Anionic constitution of 1-atmosphere silicate melts: implications for the structure of igneous melts

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.