Silica solubility and molecular speciation in water vapor at 400–800 °C

IF 5 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Geochimica et Cosmochimica Acta Pub Date : 2025-10-01 DOI:10.1016/j.gca.2024.10.022

Andri Stefánsson , Samuel W. Scott , Anna B. Bjarkadóttir , Adolph Jr. Manadao Bravo , Sigríður M. Aðalsteinsdóttir , Erlend Straume , Kono H. Lemke

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Abstract

Silica solubility and molecular speciation in hydrothermal water vapor have been determined through quartz solubility experiments at 400–800 °C and 50–270 bar using a novel U-tube flow-through reactor system and theoretical calculations. The results demonstrate that silica concentrations are low in water vapor (m_Si,tot = 0.11–4.56 mmol/kg or x_Si,tot = 8.21 × 10⁻⁵–1.98 × 10⁻⁶ mol/mol) increase with both temperature and pressure, which is attributed to the dissolution of quartz according to the reaction:

SiO_2(s) + (n + 2)H₂O_(g) ⇋ Si(OH)₄·(H₂O)_n_(g)

Thermodynamic modeling and theoretical calculations employing density functional theory (B3LYP-D3), and MP2 ab initio calculations reveal the stable structures to be Si(OH)_4(g), Si(OH)₄·(H₂O)_2(g), Si(OH)₄·(H₂O)_4(g) and Si(OH)₄·(H₂O)_7(g) (n = 0, 2, 4, 7) under the temperature and pressure conditions of interest, with higher-order hydrated structures also present at the lowest temperatures and highest pressures. Various isomers of the gaseous silica species were identified, with the number of energetically favorable structures increasing with hydration level and the motifs shifting from silanol-water bonds to complex water-water networks. Over the temperature range of interest, the logarithm of the quartz equilibrium solubility constant (logK_n) rises from −7.40 to −6.55 and −12.23 to −11.65 at 400 to 800 °C for the formation of Si(OH)_4(g) and Si(OH)₄·(H₂O)_2(g), respectively, and decreases from −16.20 to −19.15 and –22.61 to −28.74 for Si(OH)₄·(H₂O)_4(g) and Si(OH)₄·(H₂O)_7(g) at the same temperature range, respectively. Standard thermodynamic properties were derived based on the experimental results, revealing temperature-independent enthalpy (

Δ H_{n, r}^{o}

), entropy (

Δ S_{n, r}^{o})

and heat capacity (

Δ C_{p, n, r}^{o}

) of reaction for each gaseous silica species. The enthalpy of the reaction is nearly constant, whereas the entropy and heat capacity decrease with increasing hydration, resulting in higher-level hydrated species becoming less important with increasing temperature. Our quartz solubility results are in good agreement with previous experimental data and thermodynamic equations, as well as the thermodynamic properties of Si(OH)_4(g) at 25 °C and 1 bar.

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400-800 ℃水蒸气中二氧化硅的溶解度和分子分级

在 400-800 °C 和 50-270 bar 温度条件下，利用新型 U 形管流过式反应器系统和理论计算，通过石英溶解度实验确定了热液水蒸汽中二氧化硅的溶解度和分子分级。结果表明，水蒸气中的二氧化硅浓度较低（mSi,tot = 0.11-4.56 mmol/kg 或 xSi,tot = 8.21 × 10-5-1.98 × 10-6 mol/mol），随着温度和压力的升高而增加，这归因于反应过程中石英的溶解：

本文章由计算机程序翻译，如有差异，请以英文原文为准。

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来源期刊

Geochimica et Cosmochimica Acta 地学-地球化学与地球物理

CiteScore

9.60

自引率

14.00%

发文量

437

审稿时长

6 months

期刊介绍： Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.