Random and ordered mixed-layer smectite-ammonium illite (NH₄-I/S) were identified by XRD analysis in the fossil hydrothermal system of Harghita Băi (Eastern Carpathians, Romania). Morphologies of NH₄-I/S were studied by SEM and TEM. SEM image of random NH₄-I/S displays a cellular or cornflake texture. Ordered NH₄-I/S (R=1) exhibits a scalloped morphology with curled edges. Ribbon crystals of illite rise from the surfaces of plate aggregates. Two generations of NH₄-I are distinguished: one developed from a smectite precursor and another from a kaolinite precursor. Hairy illite and illite pseudomorphs after book-like aggregates of kaolinite are present.TEM images show the evolution of morphology from flake to lath habit during the smectite to illite conversion. Particles with veil characteristics and flake particles are observed in random mixed-layered NH₄-I/S. Diffuse lath shaped illite developed on the previous morphology in the expandability range 70–20 % S is observed. Euhedral lath shaped illite is present in NH₄-I/S, below 10 % S. The reaction of smectite-to-illite is continuous, leading to lath shaped illite.