We present new results on the conversion of pure, undoped synthetic ferrihydrite, wet‐annealed at pH 6.56 and 90°C without stabilizing ligands, to nanophase goethite, hematite, and an intermediate magnetic phase, nanophase maghemite. Our analyses included magnetic field and temperature‐dependent properties and characterization by powder X‐ray diffraction, Mössbauer spectra, and high‐resolution transmission electron microscopy. We sampled alteration products after 0.5 hr, and then in a geometric progression to 32 hr, yielding a detailed examination of the earliest alteration phases. There are many similarities to the latest studies of pure ferrihydrite alteration but with a significant difference: We observe early appearance of oriented nanophase goethite along with a soft magnetic contribution, while rhombohedral hematite crystals form later, as reported in previous studies. Our observations attest to the non‐uniqueness of the magnetic enhancement process and to its strong dependence on environmental conditions, with important implications for use of the hematite/goethite ratio as a paleoprecipitation proxy.

中文翻译：

---

土壤中天然氧化铁变化的模拟：磁性方法观察到的合成亚铁酸盐转化为赤铁矿而无人工掺杂剂

我们提出了在纯净的，未掺杂的合成三水铁矿（在pH 6.56和90°C的条件下进行湿退火，而没有稳定配体）转化为纳米相针铁矿，赤铁矿和中间磁相纳米磁赤铁矿的新结果。我们的分析包括磁场和温度相关的特性以及粉末X射线衍射，穆斯堡尔谱和高分辨率透射电子显微镜的表征。我们在0.5小时后对变更产品进行了采样，然后以几何级数发展到32小时，从而对最早的变更阶段进行了详细的检查。与纯铁铁矿蚀变的最新研究有许多相似之处，但有一个显着差异：我们观察到取向纳米相针铁矿的早期出现以及软磁作用，而菱形的赤铁矿晶体形成较晚，如以前的研究报道。我们的观察结果证明了磁增强过程的非唯一性及其对环境条件的强烈依赖性，这对于使用赤铁矿/针铁矿比率作为古沉淀替代品具有重要意义。