Fine-scale determination of OH in radiation-damaged zircon using Synchrotron Fourier Transform Infra Red spectroscopy (FTIR) and Attenuated Total Reflectance (ATR)

The crystal structure of zircon is gradually broken down by the decay of small amounts of U and Th present in zirconium lattice sites. It has been observed that, with increasing radiation damage, zircon can lose radiogenic Pb and at the same time gain water from the environment. The disturbance of the U-Pb system is a major problem in zircon U-Pb geochronology. Water penetration appears to be an important factor in Pb loss but to explore this further a technique is needed for making fine-scale in situ measurements of water on the polished surface of radiation damaged zircons. Our research has shown that Attenuated Total Reflectance Fourier Transform Infra Red (ATR-FTIR) spectroscopy with a synchrotron source and thermal analysis can be successfully used to map the fine-scale distribution of OH/H₂O in radiation damaged zircons. The radiation damaged zircons were found to have an OH/H₂O content varying from 0.4% to 5% with maxima occurring in the heavily damaged (metamict) zones noted for their disturbed U-Pb systems. Whilst thermal analysis confirmed the presence of OH and possibly H₂O the infrared spectra did not distinguish between them.     

Mechanism of Hydrothermal Synthesis of Zircon in a Fluoride Medium

The hydrothermal synthesis of zircon in a fluoride medium leads principally to two kinds of zircons, differentiated by their morphology, chemical composition, thermal stability and solubility. Differences in the thermodynamic parameters of solution (pH, temperature, [F–]) could explain the formation of these two kinds of zircons. The main hypothesis involves two crystallization mechanisms. Both include a step of gel dissolution with formation of soluble species that are responsible for the crystallization. In each case, the silicon species seem to be monomeric without fluorine. However, the nature of soluble zirconium species depends on the thermodynamic parameters of the solution. At acid pH and with fluoride ions, the reactive species for the crystallization seem to be fluoride rich zirconium complexes. In this case, the zircon obtained contains a large amount of fluoride presumably located in silicon sites (Zircon A). At basic pH, and whatever the concentration of fluoride, the reactive species for crystallization seems to be linear polymeric entities. In this case, the zircon obtained shows a typical morphology in the form of layered agglomerates and a high specific surface area (Zircon B).     

Annealing of natural metamict zircons: II high degree of radiation damage

In situ time-dependent high-temperature X-ray powder diffraction was used to study the amorphous to crystalline transition in natural zircons which are characterized by a high degree of radiation damage. It was possible to distinguish two stages of the annealing process: (i) the recovery of the heavily disturbed but still crystalline domains and (ii) the recrystallization of the amorphous regions. The first stage is very fast under the chosen experimental conditions and, at least apparently, is not thermally activated. The second stage is a diffusion-controlled process, whose products (zircon or zircon and zirconia phases) are strongly correlated to the annealing temperature.     

Potentials of Electron Probe Microanalysis in Studying Zircons

Potentials of electron probe microanalysis in determining hafnium, uranium, thorium, lead, niobium, titanium, manganese, iron, chromium, calcium, aluminum, tin, tungsten, yttrium, and lanthanide impurities in zircons have been studied. It was found that the background signal depends on the composition of the zircon crystal, including the concentration of hafnium. Methods have been proposed to improve the measurements of the background intensity and to optimize analytical conditions for attaining the lowest detection limits.     

Breakthrough of 2- to 3-μm scale U–Pb zircon dating using Cameca IMS-1280HR SIMS

A zircon U–Pb dating method with a high spatial resolution of 2- to 3-μm was successfully implemented by using Cameca IMS-1280HR SIMS in this study. Homemade cathode and modified intermediate pole were used to improve the intrinsic performance of duoplasmatron ion source. An O⁻ primary beam with intensity of approximately 3 μA and stability of 0.68% over 40 minutes was achieved and could be used continuously for more than 240 hours under high current conditions. Under Gaussian illumination mode, a 2- to 3-μm O⁻ primary beam was obtained with an intensity of approximately 200 pA. Four standard zircons, namely, 91500, Temora, Plešovice, and Qinghu, were dated using the 2- to 3-μm high spatial resolution U–Pb isotopic analysis, yielding U–Pb ages consistent with their recommended values, within errors. Our work shows that zircon U–Pb dating with high accuracy and spatial resolution can be achieved, on the basis of a series of duoplasmatron modification and instrumental optimization.     

