  We report new results of a 57Fe M\"ossbauer study of multiferroic 3R-AgFeO2
powder samples performed in a wide temperature range, including two points, TN1
~ 14 K and TN2 ~ 9 K, of magnetic phase transitions. At the intermediate
temperature range, TN2 < T < TN1, the 57Fe M\"ossbauer spectra can be described
in terms of collinear spin-density-waves (SDW) with the inclusion of many
high-order harmonics, indicating that the real magnetic structure of this
ferrite appears to be more complicated than a pure sinusoidally modulated SDW.
The spectra at low temperatures, T < TN2, consist of a Zeeman pattern with line
broadenings and sizeable spectral asymmetry. It has been shown that the
observed spectral shape is consistent with a transition to the elliptical
cycloidal magnetic structure. An analysis of the experimental spectra was
carried out under the assumption that the electric hyperfine interactions are
modulated when the Fe3+ magnetic moment rotates with respect to the principal
axis of the EFG tensor and emergence of the strong anisotropy of the magnetic
hyperfine field Hhf at the 57Fe nuclei. The large and temperature-independent
anharmonicity parameter, m ~ 0.78, of the cycloidal spin structure obtained
from the experimental spectra results from easy-axis anisotropy in the plane of
rotation of the iron spin. Analysis of different mechanisms of spin and
hyperfine interactions in 3R-AgFeO2 and its structural analogue CuFeO2 points
to a specific role played by the topology of the exchange coupling and the
oxygen polarization in the delafossite structures.
