Ferrihydrite is a poorly crystalline iron oxyhydroxide nanomineral that serves a critical role as the most bioavailable form of ferric iron for living systems. However, its atomic structure and composition remain unclear due in part to ambiguities in interpretation of X-ray scattering results. Prevailing models so far have not considered the prospect that at the level of individual nanoparticles multiple X-ray indistinguishable phases could coexist. Using ab initio thermodynamics we show that ferrihydrite is likely a nanocomposite of distinct structure types whose distribution depends on particle size, temperature, and hydration. Nanoparticles of two contrasting single-phase ferrihydrite models of Michel and Manceau are here shown to be thermodynamically equivalent across a wide range of temperature and pressure conditions despite differences in their structural water content. Higher temperature and water pressure favor the formation of the former, while lower temperature and water pressure favor the latter. For aqueous suspensions at ambient conditions, their coexistence is maximal for particle sizes up to 12 nm. The predictions inform and help resolve different observations in various experiments.

水铁矿是一种结晶度较差的羟基氧化铁纳米矿物，作为生命系统中最具生物利用度的三价铁形式，发挥着至关重要的作用。然而，其原子结构和组成仍不清楚，部分原因是 X 射线散射结果的解释含糊不清。迄今为止，流行的模型尚未考虑到在单个纳米粒子水平上多个 X 射线不可区分相可能共存的前景。使用从头热力学，我们表明水铁矿可能是一种具有不同结构类型的纳米复合材料，其分布取决于粒径、温度和水合作用。 Michel 和 Manceau 两种不同的单相水铁矿模型的纳米颗粒在很宽的温度和压力条件下表现出热力学等效性，尽管它们的结构水含量存在差异。较高的温度和水压有利于前者的形成，较低的温度和水压有利于后者的形成。对于环境条件下的水悬浮液，当粒径达到 12 nm 时，它们的共存达到最大。这些预测提供信息并帮助解决各种实验中的不同观察结果。