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

Ammonium-illite (NH4-I) is one of the alteration products present in a breccia structure in the fossil hydrothermal system from Harghita Bai (East Carpathians), Romania. A series from smectite (S) via ordered interstratified structures to NH4-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+-, Mg2+-, 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 degrees C show a transition from 1 Md 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 x 10(4) nm(2). The mean thickness of the NH4-I layers ranges from 4.62 to 7.89 nm. The calculated structural formula of end-member NH4-I (5%S) is: [(NH4+)(066)K0.10+Na0.Sr-+(01)0.02(+2)](0.81) (Al1.853+Fe0.013+Mg0.152+)(2.01)(Si-3.30(4+) Al-0.70(3+))(4.00)O-10(OH)(2). The fixed NH4+ 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 NH4-I-S series formed via direct precipitation from solution at different temperatures.