ALBERT

Description: The new mineral mössbauerite (IMA2012–049), $${\mathrm{Fe}}_{6}^{3+}$$ O 4 (OH) 8 [CO 3 ]·3H 2 O, 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 [ $${\mathrm{Fe}}_{6}^{3+}$$ O 4 (OH) 8 ] 2+ are intercalated with $${\mathrm{CO}}_{3}^{2-}$$ anions and water molecules. Mössbauerite is the fully oxidized analogue of fougèrite and trébeurdenite, related to them chemically by the exchange of (Fe 3+ O 2– ) with (Fe 2+ 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 μm-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/cm 3 . Synchrotron X-ray data indicate that the natural material is a nanoscale intergrowth of 2 T and 3 T polytypes; the latter probably has the 3 T 7 stacking sequence. The corresponding maximum possible space group symmetries are P 3I m 1 and P 3 m 1. Unit-cell parameters for the 3T cell are a = 3.032(7) Å, c = 22.258(4) = 3 x 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 three-dimensional 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.

Description: The new mineral mössbauerite (IMA2012−049), Fe63+O4(OH)8[CO3]·3H2O, 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 CO32− anions and water molecules. Mössbauerite is the fully oxidized analogue of fougèrite and trébeurdenite, related to them chemically by the exchange of (Fe3+O2−) 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 μm-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 Pm1 and P3m1. Unit-cell parameters for the 3T cell are a = 3.032(7) Å, c = 22.258(4) = 367.420 Å and Z = ½.Mössbauer spectroscopy at 78 K indicates that two distinct Fe3+ 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 S1m, S2m and S3m, as measured at 15 K, and shows the same intensity ratio ½:⅙:⅓ as the three doublets for fougèrite D1f, D2f, D3f 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.

Description: Ferric oxides or hydroxides are common minor components of kaolinites. If free, these minerals can be removed by Fe reduction and leaching (Mehra & Jackson, 1960), but Fe3+ ions can substitute for Al3+ ions in the octahedral sites of kaolinite (Weaver et al., 1967; Malden & Meads, 1967). Moreover, Fe2+ ions can also substitute trioctahedrally in place of dioctahedral Al, leading to a local composition of Fe-antigorite (Cuttler, 1980).The Font-Bouillant quarry (Charentes, France) is the source of disordered kaolinite known as ‘FU7, U-7-10 or FBT’ (Cases et al., 1982, 1986). The samples used in this study were considered in an earlier genetic analysis of the deposit (Delineau, 1994; Delineau et al., 1994) and were analysed previously by X-ray diffraction (XRD), infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) and UV-visible spectroscopy by Delineau et al. (1994) (Table 1).