Annealing of natural metamict zircons. I low degree of radiation damage

In-situ time dependent high temperature X-ray powder diffraction was used to investigate the ordering process occurring during annealing of natural zircons with a low degree of radiation damage. It was possible to distinguish two stages of this process. Firstly, the diffusion of defects induced by α-particles, this stage contributes only to a certain degree of relaxation in the unit cell. In the second stage there is some degree of recrystallization. A hkl-dependence in the variation of the integrated intensity is observed and the increase in the volume of crystalline zircon is therefore related to a process of migration of dislocations.     

Determination of uranium, thorium and rare-earth elements in zircon samples using ICP-MS

Zircon is an accessory mineral, which occurs at low concentrations in a wide variety of rocks and is a host for hafnium, rare-earth elements (REE) and radio active elements like uranium and thorium. The presence of uranium in zircon has led to its increased use in the age determination of rocks. Zirconium is also considered as a strategic, hi-tech element because of its various applications, especially in the manufacturing, nuclear and aerospace industries. Analysis of zircon constitutes one of the tough tasks in analytical chemistry as it is a highly resistant mineral and it is extremely difficult to achieve its complete decomposition. In the present work, inductively coupled plasma mass spectrometry has been applied to the determination of hafnium, REE, uranium and thorium in zircon samples using two different sample dissolution procedures, one employing sodium peroxide fusion and another using a fusion mixture of KHF₂ and NaF in 3:1 ratio. Some selected zircon samples originating from different places on the eastern coast of India have been analysed by both the methods and values obtained by both methods were found to be in good agreement with each other. Though a number of international zircon reference materials are available, certified or even proposed values are available only for a very few elements in them. Two zircon reference materials have also been analysed by both methods and usable values have been proposed in this paper. The values obtained by both methods were found to compare well with each other and as well with those reported in literature. The % RSD for all the estimated elements varied from 1.0 to 12.0% at different concentration levels.     

Reduction of UO2 and ThO2 content in zircon mineral from Bangka Region using calcium borate and hydrochloric acid

In an effort to reduce radiation exposure in the use of zircon minerals as opacity for the ceramic industry, it is required that the concentrations of UO₂ and ThO₂ contained in zircon must be less than 500 ppm. The purpose of this study was to reduce the concentration of UO₂ and ThO₂ in zircon minerals. The experimental investigation was initiated by synthesizing of calcium borate (CB), roasting of zircon concentrate with CB at various temperatures, and leaching with hydrochloric acid. The product quality of zircon minerals before and after roasting and leaching was characterized by Fourier Transform Infrared Spectrometer, X-Ray Diffractometer, and X-ray Fluorescence Spectrometer. The experimental results show that roasting zircon concentrate with CB at a zircon/CB ratio of 5/5 (weight/weight), a temperature of 1200 °C, a contact time of 3 h, and the leaching of the roasted results with 6 M HCl can reduce the total concentration of UO₂ and ThO₂ in zircon from 2008,1 ppm to 498.4 ppm. It can be concluded that the prototype zircon product from the experimental results has fulfilled the premium grade zircon with UO₂ and ThO₂ content of less than 500 ppm.     

Zirconia enrichment in zircon sand by selective fungus-mediated bioleaching of silica

One of the important routes for the production of zirconia is by chemical treatment and removal of silica from zircon sand (ZrSixOy). We present here a completely green chemistry approach toward enrichment of zirconia in zircon sand; this is based on the reaction of the fungus Fusarium oxysporum with zircon sand by a process of selective extracellular bioleaching of silica nanoparticles. Since this reaction does not result in zirconia being simultaneously leached out from the sand, there is a consequent enrichment of the zirconia component in zircon sand. We believe that fungal enzymes specifically hydrolyze the silicates present in the sand to form silicic acid, which on condensation by certain other fungal enzymes results in room-temperature synthesis of silica nanoparticles. This fungus-mediated twofold approach might have vast commercial implications in low-cost, ecofriendly, room-temperature syntheses of technologically important oxide nanomaterials from potentially cheap naturally available raw materials like zircon sand.     

A new equilibrium form of zircon crystal

The surface tensions of the {100}, {110}, {101} and {211} faces are calculated by using Machenzie's method. A new equilibrium form of zircon crystal is then derived on the basis of Wulf's Law. This theoretic form consists of {100} prismatic faces and {211} pyramidal faces, which is different from that of the prediction by the periodic bond chain theory. The discovery of the equilibrium form of zircon crystal provides a clue for understanding of the morphology of zircon crystals formed in deep magma chamber, indicating that zircon morphology is an indicator of crystallization conditions.     

