Low-temperature Hydrothermal Conditions Research Articles

Grain-Size Characteristics of Mullite Nanocomposites Prepared by Open-System Hydrothermal Crystallization Method

In this research, raw kaolin is processed by oxidation-reduction fining method and then baked to produce mullite precursor, which can prepared multi-phase mullite nanocrystals at low temperature hydrothermal conditions under normal pressure. According to the analysis of the controlling factors of NaOH mole ratio in the hydrothermal system, reaction temperature, holding time and the precursor concentration, etc., we studied the effect of above-mentioned factors on the grain size of product. The results show that when NaOH concentration 3mol/L, hydrothermal reaction temperature 80°C, holding time 3 h, precursor concentration 0.15g/ml, the morphology of the prepared nanocomposite with grains size of 60 ~ 90nm appears spherical-like, also the uniform dispersion can be acquired.

Tetrahydroborate sodalite nanocrystals: Low temperature synthesis and thermally controlled intra-cage reactions for hydrogen release of nano- and micro crystals

Tetrahydroborate sodalite nanoparticles were successfully synthesized under low temperature hydrothermal conditions (333 K) from high alkaline aluminosilicate gels and NaBH 4 salt. The products were characterized by X-ray powder diffraction, scanning electron microscopy, FTIR spectroscopy and thermogravimetry in comparison with the microcrystalline sample Na 8[AlSiO 4] 6(BH 4) 2· NaBH 4-sodalite of 25 nm average crystal size agglomerated by X-ray amorphous hydrosodalite type phase to bigger particles up to ∼100 nm was observed after a reaction period of 12 h. The sodalite host framework protects the BH 4 - ions from hydrolysis under open conditions at room temperature. Hydrogen release is detected via nitrate tracer reduction in temperature dependent IR (TIR) experiments. A total conversion of the BH 4 groups for the nanocrystalline sodalite sample is reached at 773 K whereas a larger amount of the BH 4 - -units remained stable within the microcrystalline sample under the same conditions. This can be explained by the smaller crystal size and the high amount of inter grown hydrosodalite type phase in the BH 4 - -sodalite nanocrystals compared to the microcrystals. The minor hydrosodalite type phase also present in the microcrystalline sample can be reloaded with water and the main hydrogen release reaction BH 4 - + 2H 2O => 4H 2 + BO 2 − can be continued.

One pot synthesis and crystal structure of aluminosilicate mixed chloro-iodo sodalite

The crystal structure of different compositions of aluminosilicate mixed chloride and iodide sodalites was studied. The sodalites have been characterized by X-ray powder diffraction using Reitveld refinement in the space group P \( \bar 4 \) 3n. FT-IR spectroscopy shows the successful enclathration of chloride and iodide anions in the β-cages of sodalite. Different sodalites are prepared under low temperature hydrothermal conditions in which zeolite A is used as starting material with variation of concentration of sodium chloride and iodide. The products obtained were washed with deionized water and dried overnight at 100°C. The increase in concentration of iodide shows that the Al-O-Si angle and distances in Na8[AlSiO4]Cl2−x I x sodalite (0< x ≤ 2) are modified considerably. X-ray diffraction study clearly shows variation in unit cell from 8.9241 to 8.9910A with increase in concentration of sodium iodide. IR study also shows pronounced shift in frequency with concentration of iodide. Amount of chloride and bromide was determined by potentiometry. SEM study shows cubic crystal morphology.

The geochemical and genetic role of organic substances in postmagmatic derivatives of alkaline plutons

