The densification of dry polar snow and firn results in a continuous increase of density with depth accompanied by significant density fluctuations within seasonal layers. Density measurements of high spatial resolution reveal a persistent minimum of density fluctuations at the vicinity of the snow-firn transition (0.55 - 0.65 g/cm3) in firn core records.In this study we give an explanation for that fluctuation minimum by applying a new method of x-ray-computer-tomography to gain three dimensional structural data of a Greenlandic firn core. At 13 different depths between 10 m and 78 m a set of 16 samples of 40 cm total length for each depth interval were measured. A reconstructed firn segment of 40 cm covers 1-2 years of snow accumulation. Using digital image analysis techniques different structural parameters are estimated including threedimensional pore and particle sizes and specific surface areas. It is shown that the densification rates of snow and firn layers consisting of coarse particles are much higher than those of layers consisting of fine particles within the same depth interval (factor 1.14 in densification rate compared to factor 1.10 in particle diameter). It causes a density crossing of fine and coarse grained layers with a minimum of density variations at the point of crossing over. The crossing over implies that formerly dense layers in the seasonal density signal are not of the same origin as dense layers in the deeper part of the firn column and that the seasonal density signal will totally change shape with depth.Furthermore we discuss the influence of structural properties on the densification process in the context of previous densification models that did not take the effect of structural changes into account. Especially the estimates for densification and pore close off during glacial times should be faced with structural properties of firn under these conditions.