and gene~~on of the res...

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