Thickness Distributions and Evolution of Growth Mechanisms of NH₄-Illite from The Fossil Hydrothermal System of Harghita Bãi, Eastern Carpathians, Romania

Abstract

Abstract The crystal growth of NH 4 -illite (NH 4 -I) from the hydrothermal system of Harghita Bãi (Eastern Carpathians) was deduced from the shapes of crystal thickness distributions (CTDs). The <2 mm and the <2-0.2 mm fractions of clay samples collected from the argillized andesite rocks consist of NH 4 -illite-smectite (I-S) interstratified structures (R1, R2, and R3-type ordering) with a variable smectite-layer content. The NH 4 -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 NH 4 -I crystals (fundamental illite particles) which make up the NH 4 -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 NH 4 -I crystallites were determined by the Bertaut-Warren-Averbach method using the MudMaster computer code. The T mean of NH 4 -I from NH 4 -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 (NH 4 -I) and from 12.1 to 24.7 nm (K-I). The CTD shapes of NH 4 -I fundamental particles are asymptotic and lognormal, whereas illites from NH 4 -I/K-I mixtures have bimodal shapes related to the presence of two lognormal-like CTDs corresponding to NH 4 -I and K-I. The crystal-growth mechanism for NH 4 -I samples was simulated using the Galoper code. Reaction pathways for NH 4 -I crystal nucleation and growth could be determined for each sample by plotting their CTD parameters on an α-ß 2 diagram constructed using Galoper . This analysis shows that NH 4 -I crystals underwent simultaneous nucleation and growth, followed by surface-controlled growth without simultaneous nucleation.