The lattice parametercof GR1 compounds is mainly determined by the geometry and size of the interlayer anion (seeFig. 9). It isc’2:385 nm in GR1(Cl) (Refaitet al., 1998a) andc’2:256 nm in GR1(CO3) (Drissiet al.,
Geochemistry of Green Rusts and Fougerite 253
1995; Abdelmoulaet al., 1996). In GR1 (CO3), the carbonate ions are parallel to the hydroxide sheets and the width of the interlayer is fixed by the diameter of the oxygen atoms of H2O and CO23. Thus, the parametercof GR1(OH) should be in the range between that of GR1(Cl) and of GR1(CO3), that is,c 2[2.256, 2.385] nm. However, it is not accessible by EXAFS spectroscopies that probe only the local environment of the nucleus target and for Fe only see the arrangement of metals in the (a, b) plane. EXAFS spectroscopy of selenate-GR, GR2(SeO4), at the Se K edge only showed that Se is sur- rounded by O in tetrahedral coordination and did not yield information on the interaction of the anion with the layer. However, a closer examination of the XRD diagrams and decomposition of peaks allowed us to determine thec parameter, which could not be done on raw diagrams.
8.1. Material and methods
Samples were taken in the Gleysol in Fouge`res every 20 cm in the beginning of October 1998, from 20 to 100 cm (Table 3). At that time, the first two samples (silty) showed oximorphic conditions, the third (silty) and fourth (saprolite) showed reductimorphic conditions, and the last (saprolite) showed oximorphic conditions. As shown previously (Feder, 2001; Feder et al., 2005), the limit between reducing and oxidized horizons fluctuates seasonally, getting deeper during winter. Clay fractions were separated by sedimentation under inert atmosphere in a glove box, and saturated with Mg, K, and ethyleneglycol or heated to 350 C, 450 C, and 550 C following the classical protocol (Robert and Tessier, 1974; Caille`re et al., 1982). Spectra were acquired with a Siemens D-500 (40 kV, 20 mA) diffractometer, equipped with a graphite monochromator. The line was Co-Ka1, and spectra were acquired from 3 to 45 (2y), by 0.02 steps, 10 s counting time per step, in scan mode. Labeling of peaks was done with Diffrac AT EVA on the basis of JCPDS ( Joint Committee on Powder Diffraction Standards) database. As the main peak of GRs is very close to the main peak of kaolinite, we used the DECOMPXR software (Lanson and Besson, 1992) in the range 3–15(2y), to decompose the peaks.
8.2. Results
The raw and decomposed diagrams are shown inFigs. 10–14.
The main peak of GR1(Cl) is at 7.97 A˚ and is masked by kaolinite at 7.13 A˚ . Quartz, K-feldspar, and plagioclase feldspars are present. Lepido- crocite is present in the deepest sample (oxidized arena) at 6.26 A˚ .
In addition to kaolinite, omnipresent at 7.13 A˚ , the peak at 10.01 A˚ that does not collapse with K can be ascribed to illite, the 14.36 A˚ peak that collapses with K (sample 1) to true vermiculite, the 14.1 A˚ peak that does not collapse with K to Al-smectite (samples 3, 4, and 5), the peak at 13.2 A˚
254 Fabienne Trolard and Guilhem Bourrie´
2q Co Ka1/degrees
0 5 10 15 20 25 30 35 40 45
Saturation magnesium
Saturation ethylene glycol
Saturation potassium Heating to 350 °C
Heating to 450 °C
Heating to 550 °C
2q Co Ka1/degrees
6 8 10 12 14 16
Intensity
Intensity 10.01Å
10.53Å
11.40Å 10.46Å
12.33Å 10.77Å
12.67Å 8.39Å8.18Å 6.71Å
12.25Å
14.38Å14.36Å 12.37Å 10.77Å
12.59Å
3.53Å
12.13Å 4.44Å
12.24Å14.38Å 12.25Å 6.71Å 2.45Å
4.70Å 3.57Å 2.38Å
3,24Å
7.13Å
9.98Å 4.97Å 4.25Å 3.19Å 2.49Å
3.34Å
14.36Å 8.13Å8.39Å 7.13Å
Heating to 350 °C
Heating to 450 °C
Heating to 550 °C
Saturation ethylene glycol
Saturation magnesium
Saturation potassium
Figure 10 Raw diagram and decomposed diagram of sample 1.
