Iron phases identified in bentonite from the Lieskovec deposit (Slovakia) by variable-temperature Mossbauer spectroscopy

The clay-mineral fraction of samples from a bentonite deposit in the Zvolenska kotlina Basin (Slovakia), formed by the alteration of andesitic pyroclastics, was dominated by Fe-bearing montmorillonite of relatively low charge. The total Fe content of the samples ranged from 3.58 to 5.81 wt.% Fe. Variable-temperature Mossbauer spectroscopy revealed that structural Fe(III) in smectite accounts for 70-90% of the total Fe in the unfiractionated samples. No structural Fe(II) was observed by Mossbauer spectroscopy, but a small amount was detected by wet chemical analysis. The remainder of the Fe is present in oxide and/or oxyhydroxide phases dominated by poorly ordered goethite and hematite. The hyperfine parameters of the Mossbauer spectra for the raw bentonite are consistent with structural Fe(III) in an alummosilicate phase mixed with varying quantities of hematite, goethite and possibly maghemite. Except in the fine fraction of sample L28, the hematite failed to undergo the Morin transition, as shown by the negative values for quadrupole splitting at 298 K and 6 K.