Fougèrite is a relatively recently described naturally occurring

green rust Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide () anion, and another anion such as carbonate (), chloride (), or sulfate (), in a layered double hydroxide str ...

mineral. It is the archetype of the fougèrite group in the larger hydrotalcite supergroup of naturally occurring

layered double hydroxides Layered double hydroxides (LDH) are a class of ionic solids characterized by a layered structure with the generic layer sequence cB Z AcBsub>''n'', where c represents layers of metal cations, A and B are layers of hydroxide () anions, and Z are l ...

. 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 In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crystal ...

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 Fougères (; br, Felger; Gallo: ''Foujerr'') is a commune and a sub-prefecture of the Ille-et-Vilaine department in the region of Brittany in northwestern France. As of 2017, Fougères had 20,418 inhabitants. The Fougères area comprises appr ...

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

, France, and recognised as a valid mineral species by the International Mineralogical Association in 2002.Fougèrite data on Mindat.org
/ref> It is blue-green to bluish-gray in colour, and resembles clay minerals 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 Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. This effect, discovered by Rudolf Mössbauer (sometimes written "Moessbauer", German: "Mößbauer") in 1958, consists of the nearly recoil-free emission and abs ...

. 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 hydr ...

, 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 ...

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

, 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