Abstract Detailed thermal experiments using relative humidity- (RH) and temperature-controlled X-ray diffraction (XRD) and H2O adsorption/desorption measurements were applied to understand the dehydration/hydration behaviors of thenardite/mirabilite (Na2SO4/Na2SO4·10H2O), tincalconite/borax (Na2B4O7·5H2O/Na2B4O7·10H2O), nahcolite (NaHCO3), and qilianshanite (NaH4 (BO3) (CO3)·2H2O or NaHCO3·H3BO3·2H2O). The thermal behaviors of thenardite/mirabilite and tincalconite/borax were evaluated at room temperature as a function of RH. Hydration of thenardite to mirabilite is very sluggish and does not begin until at least 85 % RH, whereas dehydration of mirabilite to thenardite occurs at ∼76 % RH. The reactions are accessible in the solid state and were not completely reversible on the time scale of these measurements. Qilianshanite appears stable under room conditions, as long as the RH is >20 %, supported by 2.5 months of observation during exposure to room air of known RH. Qilianshanite persists metastably for at least two days when exposed to both wet and 0% RH atmospheres at room temperature, observed during environment-controlled XRD. Below 20 % RH, qilianshanite sluggishly reacts to form tincalconite and nahcolite. When heated, qilianshanite broke down at 70 °C based on XRD results, and thermogravimetric analysis (TGA) confirmed a dehydration temperature of ∼75 °C. Tincalconite hydrated to borax at 80 % RH, and dehydration from borax to tincalconite occurred at 40–50 % RH, illustrating the effective reversibility of the reaction under room conditions. The minor phase nahcolite originally in qilianshanite remained in the residue of decomposed qilianshanite after heating to 125 °C, demonstrated by continuous XRD measurements, whereas nahcolite was largely dehydrated when heated to ∼100 °C during TGA. The difference can be attributed to the slower reaction kinetics in the environmental XRD. These results shed considerable light on the behavior of these minerals in the environment, particularly in evaporite deposits in Antarctica, where all of these minerals are reacting with the atmosphere on a diurnal-to-seasonal basis.

中文翻译：

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硫酸钠、硼酸盐和碳酸氢钠矿物的热力学和脱水/水化行为

摘要 使用相对湿度 (RH) 和温控 X 射线衍射 (XRD) 和 H2O 吸附/解吸测量的详细热实验用于了解芒硝/芒硝 (Na2SO4/Na2SO4·10H2O)、锡钙石的脱水/水合行为/硼砂（Na2B4O7·5H2O/Na2B4O7·10H2O）、钠沸石（NaHCO3）和祁连山石（NaH4（BO3）（CO3）·2H2O或NaHCO3·H3BO3·2H2O）。芒硝/芒硝和锡钙石/硼砂的热行为在室温下作为 RH 的函数进行评估。芒硝到芒硝的水合非常缓慢，直到至少 85% RH 才开始，而芒硝脱水到芒硝在 76% RH 时发生。这些反应可以在固态下进行，并且在这些测量的时间尺度上不是完全可逆的。只要相对湿度 >20%，祁连山石在室内条件下看起来很稳定，并在暴露于已知相对湿度的室内空气期间进行了 2.5 个月的观察。在环境控制的 XRD 中观察到，在室温下暴露于潮湿和 0% RH 气氛中时，祁连山铁亚稳态持续至少两天。低于 20 % RH，祁连山石反应缓慢，形成锡钙石和珠光石。加热时，根据 XRD 结果，祁连山石在 70°C 分解，热重分析 (TGA) 证实脱水温度约为 75°C。锡钙石在 80% RH 下水合为硼砂，并且在 40-50% RH 时从硼砂脱水为锡钙石，这说明了反应在室温条件下的有效可逆性。连续 XRD 测量表明，最初存在于祁连山石中的次要相纳科利石在加热至 125 °C 后仍保留在分解的祁连山石残渣中，而在 TGA 期间加热至约 100 °C 时，纳科利石大部分脱水。这种差异可归因于环境 XRD 中较慢的反应动力学。这些结果揭示了这些矿物质在环境中的行为，特别是在南极洲的蒸发岩沉积物中，所有这些矿物质都在昼夜到季节性的基础上与大气发生反应。