Solid bituminous substances (SBS) are common components of the late hydrothermal mineral assemblages of peralkaline pegmatites. SBS are formed in a reductive setting as a result of progressive sorption of minor carbon-bearing molecules (CO, CO2, CH4, C2H6, C2H4, etc.), their polymerization, transformation into aromatic compounds (reformation), and selective oxidation on microporous zeolite-like Ti-, Nb-, and Zrsilicates serving as sorbents and catalysts. The oxygen-bearing aromatic compounds with hydrophile functional groups (−OH, −C=O, −COOH, −COO) act as complexing agents with respect to Th, REE, U, Zr, Ti, Nb, Ba, Sr, Ca, resulting in transfer of these bitumenophile elements under low-temperature hydrothermal conditions in the form of water-soluble macroassociates of the micelle type. Th, REE, and to a lesser extent, U, Zr, Ti, and Nb concentrate at the late stage of the hydrothermal process as microphases impregnating SBS or macroscopic segregations of Th and REE minerals. At the final stage, homogeneous SBS break down into organic (partly together with Ca, Sr, Ba, and Pb) and mineral (with Th, Ln, Y, Ti, Nb, Ca, Na, K, Si) microphases.

Mn-kaolinite synthesis under low-temperature hydrothermal conditions

Mn-kaolinite was hydrothermally synthesized, for the first time, using silicic acid, Mn-carbonate and aluminum nitrate mixed with aqueous solutions of KOH at 100–200 °C for 24–96 h. Without the addition of Mn-carbonate to silicic acid and aluminum nitrate mixture, no crystalline material could be synthesized by hydrothermal treatment up to 200 °C for 6 days. However, by the addition of Mn-carbonate to the above precursors, Mn-kaolinite was synthesized at both 100 and 200 °C for 1 or 2 days, indicating that Mn played an important role in the formation of kaolinite. At a reaction temperature of 100 °C, the Mn-kaolinite peaks became sharper with increasing reaction duration, as expected.

Facile synthesis of ceria rhombic microplates

Interest in the fabrication of inorganic particles with controlled size and shape is driven by their potential unique and tunable physical properties and the desire to engineer advanced bulk materials from microor nano-scale building blocks [1, 2]. CeO2 is a technologically important rare earth material because of its wide applications as fast ion conductors, oxygen storage capacitors, catalysts, UV blockers, polishing materials, and optical displays [3–7]. Both single crystal and polycrystalline ceria microor nano-structures with different shapes, such as nanopolyhedra [8], nanocubes [9], nanospheres [10], nanorods/nanowires [11–16], coreshell nanowires [17], nanotubes [18, 19], and triangular microplates [20] have been synthesized by us and other research groups in recent years. It is found that the properties and applications of microor nano-materials are related to their morphologies and sizes. For example, Guo et al. [20] synthesized CeO2 triangular microplates by a thermal decomposition–oxidation process using Ce(OH)CO3 microplates as the precursor, and suggested that these materials with triangular structures could find potential applications in multiple fields as catalysts, storage hydrogen devices, and optically or electrically functional host materials. Zhong et al. [21] reported that the flowerlike micro-ceria can be used as not only an effective sorbent for the removal of pollutants in water treatment but also as an excellent support for gold nanoparticles to remove CO by catalytic oxidation, demonstrating a promising potential in environmental remediation. Sun et al. [22] reported that the monodisperse flowerlike ceria microspheres can be used as a support for catalysts with various purposes. Previous reports show that micro-ceria with the special morphology has a potential application in various fields. However, few studies have been reported on the fabrication of CeO2 rhombic microplates. Recently, we synthesized CeO2 nanoplates, nanorods, and nanotubes using CTAB as a surfactant under a low-temperature hydrothermal condition [23], and we also developed a facile template-free liquid precipitation method to synthesize CeO2 nanorods, nanotubes, nanowires, and nanocubes [24]. Herein, we first report a simple surfactant-assisted method to synthesize CeO2 rhombic microplates.

Synthesis of F-Doped Flower-like TiO2 Nanostructures with High Photoelectrochemical Activity

We report on a novel and facile approach for the direct growth of F-doped flower-like TiO(2) nanostructures on the surface of Ti in HF solutions under low-temperature hydrothermal conditions. The influence of the experimental parameters such as temperature, reaction duration, and the HF concentration on the morphology and photoelectrocatalytic activity of the formed F-doped flower-like TiO(2) nanostructures was systematically studied. The presence of HF and the reaction time play an important role in the formation of the F-doped flower-like TiO(2) nanostructures. The synthesized novel F-doped TiO(2) flower-like nanomaterials possess good crystallinity and exhibit high photoelectrochemical activity for water-splitting and photodegradation of organic pollutants compared with P-25, which is currently considered to be one of the best commercial TiO(2) photocatalysts. The approach described in this study provides a simple and novel method to synthesize F-doped TiO(2) nanostructured materials that are ready for practical applications such as the photodegradation of wastewater.