Influence of yttria addition on the phase transformations of zirconia from zircon ore by carbothermal reduction process

Zircon ore carbothermal reduction with yttria addition has been carried out. The influences of heating temperature and yttria addition on the phase transformations of zirconia from zircon ore by carbothermal reduction have been investigated in detail. The phase transformations of zirconia from zircon ore by carbothermal reduction were monitored by X-ray diffraction. The microstructure and micro-area chemical analysis of the products were characterized by scanning electron microscopy and energy dispersive spectrometer. The chemical states of Zr 3d, Y 3d and O 1s presented in the products of zircon carbothermal reduction with 10 wt% yttria addition were investigated by X-ray photoelectron spectroscopy. The results showed that the optimized heating temperature of zircon carbothermal reduction with no addition was 1600 °C, and the main phase of the products consists of m-ZrO₂, c-ZrO₂, ZrC and β-SiC. Yttria addition could be introduced into zirconia lattice and caused it to form Y₂O₃ stabilized zirconia. Zirconia in the products would be turned into partially stabilized zirconia with yttria addition from 1 wt% to 5 wt% while it would exist in the form of fully stabilized zirconia with over 8 wt% yttria addition.     

Humidity sensor using zirconium phosphates and silicates. Improvements of humidity sensitivity

Humidity-impedance characteristics were examined for amorphous and crystalline zirconium phosphate, their alkali salts as well as zircon samples calcined in the presence of phosphoric acid or monobasic alkali phosphates. The humidity-impedance characteristic was shown to be improved when the acidic protons of the sample were replaced with alkali cations. For calcined zircon, the impedance in a humid atmosphere was lowered when the sample was calcined with monobasic alkali phosphate as compared with the sample calcined with phosphoric acid, while the surface area hardly depended on the additives. In the case of amorphous zirconium phosphate-bases samples, replacing the acidic protons with alkali cations such as Na and K made the impedance lower and less dependent on the calcination temperature, while it decreased the surface area and the response time. Similar effects of alkali cations on impedance were also observed with crystalline zirconium phosphate.     

The non-isothermal kinetics of mullite formation in boehmite–zircon mixtures

In this work, we studied the kinetics of mullite formation in different composites under non-isothermal conditions using DTA. Different composites based of mullite, alumina, zircon and zirconia were prepared by reaction sintering of boehmite (as alumina source) and zircon. Several mixtures were used while varying the percentage of the boehmite from 30 to 70 mass% with a step of 10. Five compositions marked as B30, B40, B50, B60 and B70 corresponding to boehmite–zircon ratios (mass%) of 30/70, 40/60, 50/50, 60/40 and 70/30 were fabricated and studied. The DTA conducted at heating rates of 10, 20 and 30 K min^?1 showed an endothermic peak in all composites at about 1,603 K associated with mullite formation. The activation energies measured from non-isothermal treatments for five compositions (30, 40, 50, 60 and 70 mass% of boehmite) were 1,029, 1,085, 1,262, 1,508 and 1,321 kJ mol^?1, respectively. The n values (Avrami parameter) for all compositions are larger than 2.5, the mullite crystallization of these composites is followed by three-dimensional growth.     

Tailored nanostructured zirconia polymorphs are prepared from zircon concentrate decomposition products

We report on the hydrothermal synthesis of nanodisperse zirconia powders from the products of zircon concentrate decomposition by alkali solutions of various concentrations in the presence of calcium fluoride. Either tetragonal or monoclinic zirconia, or mixtures of both can be prepared depending on the chemical and phase composition of the zircon concentrate decomposition products and on the conditions under which amorphous zirconium hydroxide gel was prepared. The tetragonal phase is stabilized by calcium ions contained in the initial solutions.     

BiVO4-MCM-41复合催化剂的制备及其对亚甲基蓝降解的光催化活性

将单斜白钨矿结构的BiVO4固载于中孔MCM-41分子筛上,制备了BiVO4-MCM-41复合催化剂,并对催化剂进行了表征,考察了催化剂在光催化亚甲基蓝降解反应中的催化活性.结果表明,BiVO4首先在MCM-41分子筛上形成锆石结构或四面体白钨矿结构的结晶,通过水热处理之后转变为单斜结构的结晶.BiVO4-MCM-41催化剂不仅保持了BiVO4较高的光催化活性,而且提高了对亚甲基蓝的吸附性能,从而提高了对亚甲基蓝降解反应的光催化活性.     

