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

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Clays and Clay Minerals

Abstract

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 <2 mm and the <2-0.2 mm fractions of clay samples collected from the argillized andesite rocks consist of NH4-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 NH4-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 (Tmean) and crystallite thickness distribution (CTD) of NH4-I crystallites were determined by the Bertaut-Warren-Averbach method using the MudMaster computer code. The Tmean of NH4-I from NH4-I-S samples ranges from 3.4 to 7.8 nm. The Tmean 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-ß2 diagram constructed using Galoper. This analysis shows that NH4-I crystals underwent simultaneous nucleation and growth, followed by surface-controlled growth without simultaneous nucleation.

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Authors and Affiliations

  1. Centre of Geology, Faculty of Sciences, University of Porto, Portugal

    Iuliu Bobos

  2. Emeritus Scientist, U.S. Geological Survey, 3215 Marine Street, Suite E127, Boulder, Colorado, 80303-1066, USA

    D. D. Eberl

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Bobos, I., Eberl, D.D. Thickness Distributions and Evolution of Growth Mechanisms of NH4-Illite from The Fossil Hydrothermal System of Harghita Bãi, Eastern Carpathians, Romania. Clays Clay Miner. 61, 375–391 (2013). https://doi.org/10.1346/CCMN.2013.0610415

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