No abstract

No abstract

Abstract~he transformation of kaolinite to halloysite-7 .~ was identified in the kaolin deposit of S~o Vicente de Pereira (SVP), using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). Both the 02,1 i and 13,13 reflections show changes in the XRD patterns along the kaolinite to halloysite-7 ,~ transition, and the FTIR spectra show changes corresponding to both OH and Si-O-stretching bands and Al-O-Si-bending vibrations. The interlayer water content in the kaolinite structure increases during transition. The two-layer periodicity of well-ordered kaolinite and rolling up of kaolinite plates are observed using hi~h-resolution transmission electron microscopy (HRTEM). Long and short tubes exhibit halloysite-7 A. No structural Fe was found in the kaolinite samples. Analytical electron microscopy (AEM) indicates no substitution of AP § for Si 4+. The Si/A1 ratio shows values of --1 for the kaolinite and rolled kaolinite plates. The 27A1 magic angle spinning neutron magnetic resonance (MAS-NMR) spectra display a resonance centered at -1 ppm, assigned to six-coordinated aluminum. The transformation of kaolinite to halloysite-7 A. is controlled by surface reaction.

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.

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.