Application of thermal treatment for the interpretation of photoluminescent centres in minerals

The influence of thermal treatments on photoluminescence spectra of several minerals has been investigated. By applying step-wise heating, new luminescence centres were detected which had been not previously recognized in the corresponding minerals. Luminsecence centres appearing as result of valence changing during oxidizing heating include:

1. (UO₂)²⁺ as a result of nonluminescent U⁶⁺ transformation in zircon, barite, francolite and chert;
2. Eu²⁺ as a result of nonluminescent Eu⁺ transformation in barite.

Luminescence centres which were most stable under thermal treatment were Fe³⁺ in zircon and Mn²⁺ in barite. Luminescence centres with similar spectral-kinetic properties but with different thermal stability which allowed them to be separated and properly identified were different metaloxygen complexes (MeO_n)^m? in zircon.     

Spark ablation-inductively coupled plasma spectrometry for analysis of geologic materials

Spark ablation-inductively coupled plasma (SA-ICP) spectrometry is applied to the measurement of hafnium-zirconium ratios in zircons and to the determination of cerium, cobalt, iron, lead, nickel and phosphorus in ferromanganese nodules. Six operating parameters used for the high-voltage spark and argon-ICP combination are established by sequential simplex optimization of both signal-to-background ratio and signal-to-noise ratio. The time-dependences of the atomic emission signals of analytes and matrix elements ablated from a finely pulverized sample embedded in a pressed disk of copper demonstrate selective sampling by the spark. Concentration ratios of hafnium to zirconium in zircons are measured with a precision of 4% (relative standard deviation, RSD). For ferromanganese nodules, spectral measurements based on intensity ratios of analyte line to the Mn(II) 257.610 nm line provide precisions of analysis in the range from 7 to 14% RSD. The accuracy of analysis depends on use of standard additions of the reference material USGS Nod P-1, and an independent measurement of the Mn concentration.     

Estimation of radon and thoron caused dose at exraction and processing sites of mineral sand mining area in Vietnam (HA TINH province)

Mineral sands are mined in several countries to supply to the titanium and zircon producing industries. Coastal black mineral sands usually contain, besides ilmenite (FeTiO₃) and rutile (TiO₂), radioactive minerals such as zircon (ZrSiO₄) and monazite (RePO₄). Radon and thoron activity concentration originated from natural radioactive contents of the black mineral sand was monitored at the extraction and processing for black minerals in the coastal areas of Ha Tinh Province, one of the around 40 coastal mineral sand deposits in Vietnam. The survey was carried out with the Raduet chambers made by Radosys Ltd—Hungary. The obtained results for 25 investigated points show that the measured values are not high in the residential houses and in case of the sand extraction site as well. At the titanium processing plant the measured values were higher than outside the facility (Radon: 18–55 Bq/m³ with average of 34 Bq/³ and Thoron 33–118 Bq/m³ with average of 58 Bq/m³) but still comparable to the average concentration of the world published by UNSCEAR. The typical outdoor levels of radon and thoron gas are each of the order of 10 Bq/m³. Although the radon concentrations were low in the zircon and titanium processing plants, the thoron concentrations in the houses for separating rutile and zircon were very high. At zircon processing factory, the thoron concentration could reach 2,931 Bq/m³ and the estimated annual effective dose would be 21.4 mSv/a. Intervention has to be taken in order to reduce the thoron level in this factory since the level of thoron and its progenies corresponding to an annual occupational effective dose is beyond the action level of 6 mSv/a.     

Sol–gel modified method for obtention of gray and pink ceramic pigments in zircon matrix

A sol–gel modified method for obtention of gray and pink ceramic pigments in zircon matrix is discussed in this work. The chromophoric powders were firstly well dispersed in the solution containing zirconium and metasilicate ions before gelation and the product ceramic pigments obtained by calcining the xerogel with mineralizers. The zircon was well crystallized at 1,050 °C with the yield about 97%. The mineralization mechanism of the zircon formation was also discussed.     

Direct Analysis of Laminated Dolomite and Zircon by Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Distributions of elements in laminated dolomite and zircon crystal were obtained by laser ablation inductively coupled plasma mass spectrometry. The variation of signal intensity was normalized by an internal standard method using the sum of the signals of Ca and Mg which was assumed to be constant in every position for the dolomite sample and using Zr signal for the zircon sample. The concentrations of elements change correspondingly across the laminated structure of dolomite. U and Th show a zoned distribution in the zircon crystal.