square meters per gram. In addition to having a high surface area to volume ratio, Fh also has a high density of local or point defects, such as dangling bonds and vacancies. These properties confer a high ability to adsorb many environmentally important chemical species, including arsenic, lead, phosphate, and organic molecules (e.g., humic and fulvic acids). Its strong and extensive interaction with trace metals and metalloids is used in industry, at large-scale in water purification plants, as in North Germany and to produce the city water at Hiroshima, and at small scale to clean wastewaters and groundwaters, for example to remove arsenic from industrial effluents and drinking water. Its nanoporosity and high affinity for gold can be used to elaborate Fh-supported nanosized Au particles for the catalytic oxidation of CO at temperatures below 0 °C. Dispersed six-line ferrihydrite nanoparticles can be entrapped in a vesicular state to increase their stability. Metastability Ferrihydrite is a metastable mineral. It is known to be a precursor of more crystalline minerals like hematite and goethite by aggregation-based crystal growth. However, its transformation in natural systems generally is blocked by chemical impurities adsorbed at its surface, for example silica as most of natural ferrihydrites are siliceous. Under reducing conditions as those found in gley soils, or in deep environments depleted in oxygen, and often with the assistance of microbial activity, ferrihydrite can be transformed in green rust, a layered double hydroxide (LDH), also known as the mineral fougerite. However, a short exposure of green rust to atmospheric oxygen is sufficient to oxidize it back to ferrihydrite, making it a very elusive compound. See also Better crystallized and less hydrated iron oxy-hydroxides are amongst others: Akaganéite Feroxyhyte Goethite Hematite Lepidocrocite Limonite Maghemite Magnetite References Iron(III) minerals Iron oxide pigments Hydroxide minerals Hexagonal minerals Minerals in space group 186