2 q Co Ka1/degrees 2 q Co Ka1/degrees
0 5 10 15 20 25 30 35 40 45
Intensity
2.50Å
3,35Å
4.27Å
5.00Å
10.09Å 7.18Å 4.75Å 3.58Å 2.85Å 2.39Å
3.21Å3.25Å 2.46Å
Saturation potassium Saturation magnesium Heating to 350 °C Heating to 550 °C
13.69Å12.66Å10.92Å 3.53Å
6 7 8 9 10 11 12 13 14 15 16
Intensity
Saturation potassium Saturation magnesium Heating to 350 °C
Heating to 550 °C
7.13Å
10.05Å
10.92Å
13.69Å 7.83Å
11.00Å
12.66Å 10.99Å 7.94Å 7.65Å
10.71Å
Figure 11 Raw diagram and decomposed diagram of sample 2.
256
2 q Co Ka1/degrees 2 q Co Ka1/degrees
0 5 10 15 20 25 30 35 40 45
Intensity
Saturation potassium Saturation magnésium
Saturation ethylene glycol
Heating to 450 °C Heating to 550 °C
2.49Å
3.34Å 2.38Å
2.83Å
4.98Å 4.25Å
4.71Å
9.99Å 7.12Å
14.21Å 3.57Å
14.20Å14.02Å 3.19Å3.23Å
3.53Å
6 8 10 12 14 16
Intensity
Heating to 550 °C
Heating to 450 °C
Saturation magnesium
Saturation potassium
11.04Å
14.02Å 7.92Å
8.91Å
14.24Å 11.43Å
12.68Å 10.91Å 7.13Å
8.96Å
11.06Å 10.05Å 9.84Å10.03Å
Figure 12 Raw diagram and decomposed diagram of sample 3.
257
2 q Co Ka1/degrees 2 q Co Ka1/degrees
0 5 10 15 20 25 30 35 40 45
Intensity
Heating to 550°C
Heating to 450 °C
Heating to 350 °C
Saturation ethylene glycol
Saturation magnesium
Saturation potassium
3.33Å 2.49Å 2.38Å
4.98Å
7.13Å
10.00Å 4.25Å
4.72Å 2.85Å
3.57Å
14.06Å 3.53Å 2.45Å
3.21Å
14.04Å
6 8 10 12 14 16
Intensity
Saturation potassium Saturation magnesium
Heating to 350°C Heating to 450 °C Heating to 550 °C
7.13Å
10.01Å
10.69Å
13.38Å 8.01Å
11.06Å
14.26Å 8.01Å
10.58Å
13.95Å14.08Å 11.30Å 7.86Å7.89Å
10.92Å
14.04Å
Figure 13 Raw diagram and decomposed diagram of sample 4.
258
2 q Co Ka1 /degrees 2 q Co Ka1 /degrees
0 5 10 15 20 25 30 35 40 45
Intensity Intensity
Saturation potassium Saturation magnesium
Saturation ethylene glycol
Heating to 350 °C Heating to 450 °C Chauffage
à 550 °C
3.34Å 2.50Å
4.98Å
10.03Å12.33Å12.59Å 7.14Å 3.57Å 3.19Å
13.19Å 2.38Å
2.83Å
4.72Å
6.26Å
14.15Å14,20Å14.10Å 4.26Å 3.24Å
3.54Å3.46Å
7.90Å
6 8 10 12 14 16 18
Saturation potassium Saturation magnesium
Heating to 350 °C Heating to 450 °C Heating to 550 °C
14.10Å14.20Å 13.19Å12.59Å12.34Å 10.03Å 7.14Å 6.26Å
11.09Å11.13Å 7.83Å
10.49Å10.70Å10.64Å 7.65Å7.48Å7.69Å
Figure 14 Raw diagram and decomposed diagram of sample 5.
259
to hydroxy-Al vermiculite (sample 5), and the peak around 11 A˚ either to interstratified illite-smectites or to intergrade minerals. This complex para- genesis is classical in acid brown soils (Alocrisols) on granite in oceanic climate, and the abundance of Al in the interlayers of vermiculites and smectites in acid soils has been described long ago (Gjems, 1963; Tardy and Gac, 1968). In Fouge`res, a micropodzolization in surface superimposes on acid brown pedogenesis, and Al is present at large concentrations in solutions, and largely as ‘‘Al13’’ polymer (Bourrie´, 1981; Aurousseauet al., 1987; Bourrie´et al., 1989).
