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 < ∼ 4.0, the equilibrium is of the type Si 2 O 7 6– ⇄ SiO 4 4- + SiO 3 2- . In melts with NBO/Si ∼ 1.0 to 2.0, the equilibrium anionic species are given by 3SiO 3 2– ⇄ SiO 4 4- + SiO 5 2- . 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 Si 2 O 5 2– ⇄ SiO 3 2- + SiO 2 . In multicomponent melts with compositions corresponding to those of the major igneous rocks, the anionic species are T O 2 , T 2 O 5 , T 2 O 6 , and T O 4 , and the coexisting polymeric units are determined by the second and third of these disproportionation reactions.