Fougèrite is a relatively recently described naturally occurring green rust mineral. It is the archetype of the fougèrite group in the larger hydrotalcite supergroup of naturally occurring layered double hydroxides. The structure is based on

brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Mg( OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal vein mineral in metamorphosed limestones and chlorite schists; and ...

-like layers containing Fe²⁺ and Fe³⁺ cations, O²⁻ and OH⁻ anions, with loosely bound O₃sup>2− groups and H₂O molecules between the layers. Fougèrite crystallizes in trigonal system. The ideal formula for fougèrite is e²⁺₄Fe³⁺₂(OH)₁₂CO₃]·3H₂O. Higher degrees of oxidation produce the other members of the fougèrite group, namely trébeurdenite, e²⁺₂Fe³⁺₄O₂(OH)₁₀CO₃]·3H₂O and mössbauerite, e³⁺₆O₄(OH)₈CO₃]·3H₂O. Fougèrite was first found in forested soils near Fougères,

Brittany Brittany (; french: link=no, Bretagne ; br, Breizh, or ; Gallo language, Gallo: ''Bertaèyn'' ) is a peninsula, Historical region, historical country and cultural area in the west of modern France, covering the western part of what was known ...

, France, and recognised as a valid mineral species by the

International Mineralogical Association Founded in 1958, the International Mineralogical Association (IMA) is an international group of 40 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 5000 plus known mineral species. Th ...

in 2002.Fougèrite data on Mindat.org
/ref> It is blue-green to bluish-gray in colour, and resembles

clay minerals Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...

in habit, forming hexagonal platelets of submicron diameter. In this environment, it is intimately intergrown with trébeurdenite, to give varying overall ratios of Fe²⁺:Fe³⁺. The existence of two intergrown fixed-composition phases has been demonstrated by Mössbauer spectroscopy. The mineral is unstable in air, and decomposes by oxidation, dehydration and decarbonation, to

ferrihydrite Ferrihydrite (Fh) is a widespread hydrous ferric oxyhydroxide mineral at the Earth's surface, and a likely constituent in extraterrestrial materials. It forms in several types of environments, from freshwater to marine systems, aquifers to hydro ...

, and ultimately to

lepidocrocite Lepidocrocite (γ-FeO(OH)), also called esmeraldite or hydrohematite, is an iron oxide-hydroxide mineral. Lepidocrocite has an orthorhombic crystal structure, a hardness of 5, specific gravity of 4, a submetallic luster and a yellow-brown stre ...

or goethite, Fe³⁺O(OH).Génin J.-M. R., Aïssa R., Géhin A., Abdelmoula M., Benali O., Ernstsen V., Ona-Nguema G., Upadhyay Ch. and Ruby Ch.; 2005: ''Fougerite and Fe^II-III hydroxycarbonate green rust; ordering, deprotonation and/or cation substitution; structure of hydrotalcite-like compounds and mythic ferrosic hydroxide Fe(OH)_2+x,'' Solid State Sciences, vol. 7., no. 5, p. 545-572. .

References

{{DEFAULTSORT:Fougerite Iron(II,III) minerals Magnesium minerals Hydroxide minerals Clay minerals group Hydrates Pedology Corrosion Hydroxides Trigonal minerals Minerals in space group 166

See also

References