Fougèrite

Fougèrite is a relatively recently described naturally occurring green rust mineral.^[3] It is the archetype of the fougèrite group in the larger hydrotalcite supergroup of naturally occurring layered double hydroxides.^[4] The structure is based on brucite-like layers containing Fe²⁺ and Fe³⁺ cations, O²⁻ and OH⁻ anions, with loosely bound [CO₃]²⁻ groups and H₂O molecules between the layers. Fougèrite crystallizes in trigonal system. The ideal formula for fougèrite is [Fe²⁺₄Fe³⁺₂(OH)₁₂][CO₃]·3H₂O. Higher degrees of oxidation produce the other members of the fougèrite group, namely trébeurdenite, [Fe²⁺₂Fe³⁺₄O₂(OH)₁₀][CO₃]·3H₂O and mössbauerite, [Fe³⁺₆O₄(OH)₈][CO₃]·3H₂O.

Fougèrite
General
Category	Hydroxide mineral
Formula	[Fe2+4Fe3+2(OH)12][CO3]·3H2O
IMA symbol	Fgè[1]
Strunz classification	4.FL.05
Crystal system	Trigonal
Crystal class	3m (3 2/m) – Hexagonal scalenohedral
Space group	R3m
Unit cell	a = 3.125 Å, c = 22.5 Å
Identification
Colour	Bluish green
Luster	Earthy
Diaphaneity	Opaque
Alters to	Unstable – alters to goethite or limonite
References	[2]

Fougèrite was first found in forested soils near Fougères, Brittany, France, and recognised as a valid mineral species by the International Mineralogical Association in 2002.^[2] 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. The mineral is unstable in air, and decomposes by oxidation, dehydration and decarbonation, to ferrihydrite, and ultimately to lepidocrocite or goethite, Fe^IIIO(OH).^[5][6]

See also

• Clay minerals
• Gley soil
• Iron(II) hydroxide