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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