Mössbauerite, Fe₆³⁺ O₄ (OH) ₈ [CO₃]·3H₂O, the fully oxidized green rust mineral from Mont Saint-Michel Bay, France

The new mineral mössbauerite (IMA2012-049), Fe₆ ³⁺ O₄ (OH) ₈ [CO₃]·H ₂O, is a member of the fougèrite group of the hydrotalcite supergroup. Thus, it has a layered double hydroxide-type structure, in which brucite-like layers [Fe₆³⁺ O₄ (OH) ₈]²⁺ are intercalated with CO₃ ^2- anions and water molecules. Mössbauerite is the fully oxidized analogue of fougèrite and trébeurdenite, related to them chemically by the exchange of (Fe³⁺O^2-) with (Fe²⁺OH ^-). Mössbauerite, intimately intergrown with trébeurdenite, was discovered in intertidal gleys from Mont Saint-Michel Bay, France, along with quartz, feldspars and clay minerals. Mössbauerite is formed by the oxidation of the other members of the fougèrite group. Like them, it occurs as mm-scale platelets in gleys with restricted access to atmospheric O and decomposes rapidly when exposed to air. Identification and characterization of these minerals has relied on an electrochemical study of synthetic analogues and Mössbauer spectroscopy, which inspired the name of the new mineral. Unlike fougèrite and trébeurdenite, which are blue-green, pure synthetic mössbauerite is orange in colour. Detailed optical and other physical properties could not be determined because of the small platelet size and instability. The hardness is probably 2-3, by analogy with other members of the supergroup and the density, calculated from unit-cell parameters, is 2.950 g/cm3. Synchrotron X-ray data indicate that the natural material is a nanoscale intergrowth of 2T and 3T polytypes; the latter probably has the 3T₇ stacking sequence. The corresponding maximum possible space group symmetries are P3̄m1 and P3m1. Unit-cell parameters for the 3T cell are a = 3.032(7) Å, c = 22.258(4) = 3×7.420 Å and Z = 1/2. Mössbauer spectroscopy at 78 K indicates that two distinct Fe ³⁺ environments exist in a 2:1 ratio. These are interpreted to be ordered within each layer, but without the development of a threedimensional superlattice. Mössbauerite undergoes gradual magnetic ordering at 70-80 K to a ferromagnetic state, below which it splits into three sextets S _1m, S_2m and S_3m, as measured at 15 K, and shows the same intensity ratio 1/2:1/6:1/3 as the three doublets for fougèrite D_1f, D_2f, D_3f in the paramagnetic state at 78 K. This suggests that there is also short-range coupling of interlayer carbonate anions with respect to the octahedral layers and that the 2D long-range order of carbonates in interlayers remains unchanged.