Electron Spin Relaxation and the Mobility of Manganese(II) Exchange Ions in Smectites

The electron spin resonance spectra of Mn2 * exchange forms of vermiculite, nontronite, montmorillonite, and hectorite have been investigated at room temperature. Dipolar inter-actions between neighboring exchange ions and between the exchange ions and structural Fe3* are the most important factors affecting the Mn'z * line widths. These relaxation mechanisms op€rate whether the interlayers are hydrated or dehydrated. Oriented film samples tend to exhibit different line widths when the magnetic field direction is oriented ll and 1 to the silicate sheets, indicating that the interactions between paramagnetic centers are anisotropic. In hectorite, which contains little structural Feu*, dipolar effects have been eliminated by doping 5 percent Mn * into a Md * exchange form, and the doped mineral has been used to examine the solution-like nature of the interlayers. Under fully hydrated conditions where the Mn(HzO)o' * is present in ca 12.5 A interlayers containing several molecular layers of water, the lifetime of the complex ion between collisions with solvent molecules is estimated to be only 30 percent longer than for Mn(H,O)u+ in bulk solution. Under air-dried conditions, where the Mn(HzO)o' * ion is sandwiched between silicate sheets containing two molecular layers of water, a solutionlike spectrum is retained, but the interlayer is considerably less mobile. When the mineral is thermally dehydrated at 200'C, the Mn? ' ions move into hexagonal positions in the silicate structure, and the solution-like spectrum is replaced by one characteristic of Mn' * in certain crVstalline matrices and frozen elasses.