Study on the so-called noncrystalline free iron oxides in soils

(1)发生率：采用选择性溶解结合X射线衍射和穆斯堡尔谱的方法对火山灰和非火山灰土壤进行了研究。结果表明，灰化土中存在不同结晶度的水铁矿，而非灰化土中几乎不存在或含量极少。水镁石的存在与火山玻璃在风化早期提供了充足的Si和Fe有关，野外和实验室观测表明，当含亚铁的地下水暴露于空气中时，水镁石迅速形成。以这种方式形成的铁水石膏通常是有序性差的，因为它仅在约0.25和0.15 nm处显示出两个X射线衍射。(2)合成工作：在Fe：Si = 10：1-2，pH = 6 ~ 8的条件下，合成了结晶良好的水铁矿。在低于或高于这些极限的范围内，没有形成水铁矿，或者与其他铁矿物如针铁矿或纤铁矿共存。在pH> 11的范围内形成铁氧亚硫酸盐，这表明该矿物不能在通常pH范围的土壤中形成。用Ge代替Si，结果完全与上述相同。合成的Si-和Ge-水铁矿的d-间距的比较表明，Si（和Ge）不是固定在水铁矿的结构中，而是固定在晶畴的“外面”。有机阴离子如乙酸根和柠檬酸根对水铁矿的形成有干扰作用，尽管这种干扰作用在不同的阴离子中可能是不同的。(3)由于缺乏适当的程序，测量CEC和AEC，P-保留和比表面积的水铁矿在土壤中失败的尝试。然而，据推断，P-保留和比表面积可能大于或等于，在幅度上，粘土成分（铝硅酸盐）。

(1) Occurrence: Volcanic ash and non-volcanic ash soils were examined by means of selective dissolution combined with difference X-ray diffraction and Moessbauer spectroscopy. It was shown that ferrihydrite of various crystallinity were presented in ash soils, but it was absent or very small in amount in non-ash soils. Its presence in ash soils was considered to be related to ample supply of Si, as well as Fe, from volcanic glass in the early stage of weathering.Field and laboratory observations suggested that ferrihydrite forms rapidly when the underground water containing ferrous iron exposed to air. Ferrihydrite formed this way is usually poorly-ordered in the sense that it shows only two X-ray diffractions at about 0.25 and 0.15 nm.(2) Synthetic work: Well-crystallized ferrihydrite was synthesized from the solution containing Fe and Si in the ratios 10:1-2 and in the pH range 6-8. In the range lower or higher than these limits, ferrihydrite was not formed, or was coexisted with other iron minerals such as goethite or lepidocrocite formed. In the pH range > 11 feroxyhite was formed, suggesting that the mineral cannot form in the soils of usual pH range. Use of Ge, in the place of Si, results completely in the same as above. Comparison of d-spacings between synthetic Si- and Ge-ferrihydrite indicated that Si (and Ge) are not held in the structure of ferrihydrite, but held in the "outsides" of the crystal domains.Organic anions, such as acetate and citrate, interfered with the formation of ferrihydrite, even though the interference may be different among the kind of anions.(3) Attempts of measuring CEC and AEC, P-retention, and specific surface area for ferrihydrite in soils failed due to the lack of appropriate procedure. However, it was inferred that P-retention and specific surface area may be larger than or equal to, in magnitude, to clay constituents (aluminosilicates).