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Although ZrSiO 4 is the most well-known compound in the zircon-structured family (space group I41/amd), the experimental conditions for preparing pure and well-crystallized phases which are doped with tetravalent element via hydrothermal synthesis has never been clearly discussed in the literature. With the aim to answer this question, the experimental conditions of preparation of ZrSiO 4 and (Zr,Ce)SiO 4 were investigated in order to synthesize well-crystallized and pure phases. A multiparametric study has been carried out using soft hydrothermal conditions, with variables including reactant concentration, initial pH of the reactive medium, and duration of the hydrothermal treatment. Pure ZrSiO 4 was obtained through hydrothermal treatment for 7 days at 250°C, within a large acidity range (1.0 ≤ pH ≤ 9.0) and starting from C Si ≈ C Zr ≥ 0.2 mol·L -1 . As hydrothermally prepared zircon structured phases can be both hydrated and hydroxylated, its annealed form was also studied after heating to 1000°C. Based on these results, the synthesis of (Zr,Ce)SiO 4 solid solutions were also investigated. The optimal hydrothermal conditions to acquire pure and crystallized phases were obtained in 7 days at 250°C with initial pH = 1 and concentration of the reactants equal to 0.2 mol·L -1 . This led to (Zr,Ce)SiO 4 solid solutions with the incorporated Ce content up to 40 mol.%. Samples were characterized by multiple methods, including lab and synchrotron PXRD, IR and Raman spectroscopies, SEM, and TGA. Moreover, it was found that these phases were thermally stable in air up to at least 1000°C.
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Zircon Solubility, Metamict ZrSiO 4 Replacement, and Hydrothermal Zircon Formation at Upper Crustal Pressures
Abstract Stable and inert under most conditions, zircon can be dissolved and precipitated by aqueous fluids in the upper crust. Geochemical models using currently available thermodynamic properties for Zr aqueous species at 0.2 GPa predict that zircon solubility increases with temperature from 400 to 900°C in fluids saturated with quartz or baddeleyite. Zircon solubility is low in near‐neutral pH fluids and enhanced in acidic and alkaline fluids. Adding NaOH and to a lesser extent NaF to the solution significantly increases the solution pH values and Zr concentrations at zircon saturation. Modeled Zr concentrations are often orders of magnitude different from zircon solubilities measured experimentally under similar conditions. Metamict (amorphous) ZrSiO4is more soluble than crystalline zircon and is replaced through a coupled dissolution‐precipitation process. Reaction path kinetics models were constructed to simulate experiments described in the literature and extract rate constants for replacement of metamict ZrSiO4. Replacement is rate limited by zircon precipitation and is nearly complete after 1 week when fluid is present at 600°C, with the rate of replacement increasing with temperature. In a closed system, hydrothermal zircon may form by replacement of radiation‐damaged zircon but not fully crystalline zircon. Replacement of metamict ZrSiO4forms characteristic porosity. Geochemical models identify the conditions that promote zircon solubility, metamict ZrSiO4replacement, and the formation of hydrothermal zircon, and provide constraints on the interpretation of zircon U‐Pb dates of hydrothermal events.
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- PAR ID:
- 10599253
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 26
- Issue:
- 3
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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