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

矿物的风化和溶解过程对于了解全球元素的地质循环是重要的。为了了解这一过程，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散体矿物中溶解的溶液中离子或元素的浓度来确定矿物的溶解速率，这并没有从原子尺度上揭示元素如何从矿物中溶解的基本机理。扫描探针显微镜(SPM)可以对矿物表面进行原子尺度的原位观察，因此扫描探针显微镜(SPM)是了解这一机理的最合适工具。在这项研究中，我们用扫描电子显微镜成功地观察到了一些地球科学中最重要的矿物群--硅酸盐在强酸性溶液中的溶解特征。首先，我们尝试观察了最常见的造岩矿物之一--片状硅酸盐黑云母在pH=1的硝酸中的表面溶解。在(001)面，溶解是通过在顶层上随机形成多个蚀坑并通过扩大这些蚀坑来进行的。在顶层完全溶解后，下一层开始在同一层中溶解。有时两层或三层同时溶解，蚀坑深度超过一个单位层厚度。其次，我们成功地观察到了橄榄石(Fo_&lt；90&gt；Fa_&lt；10&gt；)在pH=1的硝酸中的(010)面的原位溶解，溶解速度取决于(010)面的取向。

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.