Table 1.
Smectite compositions and Fe redox state, as determined by XANES and Mössbauer spectroscopy.
| Smectitea | Composition | Treatment | Fe3+/ΣFeb | ||
|---|---|---|---|---|---|
| XANESc | Mössbauere | Minimum | |||
| A (1.00) | Ca0.17(Fe2+2.58Fe3+0.08Al0.27)VI(Si3.54Al0.46)IVO10(OH)2 | unaltered | 0.03 | 0.00 | 0.00 |
| air-oxidized | 0.57 | 0.24 | 0.24 | ||
| re-crystallized | 0.41 | 0.38 | 0.38 | ||
| re-oxidized | 0.94 | 0.94 | 0.94 | ||
| peroxide | n.d.d | 1.00 | 1.00 | ||
| B (0.90) | Ca0.18(Fe2+2.36Mg0.27Al0.28)VI(Si3.54Al0.46)IVO10(OH)2 | unaltered | 0.00c | 0.00 | 0.00 |
| air-oxidized | 0.38 | 0.80 | 0.38 | ||
| peroxide | n.d.d | 1.00 | 1.00 | ||
| C (0.63) | Ca0.21(Fe2+1.63Fe3+0.09Mg1.01Al0.22)VI(Si3.46Al0.54)IVO10(OH)2 | unaltered | 0.05 | 0.16 | 0.05 |
| air-oxidized | 0.27 | 0.30 | 0.27 | ||
| re-crystallized | 0.45 | 0.32 | 0.32 | ||
| re-oxidized | 0.43 | 0.42 | 0.42 | ||
| peroxide | 1.00 | 1.00 | 1.00 | ||
| D (0.33) | Ca0.18(Fe2+0.87Mg1.75Al0.28)VI(Si3.53Al0.47)IVO10(OH)2 | unaltered | 0.00 | 0.05 | 0.00 |
| air-oxidized | 0.24 | 0.40 | 0.24 | ||
| peroxide | 1.00 | 0.97 | 0.97 | ||
a
Molar ratio Fe/(Mg+Fe) is given in parentheses.
b
Uncertainty in Fe3+/ΣFe is ±0.01 for XANES and ±0.02 for Mössbauer.
c
XANES spectra of unaltered smectite B and nontronite NAu-2 were used as standards for linear combination fitting (LCF); their Fe3+/ΣFe values were defined as 0.00 and 1.00, respectively. Fe3+/ΣFe for other samples was determined by LCF.
d
No XAFS spectra were collected for peroxide-treated A or B.
e
Values for unaltered samples from Chemtob et al. (2015)