The crystal growth of NH₄-illite (NH₄-I) from the hydrothermal system of Harghita Bãi (Eastern Carpathians) was deduced from the shapes of crystal thickness distributions (CTDs). The 4-illite-smectite (I-S) interstratified structures (R1, R2, and R3-type ordering) with a variable smectite-layer content. The NH₄-I-S (40–5% S) structures were identified underground in a hydrothermal breccia structure, whereas the K-I/NH₄-I mixtures were found at the deepest level sampled (−110 m). The percentage of smectite interlayers generally decreases with increasing depth in the deposit. This decrease in smectite content is related to the increase in degree of fracturing in the breccia structure and corresponds to a general increase in mean illite crystal thickness. In order to determine the thickness distributions of NH₄-I crystals (fundamental illite particles) which make up the NH₄-I-S interstratified structures and the NH_4,-I/K-I mixtures, 27 samples were saturated with Li⁺ and aqueous solutions of PVP-10 to remove swelling and then were analyzed by X-ray diffraction. The profiles for the mean crystallite thickness (T_mean) and crystallite thickness distribution (CTD) of NH₄-I crystallites were determined by the Bertaut-Warren-Averbach method using the MudMaster computer code. The T_mean of NH₄-I from NH₄-I-S samples ranges from 3.4 to 7.8 nm. The T_mean measured for the NH₄-I/K-I mixture phase ranges from 7.8 nm to 11.7 nm (NH₄-I) and from 12.1 to 24.7 nm (K-I).

The CTD shapes of NH₄-I fundamental particles are asymptotic and lognormal, whereas illites from NH₄-I/K-I mixtures have bimodal shapes related to the presence of two lognormal-like CTDs corresponding to NH₄-I and K-I.

The crystal-growth mechanism for NH₄-I samples was simulated using the Galoper code. Reaction pathways for NH₄-I crystal nucleation and growth could be determined for each sample by plotting their CTD parameters on an α–β² diagram constructed using Galoper. This analysis shows that NH₄-I crystals underwent simultaneous nucleation and growth, followed by surface-controlled growth without simultaneous nucleation.