Synthesis of brush-like CdS nanorod arrays through a novel hydrothermal reaction of simultaneous solvent-oxidation-hydrolysis

Brush-like CdS arrays composed of well-arranged nanorods have been successfully synthesized for the first time through a novel chemical reaction under the low-temperature hydrothermal condition. The reaction can be described as the solvent-oxidation-hydrolysis reaction among Cd, H2O and thiourea. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL). TEM results showed that these nanorods were about 20 nm wide with lengths varying from 1.25 µm to 3.5 µm. XRD pattern indicated that as-obtained CdS was hexagonal phase with good crystallinity. PL spectrum showed the products had novel optical property from the bulk counterpart. The formation mechanism was also explored.

Self-assembly of organic acid–base compounds from 2-D layered network to 3-D supramolecular framework: synthesis, structure and photoluminescence

The reactions of organophosphonic acids [1-aminoethylidenediphosphonic acid (AEDPH4), 1-aminopropane-1,1,3-triphosphonic acid (APTPH6)] and 2,2′-bipyridyl-like ligands [2,2′-bipyridyl (bipy), 1,10-phenanthroline (phen)] under low temperature hydrothermal conditions yielded four acid–base compounds, namely, (AEDPH3)·(H2bipy)1/2·2H2O (1), (AEDPH3)·(Hphen)·2H2O (2), (APTPH5)·(Hbipy)·2H2O (3) and (APTPH4)·(Hphen)·(H3O)·3H2O (4). These four compounds were characterized by a single crystal X-ray diffraction method, elemental analysis (EA), infrared spectrometry (IR) and thermogravimetric analysis (TGA). They all crystallize in a triclinic system, with a P space group. In particular, the supramolecular structures of organophosphonic acids in these four compounds are different. In compounds 1 and 2, the dimer or dimer chains are formed by hydrogen bonds between adjacent phosphonate groups, while in compounds 3 and 4 two-dimensional (2-D) layered networks are constructed by the hydrogen bonds between the phosphonate groups. The diversity of supramolecular structures is due to the difference of AEDPH4 and APTPH6 ligands. Additionally, their luminescence in the solid state has also been studied at room temperature.

Isotopic signatures of extinct low-temperature hydrothermal chimneys in the Jaroso Mars analog

The present work presents a geochemical study, focused on the oxygen and carbon isotopic signatures of shallow-marine, carbonate extinct chimneys, from Jaroso Hydrothermal System (JHS). In each chimney a meticulous sampling from the central orifice to the outer rim of the structure was performed. The isotopic geochemistry study allowed to establish the origin and evolution of the fluids during the formation of the vent structures. The negative δ 13C values indicate a source of meteoric water for the Fe-rich fluids. More positive δ 13C values are present in ankerite and in some calcite, both related with marine water. δ 18O in ankerite indicates low-temperature hydrothermal conditions, while in calcite is showing either primary signatures or early diagenesis at low temperature. On the contrary, calcite displaying more negative δ 13C and δ 18O values represents a late mineral phase which was formed under meteoric diagenesis. Each chimney resulted from the precipitation of intergranular carbonate cement around a channellized flux of metal-rich fluid crossing a shallow-marine, unconsolidated, sandy-marl substrate. The paleoenvironmental interpretation carried out from the isotopic data emphasizes the importance of the stable isotopes as fluid geomarkers, also advancing in the understanding of an interesting analog for the geological and astrobiological exploration of Mars.

