Article Coexisting nanoscale phases of K-illite, NH 4 ,K-illite and NH 4 -illite-smectite: an organic nitrogen contribution in the hydrothermal system of Harghita Bãi, East Carpathians, Romania Iuliu Bobos* Institute of Earth Sciences – Porto, Faculty of Science, University of Porto, Rua do Campo Alegre 687, 4168-007 Porto, Portugal Abstract Nitrogen influx was identified in the Harghita Bãi area, where the mechanism of NH 4 + -fixation in illitic clays is relevant for the N-input budget estimation. The nanotextural features of K-illite (K-I), NH 4 ,K-I and NH 4 -illite-smectite (NH 4 -I-S) mixed layers observed in argil- lic-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 NH 4 ,K-I intergrowths show subparallel packets with clear contacts, exhibiting a diffuse contrast across layers. The thicknesses of K-I and NH 4 ,K-I packets range from 150 to 500 Å, and 1M d and 1M polytypes were identified by selected area electron diffraction patterns. Crystal chemistry of K-I, NH 4 ,K-I and NH 4 -I-S was carried out by AEM. A third interlayer cation Na + beside K + was detected in several NH 4 ,K-I packets. The NH 4 ,Na,K-I packets interleaved with NH 4 ,K-I or NH 4 -I-S (12% smectite layers) packets were also identified by TEM. The thicknesses of NH 4 ,Na,K-I packets range from 300 to 1200 Å, with abundant lenses and lenticular layer separation along the boundaries between them. The 1M d polytype dominates the NH 4 ,Na,K-I packets. Straight and parallel packets, continuous 00l layers and collapsed swelling layers at the boundary of individual NH 4 -I (5% smectite layers) packets with thicknesses ranging from 20 to 95Å were observed. The nanotextural observations indicate direct crystallization of NH 4 -I crystals within a NH 4 -I-S series from a pore fluid, where NH 4 -I packets occupy void spaces previously occupied by fluids. Keywords: ammonium-illite, one- and two-layer polytype, transmission and analytic electron microscopy, Harghita Bãi, East Carpathians (Received 4 December 2017; revised 7 October 2018) The geological cycle of nitrogen in the Earth system is slow, starting when organic matter sinks and settles in oceanic sedi- ments. Once fixed in sediments and crust, N is converted to NH 4 + via hydrolysis (Hall, 1999). Nitrogen may be carried into subduction zones, where it is volatilized (Elkins et al., 2006), or carried into the mantle, where it is mostly recycled (Marty, 1995). Either subduction or volcanic–basement interaction zones with high geothermic gradients favour volatilization, where N as NH 4 + may be incorporated in clay minerals from argil- lic envelopes (i.e. low- or high-sulfidation zones) related to the fossil hydrothermal systems (Bobos & Williams, 2017). Because NH 4 + has the same charge as K + and a similar ionic radius (1.43 Å and 1.38 Å, respectively), it may substitute K + in the mineral lattice sites; illite (I) is one of the minerals where the NH 4 + may replace K + , forming NH 4 -I (Stevenson & Dhariwal, 1959; Higashi, 1982; Williams & Ferrell, 1991). Several NH 4 -I occurrences have been reported around the world in a wide variety of geological environments (i.e. sediment- ary, metamorphic and hydrothermal). The structure, morphology, crystal chemistry and formation of NH 4 -I in diagenetic to low- grade metamorphic shales, and anthracite-rank coal environ- ments have been extensively studied by X-ray diffraction (XRD), infrared (IR) spectroscopy and electron microscopy techniques (Sterne et al., 1982; Cooper & Evans, 1983; Juster et al., 1987; Daniels & Altaner, 1990; Compton et al., 1992; Lindgreen, 1994; Šucha et al., 1994; Schroeder & McLain, 1998; Lindgreen et al., 2000; Nieto, 2002; Drits et al., 2002, 2005; Árkai et al., 2004; Bauluz & Subías, 2010). The NH 4 -I samples in anthracite-rank coals described by Juster et al. (1987) were studied by Jiang et al. (1990a), where NH 4 -I and K-I intergrown packets without smectite layers were identified by TEM. In very-low-grade metapelites from the Douro-Beira Carboniferous basin in Portugal, Nieto (2002) described the NH 4 - and K-micas segregated into well-separated packets with few interstratifications. In addition, the NH 4 -bearing mica layers intergrown with K-mica layers were identified in low- grade metaclastic rocks and polymetamorphic schists from the Betic Cordillera (Ruiz-Cruz & Sanz de Galdeano, 2008, 2010). X-ray diffraction (XRD) is an accurate method for detecting the interstratifications between NH 4 -I and other 2:1 layer types such as K-I, smectite or vermiculite (Drits et al., 1997a). Thus, *E-mail: ibobos@fc.up.pt Guest Associate Editor: S. Potel This paper was originally presented during the session: ‘GG01 – Clays in faults and frac- tures+ MI-03 Clay mineral reaction progress in very low-grade temperature petrologic studies’ of the International Clay Conference 2017. Copyright © Mineralogical Society of Great Britain and Ireland 2019 Cite this article: Bobos I (2019). Coexisting nanoscale phases of K-illite, NH 4 ,K-illite and NH 4 -illite-smectite: an organic nitrogen contribution in the hydrothermal system of Harghita Bãi, East Carpathians, Romania. Clay Minerals 1–14. https://doi.org/10.1180/ clm.2019.4 Clay Minerals (2019), 1–14 doi:10.1180/clm.2019.4