The GRs are a family of labile Fe(II)–Fe(III) hydroxysalts which can be synthesized in the laboratory (Murad, 1990). They can form by oxidation of Fe(II) solution (Tamaura et al., 1984; Vins et al., 1987; Hansen et al., 1996; Lewis, 1997), by interaction of soluble Fe(II) with Fe(III) oxides (Taylor and Mc Kenzie, 1980), by partial oxidation of Fe(OH)2 (Ge´ nin et al., 1994), or by reduction of Fe(III) oxides with bacteria (Tuovinen et al., 1980; Bigham and Tuovinen, 1985; Fredrickson et al., 1998). GRs consist of brucite-like hydroxide layers positively charged due to the presence of Fe(III) cations, which alternate regularly with interlayers made of anions and water molecules. The structure of GRs and parent minerals can accommo- date a variety of anions, such as OH , Cl ,CO23; and SO 24 (Table 3).
Depending o n t he nature of anion, two types of GRs could be di stinguished by means of XRD (Bernal et al. , 1959; Lewis, 1997) and Mo¨ ss ba uer spectr om- etry. Synthetic Fe(II)–Fe(III) GRs show two main doublet s at 78 K by classical transmission Mo¨ ssbauer s pectroscopy (TMS): D1 (i somer s hift d ’ 1 :27 mm s 1 , quadrupole splitting D EQ ’ 2: 87 an d 2 :92 mm s 1 ) and D3 ðd ’ 0 :47 0:48 mm s1ị,DEQ ’0:38 and 0:43 mm s 1 ). D1 has been ascribed to Fe2þ and D 3 to Fe 3þ. A slightly better fit was sometimes obtained by considering a second ferrous doublet D2ðd’1:271:28 mm s1; DEQ’2:552:69 mm s1ị; this occurs when the structure is GR1.
6.1. Structure of GR1s
The GR1s present a stacking sequence similar to that of pyroaurite, . . . AcBiBaCjCbAk. . ., where A, B, C are planes of OHions; a, b, c, planes of Fe atoms; and i, j, k, the interlayers. The lattice is rhombohedral and the space group R3m (Fig. 9 A, Tables 4–6).
The interlayered anions are spherical or planar, for example, Cl, Br, I, CO23;C2O24;SO23. The rhombohedral structure is due to the
246 Fabienne Trolard and Guilhem Bourrie´
Table 3 Structural formula of natural minerals of the fougerite group and nature of the interlayer anion
Mineral Structural formula Interlayered anion
Fougerite ẵðFe2ỵ;Mgị1xFe3xỵðOHị2 ẵx=nAn;mH2O ;1=4x1=3 Discussed: OH, Cl, CO32
Meixnerite [Mg6Al2(OH)16][(OH)2, 4H2O] OH
Woodallite [Mg6Cr2(OH)16][Cl2, 4H2O Cl
Iowaite [Mg4FeIII(OH)10][Cl2, H2O] Cl
Takovite [Ni6Al2(OH)16][(OH, CO3), 4H2O] OH, CO23
Hydrotalcite [Mg6Al2(OH)16][CO3, 4H2O] CO32
Pyroaurite [Mg6Fe2III(OH)16][CO3, 4H2O] CO32
FromTrolardet al.(2007).
stacking sequence of layers AB-BC-CA. Fe(II) and Fe(III) cations are distributed randomly among the octahedral positions. Water molecules in the interlayer are situated close to the threefold axis that connects the two OHions of the adjacent hydroxide layers.
The general following formula could be proposed for GR1s:
ẵFeII1xFeIIIx ðOHị2
ỵxẵx=nAn;m=nH2Ox;where 1=4x1=3:
Figure 9A shows an example of the crystal structure of a GR1 such as the hydroxychloride, GR(Cl) ( Refait et al., 1998; Ge´ nin et al., 2001). More specifically, the XRD study of GR1(Cl) (Refaitet al., 1998a) showed that the structure was close to that of pyroaurite ẵMgII6FeIII2 ðOHị12
2þ
ẵCO23 ;4H2O and iowaite, a Mg(II)–Fe(III) pyroaurite-like mineral incorporating Cl.
6.2. Structure of GR2s
The GR2s present a stacking sequence similar to the crystal structure of Fe (OH)2, . . . AcBiA . . ., with the same notations as above. The lattice is trigonal and the space group is P3m1 (Fig. 9C) (Simon et al., 2003).