Photocatalytic performance of pure anatase nanocrystallite TiO 2 synthesized under low temperature hydrothermal conditions

Photocatalytic performance of a hydrothermally synthesized pure anatase TiO 2 with 8 nm average crystallite size for decomposition of Reactive Red 141 was examined by investigating the effects of UV-light irradiation time, irradiation power, amount of TiO 2 and initial dye concentration. Change in the UV absorbance of the dye during irradiation was monitored. One wt.% TiO 2 in 30 mg/l Reactive Red 141 aqueous solution was found adequate for complete decolorization in 70 min at 770 W/m 2 irradiation power. It was realized that, compared to Degussa P-25, the synthesized nano-TiO 2 can be repeatedly used as a new catalyst. The results also proved that Reactive Red 141 is decomposed catalytically due to the pseudo first-order reaction kinetics.

Synthesis of MnOOH nanorods by cluster growth route from [Mn 12O 12( RCOO) 16(H 2O) n] ( R=CH 3, C 2H 5). Rational conversion of MnOOH into Mn 3O 4 or MnO 2 Nanorods

Single-crystalline nanorods of γ-MnOOH (manganite) phase with diameters of 120 nm and lengths of 1100 nm have been prepared using a new cluster growth route under low-temperature hydrothermal conditions starting from [Mn 12O 12(CH 3COO) 16(H 2O) 4]·2CH 3COOH·4H 2O or [Mn 12O 12(C 2H 5COO) 16(H 2O) 3]·4H 2O without any catalyst or template agents. The so-obtained nanorods were studied by X-ray diffraction (XRD), infrared (IR) spectroscopy, Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Their thermal conversion opens an access to Mn 3O 4 (hausmannite) and β-MnO 2 (pyrolusite) nanorods, respectively, under argon or air atmosphere. A coercive field of 12.4 kOe was obtained for the Mn 3O 4 nanorods.

Transport of uranium, thorium, and lead in metamict zircon under low-temperature hydrothermal conditions

Understanding the stability of radiation-damaged zircon under low-temperature hydrothermal conditions is crucial to the application of zircon for U–Pb geochronology and as a host phase for the disposal of plutonium waste. We report the results of an investigation of the stability of partially metamict zircon by leaching experiments at 175 °C with a 2 M AlCl 3 and a 1 M HCl–CaCl 2 solution as hydrothermal fluids for 1340 h. Cathodoluminescence (CL) and backscattered electron (BSE) images show that the zircon grains have developed a reaction rim several micrometers thick or deeply penetrating reticulated alteration zones with sharp boundaries to unaltered metamict zircon. These zones have experienced severe loss of Si, U, Th, and Pb, and gain of Al or Ca, and a water species as revealed by electron microprobe, sensitive high-resolution ion microprobe (SHRIMP) analyses, and infrared spectroscopy. Micro-Raman and infrared measurements on the altered areas show that disordered crystalline remnants of the partially metamict zircon structure were partially recovered, whereas recrystallization of the embedding amorphous phase was not observed. No detectable structural or chemical changes were detected inside the unaltered areas. Intensive fracturing, which was most intense in the HCl–CaCl 2 experiment, occurred inside the altered areas due to the volume reduction associated with the recovery of the disordered crystalline material and probably with the leaching reactions. We explain the formation of deep penetrating alteration zones by a percolation-type diffusion model, which is based on the assumption that percolating interfaces or areas of low atomic density between crystalline and amorphous regions as well as between amorphous domains exist along which fast chemical transport is possible. The idea of the existence of fast diffusion interfaces is supported by the sharp chemical gradients at the margin to unreacted zircon. The model was used to estimate for the first time diffusion coefficients for U, Th, and Pb diffusion in amorphous zircon at 175 °C by assuming that volume diffusion inside the amorphous domains is the loss rate-limiting process. These estimates show that volume diffusion in metamict zircon can cause significant loss of U, Th, and especially loss of radiogenic Pb over geological time scales, even at temperatures as low as 175 °C. Our results show that recent Pb loss discordias can be generated (1) by predominate Pb loss from metamict zircon through volume diffusion at low temperatures where thermal healing of the structure is insignificant, and (2) by leaching of Pb (and U and Th) from metamict zircon through an external fluid.

