Oxide solution mechanisms in silicate melts; systematic variations in the activity coefficient of SiO2

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doi: 10.1016/0016-7037(85)90159-0
Authors:Ryerson, F. J.
Author Affiliations:Primary:
Univ. Calif., Lawrence Livermore Natl. Lab., Livermore, CA, United States
Volume Title:Geochimica et Cosmochimica Acta
Source:Geochimica et Cosmochimica Acta, 49(3), p.637-649. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037
Publication Date:1985
Note:In English. 77 refs.; illus.
Summary:Phase equilibria and spectroscopic data are used to develop a simple model for the interaction of various oxide components and molten SiO2. Network-modifying oxides, MxOy produce non-bridging oxygens thereby depolymerizing the SiO2 network. The energetics of non-bridging oxygen formation are least favourable when the field strength of the metal cation is high. This produces relatively strong M-O and Si-O-Si bridging bonds at the expense of weaker Si-O-M bonds. This relationship is manifested by an increase in positive deviations from ideality with increasing cation field strength in MxOy-SiO2 systems; the activity coefficient of SiO2 is inversely correlated with Si-O-M bond strength. Network-forming oxides (aluminates, phosphates, titanates, zirconates, etc.) may copolymerize with the SiO2 network. Mixing on the same quasi-lattice produces solutions which approach ideality. Deviations from ideality in such solutions can be linked to distortions in the SiO2 network. Discrete anion formers (phosphates, titanates, chromates, zirconates) complex with metal oxides other than SiO2 to form discrete structural units which do not copolymerize with SiO2. The SiO2 network is essentially shielded from the high charge density cations in such systems and unmixing is common. As a result, the relative deviations from ideality in such melts are high. It is important to recognize that oxides such as P2O5, TiO2 and ZrO2 may act as either network-formers or discrete anion formers, depending upon melt composition, and are probably distributed between these two 'sites' in most geologically important liquids. The latter structural role is favoured in more basic compositions. (Author's abstract) [C.M.B.H.]
Subjects:Bonding; Chain silicates; Clinoenstatite; Clinopyroxene; Coexisting minerals; Coordination; Cristobalite; Crystal chemistry; Experimental studies; Forsterite; Framework silicates; Ion exchange; Lattice; Melts; Mineral assemblages; Mixing; Nesosilicates; Olivine group; Order-disorder; Orthosilicates; Partitioning; Phase equilibria; Pyroxene group; Silica minerals; Silicates; Substitution; Tridymite; Reactions
Abstract Numbers:86M/0632
Record ID:1985049612
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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