Nitrogen influx was identified in the Harghita Bãi area, where the mechanism of NH4+-fixation in illitic clays is relevant for the N-input budget estimation. The nanotextural features of K-illite (K-I), NH4,K-I and NH4-illite-smectite (NH4-I-S) mixed layers observed in argillic-altered andesitic rocks from the hydrothermal area of Harghita Bãi (East Carpathians) were studied by X-ray diffraction, infrared spectroscopy and transmission and analytical electron microscopy (TEM-AEM). The texture of undisturbed argillic-altered andesite rocks exhibits chaotic intergrowths of randomly oriented and curved illitic packets with abundant pore spaces and high porosity between packets. The TEM images of K-I and NH4,K-I intergrowths show subparallel packets with clear contacts, exhibiting a diffuse contrast across layers. The thicknesses of K-I and NH4,K-I packets range from 150 to 500 Å, and 1Md and 1M polytypes were identified by selected area electron diffraction patterns. Crystal chemistry of K-I, NH4,K-I and NH4-I-S was carried out by AEM. A third interlayer cation Na+ beside K+ was detected in several NH4,K-I packets. The NH4,Na,K-I packets interleaved with NH4,K-I or NH4-I-S (12% smectite layers) packets were also identified by TEM. The thicknesses of NH4,Na,K-I packets range from 300 to 1200 Å, with abundant lenses and lenticular layer separation along the boundaries between them. The 1Md polytype dominates the NH4,Na,K-I packets. Straight and parallel packets, continuous 00l layers and collapsed swelling layers at the boundary of individual NH4-I (5% smectite layers) packets with thicknesses ranging from 20 to 95 Å were observed. The nanotextural observations indicate direct crystallization of NH4-I crystals within a NH4-I-S series from a pore fluid, where NH4-I packets occupy void spaces previously occupied by fluids.