The peak of fougerite should be near 7.97 A˚ . The peak at 8.13–8.39 A˚
in sample 1 is very small and cannot be interpreted. However, samples 2–4 (K-saturated) clearly show peaks at 7.94, 7.92, and 7.89 A˚ , respectively.
This peak disappears on heating and shifts to either smaller or larger values with Mg saturation. In sample 5, the peak present at 7.48 A˚ shifts to 7.65 A˚
with Mg and larger values when heating, until it disappears at 550C.
8.3. Discussion
We propose to ascribe the peaks observed in samples 2–4 to fougerite. We exclude the Mg-saturated samples for the following reason: large concen- trations of Mg in the milieu can result in an absorption of Mg by the mineral, with release of Fe, which can explain the variability observed.
This is not likely to occur with K, due to the larger size of this cation. The main peak of GR1 is ascribed tod003, which leads tocẳ3d003ẳ2.375 0.0075 nm, from the three experimental values above. This value is close to the value of GR1(Cl), that is, c ẳ2.3856 nm (Table 4), but smaller; it is much larger than the value for GR1(CO3), c ẳ 2.256 nm (Abdelmoula et al., 1996), which would shift the main peak tod003ẳ7.52 A˚ . Fougerite is, thus, closer to GR1(Cl) than to GR1(CO3).
Abdelmoulaet al., (1996) obtainedcẳ2.256, 2.255, and 2.274 nm, in the range classically admitted for GR1(CO3), [2.25–2.28] nm. The confi- dence interval of the average of our three measurements is given byts= ffiffiffi
pn . Withnẳ3,nẳ2 degrees of freedom,tẳ2.92 from Student distribution, and s ẳ 0.0075 nm, at the probability level a ẳ 0.05, the confidence interval for the average of cis [2.36–2.39], which is completely out of the range given above [2.25–2.28]. The average of our measurements cannot, thus, be taken as an estimate of c of GR1(CO3), and we can rule out definitively CO23as the interlayer anion inFouge`res—fougerite.
For GR1(Cl), the best values are derived from the refinement of the structure (Refait et al., 1998a, Table 4): c ẳ 2.385 0.006 nm. The confidence interval on the average of three measurements of a GR1(Cl) is given byts= ffiffiffi
pn
, in which we can usetẳ1.96, at the probability levelaẳ 0.05, from the normal distribution, asc is derived from a large number of measurements and statistical refinement. The confidence interval on the average is [2.378–2.392]. The average obtained, c ẳ2.375 nm, is outside
260 Fabienne Trolard and Guilhem Bourrie´
this confidence interval, though close to it. We cannot rule out definitively Cl, but solutions are always largely undersaturated with respect to chloro- fougerite (Federet al., 2005, and this study), and it is thus highly doubtful that Cl be the interlayer anion. As Cl is larger than OH (0.181 vs 0.135 nm), the decrease observed incis logical, and in favor of OHas the interlayer anion in Fouge`res—fougerite, as previously assumed on the basis of mineral/solutions equilibria (Ge´nin et al., 1998; Bourrie´ et al., 1999;
Federet al., 2005). The further decrease incwith carbonate in the interlayer cannot be ascribed simply to the radius of carbonate as compared to OH, as oxygen anion in both cases is the larger atom. We could ascribe it to a less compact arrangement with OHdue to hydrogen bonding.
For sample 5, the data are more complex. This sample is oxidized and showed the presence of lepidocrocite. Lepidocrocite is known as a product of oxidation of GR (Schwertmann and Fechter, 1994; Lin et al., 1996;
Srinivasanet al., 1996), in this case fougerite. We can, thus, ascribe the peak in sample 5 to a precursor of lepidocrocite. A good candidate would be the ill-defined ‘‘ferric GR,’’ a Fe(III) compound keeping the layered structure of GRs obtained with partial deprotonation of OH (oxolation) (see below).
Abdelmoulaet al.(1996)obtainedcẳ2.202 nm, that is,d003ẳ7.34 A˚ , for this compound. The value observed here is 7.48 A˚ , intermediate between fougerite and ‘‘ferric GR.’’ One would, thus, see the beginning of the transformation of fougerite into lepidocrocite through ‘‘ferric-GR’’ (or
‘‘proto-lepidocrocite’’), the oxidation leading to a contraction of the cell, consistent with the smaller size and larger charge of Fe3þ.
The criteria for identification of fougerite are reported inTable 7.