Hydrothermal Synthesis and Characterization of Bi2Pb2O7 with Pyrochlore Structure

Pyrochlore Bi2Pb2O7 was fabricated under low-temperature hydrothermal conditions and characterized by XRD, IR, SEM DTA–TG, and XPS. XRD analysis shows that the structure of Bi2Pb2O7 is cubic, which is similar to that obtained from high-pressure reactions. XPS measurements suggest that the valences of Bi and Pb ions are +3 and +4, respectively. The cubic Bi2Pb2O7 is a metastable phase; it decomposes into a new phase with liberation of O2 at 450°C.

The Crystallisation Kinetics of Nitrate Cancrinite Na7.6[AlSiO4]6(NO3)1.6 (H2O)2 Under Low Temperature Hydrothermal Conditions

Investigations on the formation of nitrate enclathrated cancrinite were performed under low temperature hydrothermal conditions (353 K, autogeneous pressure). The alkaline transformation of kaolinite in the presence of sodium nitrate was selected as the preferred method of synthesis. All experiments were carried out at high alkalinity, using a 16 molar NaOH solution as well as at low NaOH concentrations (2 molar NaOH). Qualitative phase analysis was performed by X-ray-powder diffraction and IR-spectroscopy. Besides cancrinite formation no further intermediate phases were formed. Quantitative data of the educt conversion into cancrinite was determined by 27Al MAS NMR spectroscopy and integration of the resonance signals of four- and six-coordinated aluminium. Whereas full conversion could be found already after 24 h of reaction under highly alkaline conditions, only an insufficient progress of the reaction resulted in the case of low alkalinity.

Synthesis and crystal structure of carbonate cancrinite Na 8[AlSiO 4] 6CO 3(H 2O) 3.4, grown under low-temperature hydrothermal conditions

The synthesis of cancrinite in the system Na 2O–SiO 2–Al 2O 3–Na 2CO 3–H 2O was studied under low-temperature hydrothermal conditions in the 353 K< T<473 K interval. The aim was to reveal the suitable range for the crystallization of pure-phase carbonate cancrinite with the ideal composition Na 8[AlSiO 4] 6CO 3(H 2O) 2 without cocrystallization of sodalite or intermediate disordered phases between cancrinite and sodalite. It was found that cancrinite formation reacts very sensitive on the temperature within the autoclaves whereas the concentration of reactants and the alkalinity of the hydrothermal solution have a much lower influence on the phase formation. Thus the temperature of crystallization of carbonate cancrinite without any by-products should not remain below 473 K. At the lower reaction temperature of 353 K the formation of a disordered intermediate phase between the cancrinite and the sodalite structure has been obtained in every case, independent of the template concentrations and the base. Some problems to detect this in a typical powder product mixture are discussed. Besides the 29Si and 27Al MAS NMR characterization of the products, the crystal structure refinement of pure carbonate cancrinite of ideal composition Na 8[AlSiO 4] 6CO 3(H 2O) 3.4, has been carried out from X-ray powder data using the Rietveld method: P6 3, a=1271.3(1) pm, c=518.6(1) pm, R WP=0.073, R F=0.016 for 347 structure factors and 45 variable positional parameters.

Subsolidus rubidium-dominant feldspar from the Morrua pegmatite, Mozambique: paragenesis and composition

Abstract At the Morrua pegmatite, Mozambique, alkali feldspar has replaced pollucite under low-temperature (250–150°C) hydrothermal conditions. Fluids invading a fracture system in pollucite formed round granular aggregates of (K-Rb)-feldspar in three stages: (1) a compositionally heterogeneous core of the feldspar cluster (+cookeite ± apatite) with 7–20 mol.% RbAlSi3O8, grading outward into a Rb-dominant feldspar with 66 mol.% RbAlSi3O8 (20 wt.% Rb2O); (2) an intermediate layer of non-porous, inclusion-free, end-member K-feldspar; (3) an outer layer of porous end-member K-feldspar. Feldspars of all three stages seem to be monoclinic and disordered, with metastable sanidine structure. Zoning in K/Rb, preserved on a fine scale, was formed during growth at a temperature too low for subsequent alkali-cation diffusion or (Al,Si)-ordering.

