Dissolution mechanisms of water in depolymerized silicate (peridotitic) glasses based on infrared spectroscopy

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Bondar, Dmitry ; Withers, Anthony ; Whittington, Alan G. ; Fei, Hongzhan ; Katsura, Tomoo:
Dissolution mechanisms of water in depolymerized silicate (peridotitic) glasses based on infrared spectroscopy.
In: Geochimica et Cosmochimica Acta. Bd. 342 (2023) . - S. 45-61.
ISSN 0016-7037
DOI: https://doi.org/10.1016/j.gca.2022.11.029

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Abstract

The dissolution of water changes the chemical and physical properties of melts. The effect of water dissolution depends on the bulk H2O content and dissolution mechanisms. Previous research mainly focused on relatively polymerized glasses, whereas dissolution mechanisms of water in highly depolymerized glasses have not been well studied. We investigated three sets of hydrous (up to 5 wt H2O), highly depolymerized glasses using Fourier transform infrared spectroscopy. The three suites of glasses can be classified as tephritic (non-bridging oxygens to tetrahedral cations, NBO/T ≈ 0.9), basanitic (NBO/T ≈ 1.5), and peridotitic (NBO/T ≈ 2.5). The comparison of infrared spectra indicated four prominent features. First, the combination band related to molecular H2O at ∼ 5200 cm−1 decreases in relative intensity with increasing depolymerization. At 2 wt bulk H2O, the proportion of H2O molecules decreases from the tephritic through basanitic to peridotitic glasses, where the proportion of H2O molecules is below 10 . At 5 wt bulk H2O, the proportion of H2O molecules in the peridotitic glass is around 20 , whereas it is around 50 in the basaltic glass with the same H2O content. Second, a second peak appears at 4270–4180 cm−1, next to the (Si,Al)OH combination peak (network hydroxyl), with increasing CaO and MgO contents in basanitic (∼25 wt CaO) and peridotitic (∼40 wt MgO) glasses, respectively. We assign this peak to the (Mg,Ca)OH group (free hydroxyl); this is the first identification of this group based on infrared spectroscopy. This assignment is also supported by IR data on Mg(OH)2 and Ca(OH)2 crystalline phases from the literature. Third, the ratio of (Si,Al)OH and (Mg,Ca)OH peak absorbances remains nearly constant with increasing water content in basanitic and peridotitic glasses, suggesting no effect of water on polymerization of extremely depolymerized melts. Fourth, the (Si,Al)OH peak shifts to lower wavenumbers with increasing depolymerization. As a result, the (Si,Al)OH peak position could potentially be used to evaluate the degree of melt polymerization in general, and the effect of water on melt polymerization in particular.