American Mineralogist, Volume 97, pages 962–982, 2012 0003-004X/12/0506–962$05.00/DOI: http://dx.doi.org/10.2138/am.2012.3801 962 Characterization of smectite to NH 4 -illite conversion series in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania IulIu BoBos* Centre of Geology, Faculty of Sciences, University of Oporto, Rua do Campo Alegre 687, 4169-007 Oporto, Portugal ABstrAct Ammonium-illite (NH 4 -I) is one of the alteration products present in a breccia structure in the fossil hydrothermal system from Harghita Bãi (East Carpathians), Romania. A series from smectite (S) via ordered interstratified structures to NH 4 -I (40 to 5%S) was characterized by X-ray diffrac- tion (XRD), Fourier transform-infrared spectroscopy (FTIR), scanning and transmission electron microscopy (SEM and TEM), and chemical analyses. Calculation of one-dimensional X-ray patterns was simulated with the NEWMOD code. Transition from two- to one-water smectite interlayer was identified by XRD. Selected samples were saturated with K + -, Mg 2+ -, and Li + -cations to differentiate low- to high-charge smectite or beidelite layers. X-ray patterns of random powders of K + -saturated samples, heated at 300 °C show a transition from 1Md to cis- and trans-vacant 1M polytype. The cell parameters of the cis-vacant and trans-vacant 1M polytype were calculated by oblique texture electron diffraction. The vibration frequencies at 1430 cm –1 of the N-H bond were identified in the samples analyzed. Scanning and transmission electron microscopy images show morphological changes from flaky to lath-like shapes. The mean shape ratio of lath crystals ranges from 6 to 5.42 nm and the mean area from 7.8 to 24 × 10 4 nm 2 . The mean thickness of the NH 4 -I layers ranges from 4.62 to 7.89 nm. The calculated structural formula of end-member NH 4 -I (5%S) is: [(NH 4 + ) 0.66 K + 0.10 Na + 0.01 Sr 2+ 0.02 ] 0.81 (Al 3+ 1.85 Fe 3+ 0.01 Mg 2+ 0.15 ) 2.01 (Si 4+ 3.30 Al 3+ 0.70 ) 4.00 O 10 (OH) 2 . The fixed NH 4 + content quantified ranges from 0.39 to 0.66 atoms per half unit cell [O 10 (OH) 2 ]. Tetrahedral and octahedral substitutions took place as the %S decreases. The NH 4 -I-S series formed via direct precipitation from solution at different temperatures. Keywords: Ammonium-illite, chemistry, electron microscopy, Harghita Bãi, infrared spectroscopy, polytypes, cell parameters, two- to one-water smectite interlayer, X-ray diffraction IntroductIon The atmosphere, biosphere, continental crust, and mantle are the four major sources providing nitrogen that can be in- corporated in mineral structures by diagenetic, metamorphic, magmatic, and hydrothermal processes. Ammonium is a cation-complex with low-hydration energy and its ionic radius (1.44 Å) is approximately equal to that of K + (1.34 Å). Since the early 1960s, numerous studies have shown that NH 4 + may substitute for K + or other alkali cations in feldspar (Erd et al. 1964; Loughnan et al. 1983; Hori et al. 1986), biotite, muscovite (Vedder 1965; Yamamoto and Nakahira 1966), illite (Stevenson and Dhariwal 1959; Higashi 1978, 1982), and illite-smectite (I- S) interstratified structures (Cooper and Abedin 1981; Nadeau and Bain 1986; Williams and Ferrell 1991). Also, either NH 4 + - exchangeable cation in smectite layers (Čičel and Machajdik 1981) or NH 4 + -fixed cation may occur in I-S layers (Eberl et al. 1986; Lindgreen 1994). Ammonium is a good geochemical tracer in mica and feld- spars (Hall and Neiva 1990; Neiva and Hall 1993) or even in zeolites (Altaner et al. 1991), often being an indicator for gold and mercury (Bloomstein et al. 1987; Wilson and Parry 1990), silver (Ridgway et al. 1991), stratiform base metal deposits (Sterne et al. 1982), and/or sedimentary exhalative ore deposits (Ridgway et al. 1990; Williams et al. 1987). Moreover, the presence of NH 4 -I points to present or past occurrences of oil in sediments (Williams et al. 1989, 1992). Several hydrothermal occurrences of NH 4 -I are known around the world: Japan (Yamamoto 1967; Higashi 1978, 1982; Kawano and Tomita 1988), Central and Eastern Europe (Kozac et al. 1977; Bobos and Ghergari 1999; Šucha et al. 2007), United States of America (Wilson et al. 1992), and New Zealand (Yang et al. 2001). The most common occurrences of NH 4 -I are known in anthracite-rank coals, shales, and mudrocks, associated with diagenetic and anchimetamorphic environments (Cooper and Evans 1983; Sterne et al. 1982; Juster et al. 1987; Daniels and Altaner 1990; Compton et al. 1992; Šucha et al. 1994; Schroeder and McLain 1998; Lindgreen et al. 2000; Nieto 2002; Drits et al. 2002, 2005). Therefore, it could be stated that NH 4 -bearing minerals may be formed by a multitude of mineralogical reactions and may occur in a wide variety of geological environments, where either organic or inorganic origins provided the NH 4 + molecules fixed in aluminosilicates. The reaction of smectite illitization has been extensively studied in either diagenetic or hydrothermal environments, being viewed as a statistically homogeneous and continuous reaction associated with a steady increase of illite interlayers in I-S interstratified structures. Nevertheless, smectite illitization in diagenetic environments implies a heterogeneous reaction via a sequence of intermediate phases characterized by physical * E-mail: ibobos@fc.up.pt