A thermodynamic model for the system SiO2-H2O near the upper critical end point based on quartz solubility experiments at 500-1100 °C and 5-20 kbar
A thermodynamic model for the system SiO2-H2O near the upper critical end point based on quartz solubility experiments at 500-1100 °C and 5-20 kbar
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DOI：
10.1016/j.gca.2012.03.006
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发表时间：
2012-06-01
影响因子：
5
通讯作者：
Manning, Craig E.
中科院分区：
文献类型：
--
作者：
Hunt, Jonathan D.;Manning, Craig E.
基于石英在水中溶解度的新的和现有的实验数据，建立了SiO2-H2O在亚超临界流体中混合的热力学模型。为了补充以前发表的数据，我们使用热液活塞-气缸方法在15和20 kbar和900-1100 ℃下进行了新的溶解度实验。在低于类似于10mol%SiO2的浓度下，通过单晶重量损失测量溶解度。在较高浓度下，通过使用具有不同SiO2-H2O比率的多个等温和等压运行在淬火电荷中包围石英的存在和不存在来确定溶解度。这些数据与以前发表的结果相结合，构建SiO2-H2O混合的热力学模型。在硅酸盐熔体的研究之后，该模型将流体中的氧以三种形式存在：自由的、分子H2O、Si桥氧（O-br（2-））和硅烷醇基团的末端羟基（OHtm-）。氧在这些形式之间的平衡交换可以写作1/2 H（2）O + 1/2 O（br）（2-）= OHtm-。该反应的标准吉布斯自由能变化（Δ G度）被纳入到SiO2液体和H2O流体混合的亚规则溶液模型中。Δ G度和交换能的P-T依赖性由误差最小化算法导出，产生十三个独立的拟合参数。该模型适用于5 - 20 kbar和500 ℃的石英干熔融曲线。它重现实验得出的石英溶解度数据，平均为3.8%（1西格玛= 5.3%）。该模型还预测含水熔融石英，临界熔体-蒸汽混合，活性-浓度关系，偏摩尔体积和熵的含水二氧化硅，水的形态，和热膨胀系数，等温压缩性，和等压热容量的流体与石英平衡。该模型预测了SiO2-H2O系统在1067 ℃和9.33 kbar下的临界终点，与类似于1080 ℃和9.5-10 kbar的公认位置非常一致。该模型也是在良好的协议与以前的估计二氧化硅聚合的程度。这项研究的结果清楚地表明，有一个明确的聚合化学和硅酸盐-H2O溶液的临界混合之间的联系。(C)2012爱思唯尔有限公司保留所有权利。
A thermodynamic model of SiO2-H2O mixing in sub- and supercritical fluids has been developed based on new and existing experimental data on the solubility of quartz in H2O. To supplement previously published data, we conducted new solubility experiments at 15 and 20 kbar and 900-1100 degrees C using hydrothermal piston-cylinder methods. At concentrations below similar to 10 mol% SiO2, solubility was measured by single-crystal weight loss. At higher concentrations, solubility was determined by bracketing the presence and absence of quartz in quenched charges using multiple isothermal and isobaric runs with varying SiO2-H2O ratios. These data were combined with previously published results to construct a thermodynamic model of SiO2-H2O mixing. Following studies of silicate melts, the model takes oxygen in the fluid to be in three forms: free, molecular H2O, Si-bridging oxygens (O-br(2-)), and the terminal hydroxyls (OHtm-) of silanol groups. The equilibrium exchange of oxygen between these forms can be written 1/2H(2)O + 1/2O(br)(2-) = OHtm-. The standard Gibbs free energy change of this reaction (Delta G degrees) was incorporated into a subregular solution model for mixing of SiO2 liquid and H2O fluid. The P-T dependences of Delta G degrees and interchange energies were derived by an error minimization algorithm, producing thirteen independent fit parameters. The model is applicable from 5 to 20 kbar and 500 degrees C to the dry melting curve of quartz. It reproduces experimentally derived quartz solubility data to 3.8% on average (1 sigma = 5.3%). The model also predicts hydrous melting of quartz, critical melt-vapor mixing, activity-concentration relations, partial molar volume and entropy of aqueous silica, water speciation, and the thermal expansivity, isothermal compressibility, and isobaric heat capacity of a fluid in equilibrium with quartz. The model predicts a critical end point in the SiO2-H2O system at 1067 degrees C and 9.33 kbar, in very good agreement with the accepted location at similar to 1080 degrees C and 9.5-10 kbar. The model is also in good agreement with previous estimates of the extent of silica polymerization. The results of this study clearly demonstrate that there is an explicit link between polymerization chemistry and critical mixing of silicate-H2O solutions. (C) 2012 Elsevier Ltd. All rights reserved.