Stacking A Bc
A Bc
C
b
Interlayer Hydroxide layer
b b
Interlayer Hydroxide layer Interlayer Hydroxide layer c/3
Stacking C Ba B Ac
c/3 Cl-
H2O H2O
S C A
Stacking C Ba B Ac
OH- H2O B
Fe(II) Fe(III) OH-
Fe(II) Mg(II) Fe(III) OH-
Fe(II) Fe(III) OH- O
Figure 9 Stacking sequences and positions of water molecules and anions in an interlayer viewed along [001] of: (A) GR1Cl, hydroxychloride green rust (adapted from Ge´ninet al., 2001); (B) fougerite with OHas interlayer anion and (C) GR2, hydroxysulfate green rust (adapted from Simon et al., 2003).
248 Fabienne Trolard and Guilhem Bourrie´
Table 4 X-ray diffraction data (CoKa1radiation) compared with computed interplanar distances (nm) and intensities of synthetic GR1(Cl)
hkl daobs dcalc dobs-dcalc Iobs/I1 Icalc/I1 Iobs/Icalc
003 0.797 (3) 0.7950 0.002 100 100 1
006 0.3966 (8) 0.3975 0.0009 31.5 31.5 1
101 0.2744 (6) 0.2744 0
012 0.2692 (4) 0.2691 0.0001 34 30.8 1.104
009 0.264 (1) 0.2650 0.001
104 0.253 (2) 0.2507 0.0023 4.5 5.4 0.833
015 0.2392 (3) 0.2391 0.0001 21 21.5 0.977
107 0.216 (2) 0.2146 0.0014 2 3.0 0.666
018 0.2027 (3) 0.2026 0.0001 19.2 21.4 0.897
0012 0.198 (1) 0.1988 0.0008
1010 0.1808 (4) 0.1805 0.0003 5.5 6.3 0.873
0111 0.1702 (5) 0.1706 0.0004 4.5 6.1 0.738
110 0.1595 (1) 0.1595 0 9.0 8.7 1.034
113 0.1563 (1) 0.1564 0.0001 10.4 11.3 0.920
1013 0.1526 (7) 0.1528 0.0002 5 3.9 1.282
116 0.1479 (1) 0.1480 0.0001 4.3 5.9 0.729
0114 0.1448 (3) 0.1450 0.0002 3 2.6 1.154
021 0.1375 (3) 0.1379 0.0004
202 0.1364 (2) 0.1372 0.0008 5 5.3 0.943
024 0.1348 (3) 0.1346 0.0002 1.5 0.8 1.875
205/0018 0.1323 (3) 0.1326 0.0003 6 6.2 0.968
1016 0.1309 (4) 0.1312 0.0003
(continued)
Table 4 (continued)
hkl daobs dcalc dobs-dcalc Iobs/I1 Icalc/I1 Iobs/Icalc
027 0.1284 (2) 0.1280 0.0004 0.5 0.8 0.625
208/0117 0.1251 (2) 0.1252 0.0001 5 3.8 1.316
1112 0.1240 (3) 0.1244 0.0004
Weighted average þ0.0001 1.002
FromRefaitet al.(1998a).
The structure is R3m with parametersaẳ0.3190(1) nm andcẳ2.385(6) nm (Vẳ0.2102 nm3).
The digit in parentheses is the uncertainty (1s) on the last digit.
250
Table 5 Reduced coordinates of atoms in synthetic G R1(Cl) and temperature factor, from Refait et al., (1998a )
x y z U11 U22 U33 U23 U13 U12
Fe 0 0 0 0.009 0.009 0.025 0 0 0.0045
O (OH) 0 0 0.375 0.014 0.014 0.06 0 0 0.007
O (H2O) 0.1 0.1 0.5 0.15 0.15 0.08 0.02 0.02 0.1
Cl 0.25 0.25 0.5 0.3 0.3 0.08 0.02 0.02 0.1
From Refait et al. (1998a).
251
The interlayered anions are three-dimensional, for example, SO24;SeO24 . . .Moreover, an ordering of Fe(II) and Fe(III) was observed and the interlayer contained two layers of water molecules and ordered anions inducing a stoichiometry. The D1doublet has been assigned to Fe(II) in the octahedral sites of the brucite-like sheets, as its hyperfine parameters were close to those of Fe(OH)2and the D2doublet to Fe(II) atoms situated close to the anions. Thus, the following formula could be proposed:ẵFeII4FeIII2 ðOHị12
2ỵẵA;8H2O2, where A is one of the three-dimensional divalent anions cited above.