X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering
X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering
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X射线光电子研究风化过程中硅酸铁溶解机理
DOI：
10.1016/0016-7037(83)90046-7
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发表时间：
1983
影响因子：
5
通讯作者：
R. Berner
中科院分区：
文献类型：
--
作者：
J. Schott;R. Berner
铁硅酸盐矿物（青铜矿、铁矾石）在实验室中，在室温、pH值为1、1.5和6的条件下，暴露于水溶长达60天，利用XPS研究了表面成分变化的证据，并将这些结果与溶液化学分析所得结果进行了比较。在低pH（1-1.5）下没有溶解的o2时，溶解在初始形成薄（<10 Å）质子化表面层后进行，相对于Si，铁含量减少。这一层是不稳定的，不会随着时间的推移而增长，这一点可以通过长期的全等溶解和在pH 1和1.5下形成无定形二氧化硅表面分解产物来证明。在铜铜矿中，由于Fe+2优先占据了键合较弱的m2位点，镁层也有轻微的耗尽，但比铁层少得多。该层的行为与先前在无铁辉石中发现的相似（Schottet al., 1981）；换句话说，由于它的薄和不稳定性，它不能抑制扩散或保护溶解。在溶解氧存在的情况下，就像在大多数风化溶液中一样，铜铜矿和铁矾石的溶解导致两层表面层的形成，这两层的组成可以通过测量XPS结合能推断出来。由含水氧化铁组成的外层很容易通过超声波清洗去除，而且很可能对溶解没有保护作用。内层由铁+3组成，在质子化或羟基化的硅酸盐（在青铜矿的情况下是镁硅酸盐）基质中。在实验的时间尺度上，这一层似乎阻碍了溶解，抛物线溶解速率证明了这一点。然而，在风化的时间尺度上，这一层并没有继续增长，因为超声波清洗的土壤颗粒（BernerandSchott, 1982）的表面成分与目前长达一个月的实验室实验中发现的相似。换句话说，在大多数酸性土壤中，厚的、高度改变的、抑制扩散的保护性表面层不会形成。
Iron silicate minerals (bronzite, fayalite), exposed to aqueous dissolution in the laboratory for up to 60 days at room temperature and pH 1, 1.5, and 6, have been studied for evidence of changes in surface composition, using XPS, and these results compared with those obtained from solution chemical analysis. In the absence of dissolved O2or at low pH (1–1.5) dissolution proceeds congruently after the initial formation of a thin (<10 Å) protonated surface layer depleted in Fe relative to Si. This layer is unstable and does not grow with time as attested to by long term congruent dissolution and by the formation of an amorphous silica surficial breakdown product at pH 1 and 1.5. In bronzite the layer is also slightly depleted in Mg but much less than it is in Fe due to the preferential occupation by Fe+2of more weakly bonded M2sites. The behavior of the layer is similar to that found earlier on iron-free pyroxene (Schottet al., 1981); in other words, because of its thinness and instability it is not diffusion-inhibiting or protective toward dissolution.In the presence of dissolved O2, as would be the case in most weathering solutions, dissolution of bronzite and fayalite results in the formation of two surface layers whose compositions were deduced by measurements of XPS binding energies. The outer layer, consisting of hydrous ferric oxide, is readily removed by ultrasonic cleaning and, most likely, is not protective toward dissolution. The inner layer consists of Fe+3in a protonated or hydroxylated silicate (Mg-silicate in the case of bronzite) matrix. This layer appears to impede dissolution over the time scale of the experiment as attested to by parabolic dissolution rates. However, the layer does not continue to grow on the time scale of weathering because ultrasonically cleaned soil grains (BernerandSchott, 1982) exhibit surface compositions similar to those found in the present month-long laboratory experiments. In other words, athick, highly altered, diffusion-inhibiting, protective surface layer does not form at the acidic pH of most soils.