Phase stability of (Y,NB)-TZP/Al 2O 3 composites under low temperature hydrothermal conditions

Y 2O 3 and Nb 2O 5 co-doped tetragonal zirconia polycrystals (TZP) containing 10 to 30 vol.% Al 2O 3 ((Y,Nb)-TZP/Al 2O 3) were sintered for 5 h at 1550°C in air and hydrothermal stability of the composites was evaluated after aging for 5 h at 180°C in 0.3 MPa H 2O vapor pressure. (Y,Nb)-TZP/Al 2O 3 composites showed superior phase stability under the hydrothermal condition as compared with 3Y-TZP/Al 2O 3 composites due to the effects of Y–Nb ordering in t-ZrO 2 and the Al 2O 3 addition. The optimized strength and fracture toughness were 670 MPa and 7.3 MPa m 1/2, respectively, when 20 vol.% of 2.8 μm Al 2O 3 particles were added.

Bariosincosite, a New Hydrated Barium Vanadium Phosphate, from the Spring Creek Mine, South Australia

Bariosincosite is a new barium vanadium phosphate hydrate from the Spring Creek Mine, near Wilmington, South Australia. The new mineral occurs as irregular clusters of pale green, very thin platey crystals up to 250 µm across and 2 to 5 µm thick. The tetragonal crystals are tabular on {001} and the other form present is {100}. Associated with bariosincosite are quartz, cuprite, native copper, fluorapatite, whitlockite, baryte and springcreekite, BaV33+ (PO4)2(OH,H2O)6. Bariosincosite appears to have formed under supergene or low-temperature late-stage hydrothermal conditions. Electron microprobe analysis yielded: BaO 23.20; SrO 4.19; CaO 0.36; VO2 31.55; Fe2O3 0.20; Al2O3 0.50; P2O5 28.15; H2O 13.93 (calculated). These data give an empirical formula of (Ba0.77Sr0.20Ca0.03)∑1.00[(V0.964+ Al0.03Fe0.013+)∑1.00O(PO4)]2·4H2O, calculated on the basis of two P atoms. The simplified formula is Ba(V4+OPO4)2·4H2O. The mineral is transparent with a very pale green streak, a vitreous lustre and an estimated Mohs hardness of 3. The strongest lines in the X-ray powder pattern are [dobs (Iobs) (hkl)] 6.414 (20) (110, 002); 5.748 (70) (111); 4.552 (30) (112, 200); 3.198 (20) (220, 004); 3.100 (100) (203, 221); 2.847 (40) (222, 114); 2.786 (80) (311); 2.368 (30) (313, 115); and 2.017 (100) (420, 332, 116). These data were indexed on a tetragonal cell, with a = 9.031(6), c = 12.755(8) Å and V = 1040(1) Å3; the space group is probably P4/n or P4/nmm. For Z = 4 and using the empirical formula, the calculated density is 3.306 gm/cm3. Bariosincosite is uniaxial negative with ω = 1.721(2) and ε = 1.715(2) (white light); pleochroism is weak from colourless (E) to pale green (O), absorption O &gt; E. The mineral is named for the relationship to sincosite, Ca(V4+OPO4)2·4H2O.

Crystallization kinetics of Na7.4(AlSiO4)6(CO3)0.7·4H2O, an intermediate phase between cancrinite and sodalite, grown under low temperature hydrothermal conditions

During low temperature hydrothermal alkaline conversion of kaolinite in the presence of sodium carbonate a disordered phase crystallizes showing structural features between the sodalite and cancrinite minerals. A detailed study of its formation field and the kinetics of crystallization is the first step for further investigations of this zeolite-like material. The crystallization kinetics was investigated in the early stage of the reaction for times up to 192 h at a temperature of 353 K. Besides X-ray powder diffraction27Al MAS NMR has been found to be the preferred method to follow the reaction kinetics because a signal assigned to six-coordinated aluminium of the starting kaolinite can be clearly distinguished from a second resonance, being typical for aluminium in four-coordinate position of an aluminosilicate framework structure. Important conclusions can also be drawn from the experiments for the synthesis of ideal ordered cancrinites.