57Fe Moessbauer spectra and magnetic data from the kagome antiferromagnet H3O-jarosite

57Fe Moessbauer spectra are presented from (H3O)Fe3(SO4)2(OH)6 (or H3O-jarosite), which is a model kagome antiferromagnet which features geometrical frustration and spin-glass-like behaviour. Dynamic scaling of the freezing temperature as a function of frequency is observed over a large frequency range, which indicates the presence of a spin-glass transition. A fast relaxation model between 'up' and 'down' states, separated by an energy gap, is presented to account for the shape of the Moessbauer spectra below the freezing temperature. From a calculation of the Electric Field Gradient tensor, it is suggested that H3O-jarosite is an XY-Heisenberg antiferromagnet, where the Fe3+ moments lie in the kagome planes.