Na-form synthetic fluorine micas have very high selectivity for alkaline metal ions in ion-exchange reactions. Na-form taeniolite (NaT, NaMg2LiSi4O10F2.nH2O), exchanges its Na+, the interlayer cation, for K+, Rb+, NH4+, etc. in solutions. Na-form hectorite (NaH, Na1/3Mg8/3Li1/3Si4O10F2.nH2O) exchanges for Rb+, Cs+, and other cations. NaT shows specially high selectivity for K+, Rb+, Cs+, and NH4+, and NaH shows the same for Rb+ and Cs+. NaT and NaH were exchanged for Rb+ and XAFS of the exchanged ion were measured. The structural changes were studied with powder X-ray diffraction patterns to reveal the relation of their ion-exchange selectivity and the local structure around interlayer ions.
Fluorine micas were used after removing impurities such as SiO2(tetra) or starting materials by washing with water and separation with centrifugation. RbCl solutions were used for the reactions under 25°C. Exchange ratios of Rb+ to Na+ in the micas were varied from 0.5% to the maximum. X-Ray absorption spectra of Rb K-edge were obtained at BL-10B. X-Ray powder diffraction patterns were measured by Rigaku RINT Ultima+. Samples were prepared to be wet with the exchange reaction solution for both measurements to avoid their structural changes by drying.
Figure 1 shows XANES spectra of Rb K-edge uptaken into NaT and NaH and 0.1 mol/dm3 RbNO3 solution. It shows that Rb takes another local structure than in the solution state. All of the exchanged NaT of different exchange ratio showed same XANES spectra. XANES for NaH of low exchange ratios were the same as NaT, but those of the high ratios become somewhat similar to that for the aqueous solution. EXAFS Fourier transforms of exchanged micas are shown in Fig. 2. In Fig. 2, many peaks can be assigned according to the crystal structure of taeniolite (its original K-form), KMg2LiSi4O10F2) There is no peak or shoulder for the hydrated water oxygen at the position (about 0.28 nm) for the aqueous solution. Taeniolite (K-form) has no interlayer water molecule and gives similar interatomic distances as shown in Fig. 2. After drying at 500°C, the maximum-exchanged NaT also shows the same characters to Figs. 1 and 2. Therefore, Rb+ is placed as the anhydrous form in Rb-exchanged NaT at all exchange ratios and in NaH at low ones.
X-Ray diffraction patterns around basal plane reflection, i.e., (001) diffraction of 1M-type mica, of NaT and NaH after Rb-exchange reaction for the low and the high exchange ratios are shown in Fig. 3. NaT show two- (2theta = 5.8°), one- (7.4°), or no-water layer (9.2°) phase of Na-form taeniolite and anhydrous Rb-form phase (8.7°) owing to their exchange ratio. NaH give only Rb-form hectorite phase with two- (2theta = 5.7°), one- (6.9°), or no-water (8.6°) layers. In both micas, the two-water layers decrease with increasing the ratio, the anhydrous layers increase instead. From the diffraction pattern changes with the exchange ratios, the layers of the micas containing water can exchange its Na+ and they become an anhydrous form.
The diffraction pattern changes of NaH accord with those of NaT except the species of the interlayer cation. But, it is not consistent with the XAFS results. This inconsistency may be come from the difference of selectivity of the two methods. At the low exchange ratio, most of the Rb-containing layers of NaH are anhydrous but would be spread randomly in the sample. Therefore, there is no diffraction peak at 2theta = 8.6° at low exchange ratios.
Photon Factory Activity Report, 13, 157 (1997).
Questions or comments are welcome.