三种水合氯化镁（十二水、八水、六水氯化镁）在类火星温湿条件下的稳定性质研究

Various salts have been found on Mars，thus the study of salts becomes essential for Mars science. To date, the observations reveal that the chloride bearing materials are widely distributed on Mars. Magnesium chloride, which is one of the common chlorides on Earth, may also exist on Mars. Low temperature phases of hydrated magnesium chlorides have high structural water content, thus may contribute an essential part to the martian subsurface water. Magnesium chloride has low eutectic point (-33.65℃), thus it might be one of the major chlorides that supposed to cause the morphological feature Recurring Slope Lineae (RSL) recently observed on the martian surface. The stability conditions (relative humidity and temperature, RH-T) for hydrated magnesium chlorides can be used to evaluate 1) the feasibility for their occurrence on current martian surface and subsurface; 2) which phase can occur under current martian conditions; and 3) where these hydrated magnesium chlorides can exist. On the basis of the evaluation in the previous step, the significance for their contribution to the martian subsurface water content can be discussed. The transition pathways of hydrated magnesium chlorides, RH-T conditions for the solid-solution transitions and for the rehydration of hydrated magnesium chlorides are critical for understanding the formation mechanism of RSL on Mars. Previous studies mainly focus on the solubility of magnesium chloride at different temperatures, and to our knowledge, little work has been done on their RH-T stability properties. In this project, we will use the humidity buffer method to obtain 1) the transition pathways of three hydrated magnesium chlorides (MgCl2o6H2O, MgCl2o8H2O and MgCl2o12H2O); 2) the phase boundaries between three hydrated magnesium chlorides, as well as 3) the phase boundaries between the three hydrated magnesium chlorides and their solutions at 0.1 MPa.

火星发现了大量盐类矿物，盐类研究对火星科学具有重要意义。火星氯化物分布广泛，作为地球常见沉积盐类之一，氯化镁也很有可能在火星上存在。低温相水合氯化镁含水量高，可能对火星次地表水含量有较大贡献。氯化镁共晶点低，可能是造成火星反复出现的斜坡沟渠（RSL）的主要氯化物之一。水合氯化镁稳定存在所需温湿条件可用于评估水合氯化镁在火星地表次地表环境下存在的可能性，可能的赋存状态及存在区域，进一步可用于讨论其对火星次地表水含量的贡献。其相变路径、固-液转化条件及其吸水条件对于研究火星RSL的产生机理非常重要。已有研究主要关注氯化镁不同温度下的溶解度，据笔者所知，目前还少有水合氯化镁在不同温湿条件下稳定性质的研究。本项目将利用湿度缓冲器技术明确标压下三种水合氯化镁（MgCl2o6H2O、MgCl2o8H2O和MgCl2o12H2O）不同温湿度下的相变路径，厘定三者的相变边界及三者固-液转化的温湿条件。

本项目旨在开展水合氯化镁在不同温湿条件下稳定性质的研究。根据实验设计，我们搭建了开展水合盐类温湿度相图研究的实验平台。依托实验平台，我们开展了三种水合氯化镁矿物（MgCl2•6H2O、MgCl2•8H2O 和 MgCl2•12H2O）的合成实验。依据氯化镁溶解度相图，在三种不同温度下合成水合氯化镁矿物后，观测到两个低温环境下合成的矿物（可能是MgCl2•8H2O 和 MgCl2•12H2O）在脱离合成环境后立即潮解，设计的水合氯化镁温湿度相图无法继续进行。在回顾了实验设计后，我们发现过去的温湿度相图实验存在理论缺陷。通过文献分析，结合与盐类物理化学领域的专家讨论后，我们对过去水合盐类稳定性的基础理论提出了质疑，并提出了一些猜想，可作为该领域的学者进一步深入研究的参考。然后，我们对察尔汗团结湖水氯镁石类火星条件下的次生变化进行了观测，发现水氯镁石（MgCl2•6H2O）在较大温差和湿度变化范围内仍然稳定，在低于-10℃的温度下任然没有观测到低温相矿物(MgCl2•8H2O 或 MgCl2•12H2O)。此外，该区水氯镁石曝露在紫外辐射较强的的环境中，没有观测到其氧化产物（氯酸盐、高氯酸盐）的产生，这些观测结果可以火星氯化物及高氯酸盐的研究重要参考。