Characterization of smectite to NH4-illite conversion series in the fossil hydrothermal system of Harghita Bai, East Carpathians, Romania

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 diffraction (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.

Coexisting nanoscale phases of K-illite, NH4,K-illite and NH4-illite-smectite: an organic nitrogen contribution in the hydrothermal system of Harghita Bãi, East Carpathians, Romania

“…In addition, the NH 4 + estimated for the NH 4 ,K-I (sample HB-9) is ∼0.43 apfu (Clauer et al, 2010). The Na + and K + contents determined by flame photometry in the <2 µm clay fraction of NH 4 -I-S (samples HB-18 and HB-12) and the K-I/NH 4 ,K-I mixture (Clauer et al, 2010;Bobos, 2012) are comparable with the AEM data obtained in this study.…”

“…The average compositions determined by AEM for the various specimens are in agreement with the chemical compositions of the <2 µm fractions (Bobos, 2012). Chemical data (Tables 1, 2) show that those compositions have, on average, very scattered values for all elements.…”

“…Sample HB-12 was revisited because greater amounts of Na 2 O than K 2 O were measured in the <2 µm clay fractions by flame atomic absorption spectroscopy (see Supplementary Material, available online) and related XRD patterns did not reveal a possible mixture with K-I. Sample HB-18 was selected due to a higher NH 4 + content reported elsewhere (Bobos, 2012). The andesitic samples with argillic alteration were crushed and sieved to a grain size of <177 µm.…”

“…The chemical compositions of K-I and NH 4 ,K-I (specimen HB-9), NH 4 ,Na,K-I (specimen HB-9a*), NH 4 ,K,Na-I (specimen HB-9a) and NH 4 -I-S mixed layers (samples HB-18 and HB-12) show large compositional variation (Tables 1, 2) in alkali elements. The mean value of NH 4 + fixed in illite for the <2 µm fractions of NH 4 -I-S in samples HB-18 and HB-12 is 0.60 apfu (Bobos, 2012) and 0.52 apfu (see Supplementary Material), respectively. In addition, the NH 4 + estimated for the NH 4 ,K-I (sample HB-9) is ∼0.43 apfu (Clauer et al, 2010).…”

“…The latter case represents a mixed-layer structure of K-I and NH 4 -I layers and the former is similar to the formation mechanism of tobelite (NH 4 -mica) described by Higashi (1978Higashi ( , 1982, in which a progressive substitution of K + for NH 4 + was proposed. The K-I and NH 4 ,K-I physical mixture and NH 4 -I-S mixed layer with a percentage of smectitic layers (%S) ranging from 5% S to 40% S were identified in the hydrothermal area of Harghita Bãi from the Eastern Carpathians (Bobos, 2012;Bobos & Eberl, 2013). The structure, morphology and chemistry of the NH 4 -I-S mixed layer (the <2 µm clay fractions) were studied to provide new insight into the mechanism of smectite tobelitization in hydrothermal systems (Bobos & Ghergari, 1999;Bobos, 2012).…”