Investigation of the dissolution mechanism of silicate minerals at the atomic scale by using scanning probe microscopy

矿物的风化和溶蚀过程对认识全球元素地质循环具有重要意义。为了了解这些过程，在过去的几十年里，许多小组进行了实验室实验。然而，这些实验基本上是测量从大块矿物中溶解的溶液中离子或元素的浓度，以确定矿物的溶解速率，这并没有在原子尺度上揭示元素如何从矿物中溶解的基本机制。扫描探针显微镜（SPM）可以在原子尺度上对矿物表面进行原位观察，是了解这一机制最合适的工具。在本研究中，我们利用SPM成功地观察了地球科学中最重要的矿物群硅酸盐在强酸溶液中的溶解特征。首先，我们试图观察黑云母（一种硅酸盐薄片，也是最常见的造岩矿物之一）在pH=1的硝酸中的表面溶解。在（001）表面，溶解是通过在顶层随机形成许多蚀刻坑并扩大这些蚀刻坑来进行的。在第一层完全溶解后，下一层开始以同样的方式溶解。有时两层或三层同时溶解，蚀坑深度超过一个单位层厚度。这可能与黑云母的晶体生长机制有关。其次，我们成功地观察了同样是重要的造岩矿物的橄榄石（Fo_<90>Fa_<10>）在pH=1的硝酸中（010）表面的原位溶解。溶解速度与（010）表面的方向有关。在此基础上，用SPM法测定了水镁石（Mg (OH)_2）溶解速度随温度的变化规律。

Weathering and dissolution processes of minerals are importan to understand the global gelogic cycle of elements. In order to understand the processes, laboratory experiments were perfoumed by many groups for the last several decades. However, these experiments were basically to mearure the concentrations of ions or elements in solutions dissolved from bulk minerals to determine mineral dissolution rates, which did not suggest basic mechanism how the elements dissolve from mineral surgaces at the atomic scale. Scanning probe microscopy (SPM) is the most suitable tool to obtain the insights for this mechanism because SPM can make atomic scale in-situ obserbation of mineral surfaces in folution. In this research, we have observed successfully the dissolution feature of some silicates, which are the most important mineral group in earth science, in strong acid solution using SPM.First, we tried to observe the surface dissolution of biotite, a sheet silicate and one of the most common rock-forming minerals, in nitric acid at pH=1. On the (001) surface, the dissolution proceeds by forming many etch-pits randomly on the top layer and by expanding these etch-pits. After the top layer dissolved completely, the next layer starts to dissolve in the same mnner. Sometimes two or three layers dissolve simultaneously with etch-pits deeper than one unit layer thicness. Probably this is related to the crystal growth mechanism of biotite.Secondly we have succeeded to observe the in-situ dissolution of (010) surface of olivine (Fo_<90>Fa_<10>), also an important rock-forming mineral, in nitric acid at pH=1. The dissolution speed depends on the dirctions in (010) surface.Beides theseresults, temperature dependence of the dissolution speed of brucite, Mg (OH)_2, was determined using a hot specimen stage for the SPM measurement.