Tightly bound water in smectites
Tightly bound water in smectites
复制标题
DOI：
10.2138/am-2017-5918
复制
发表时间：
2017-05
影响因子：
3.1
通讯作者：
Artur Kuligiewicz;A. Derkowski
中科院分区：
文献类型：
--
作者：
Artur Kuligiewicz;A. Derkowski
摘要蒙脱石能够在100℃以上的温度下保持分子紧密结合水(TBW)，即使在长时间干燥后也是如此。TBW的存在影响了蒙脱石和富蒙脱石样品的稳定同位素比值和脱羟基行为，并对测量富粘土岩石的各种性质具有重要意义。对5种镁、钙、钠和铯交换形式的参考蒙脱石进行了不同的干燥处理，然后用精密热重分析法(TG)测定了其TBW含量。在非等温热失重实验中，用模型无关的方法测定了不同水分组分在1000℃以下脱除的活化能。此外，在这两种情况下，4A和13X沸石都被检测为明显的无OH参照物。在110°C干燥后，所有蒙脱石的每个层间阳离子仍含有多达3个水分子。蒙脱石中的TBW含量主要取决于等温干燥温度。在一定温度下，TbW含量随层间阳离子类型的变化顺序为：Mg>；Ca>；Na>；Cs。干燥时间和蒙脱石层装药量的影响可以忽略不计。由Friedman法测定的100℃以下的脱水的Ea在45-60kJ/mol范围内相当恒定。在90~180kJ/mo1范围内，脱氢表观活化能随反应程度的增加而增加，脱羟基表观活化能在159~249kJ/mo1范围内，与样品的八面体片层结构和层间阳离子密切相关。镁离子可以容纳甚至超过550°C的H2O分子，使其在脱羟基过程中可用，或者-用于地质规模的反应-将H2O传递到变质条件下。蒙脱石和阳离子(低Si/Al-)沸石的TBW含量与脱水Ea具有很高的相似性，说明蒙脱石中的TBW在阳离子键合和扩散特性上与沸石水极为相似。蒙脱石的最佳干燥方案是用低水合热的阳离子(如Cs)取代层间阳离子，并在样品缺铁的情况下在300°C下干燥样品。富铁蒙脱石应在200°C干燥，以避免在300°C以下发生脱羟基。
Abstract Smectites are able to retain molecular tightly bound water (TBW) at temperatures above 100 °C, even after prolonged drying. The presence of TBW affects the stable isotope ratios, the dehydroxylation behavior of smectites and smectite-rich samples and also has implications in measuring various properties of clay-rich rocks. Five reference smectites, in Mg-, Ca-, Na-, and Cs-exchanged forms were subjected to different drying protocols followed by the determination of TBW contents using precise thermogravimetric (TG) analysis. Activation energies (Ea) of the removal of different water fractions at temperatures up to 1000 °C were determined in non-isothermal TG experiments using model-independent methods. Additionally, 4A and 13X zeolites were examined in both cases as apparent OH-free references. After drying at 110 °C, all smectites still contained up to 3 water molecules per interlayer cation. The TBW contents in smectites were found to be primarily dependent on the isothermal drying temperature. For a given temperature, TBW contents decreased with respect to the type of interlayer cation in the following order: Mg > Ca > Na > Cs. The influence of the time of drying and the smectite layer charge were found to be negligible. The Ea of dehydration below 100 °C, as determined by the Friedman method, was quite constant within the 45–60 kJ/mol range. The Ea of TBW removal increased along with the degree of reaction from 90 to 180 kJ/mol, while the Ea of dehydroxylation was found in the 159–249 kJ/mol range, highly depending on the sample’s octahedral sheet structure and the interlayer cation. The Mg2+ cation can hold H2O molecules even beyond 550 °C, making it available during dehydroxylation or—for geologic-scale reactions—pass H2O to metamorphic conditions. High similarities between the TBW contents and the Ea of dehydration for smectites and cationic (low Si/Al-) zeolites lead to the conclusion that TBW in smectites is remarkably similar to zeolitic water in terms of cation bonding and diffusion characteristics. The optimal drying protocol for smectites is to substitute interlayer cations with cations of a low-hydration enthalpy, such as Cs, and to dry a sample at 300 °C, provided that the sample is Fe-poor. Fe-rich smectites should be dried at 200 °C to avoid dehydroxylation that occurs below 300 °C.