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[SCG57-P05] Crystal growth for reference materials based on the flux method:Attempt to use natural monazite as starting materials
Keywords:Reference material, Flux method, Crystal growth, Monazite
This study aims to grow synthetic crystals into reference materials for geochemical analysis and dating. In general, internationally recognized natural minerals or artificial glasses (e.g., NIST SRM glass: Pearce et al. 1997, Geostand. Newslett.) are often used as reference materials. However, natural minerals are not always ideal due to solid/fluid inclusions or heterogeneous composition resulting from initial zoning, secondary metamorphism, or alternation. In mass spectrometry, unknowns and standards should be the same compositions to reduce the matrix effect. Therefore, we have attempted to synthesize flawless (i.e., minimal voids and cracks) crystals with homogeneous chemical compositions based on the flux method. This presentation will provide the results on monazite (LREE(PO)4), a target mineral for U-Th-Pb, (U-Th)/He, and fission track dating.
Flux synthesis is a method of crystal growth from the liquid phase. The starting material and solvent (flux) are mixed and heated, and the target single crystal is grown by cooling the solution or evaporating the flux (e.g., Hasegawa, 1968, Jour. Mineral.). This method can control the crystal morphology (e.g., euhedral crystal, size, amount of crystal strain) and the number of crystals produced by manipulating the growing conditions. Such conditions include starting materials, atmosphere, flux type, temperature, heating hold time, and cooling patterns. Given the prevalent use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or electron probe microanalyzer (EPMA) for dating, the synthetic crystal should meet the following conditions : (1) homogeneous major and trace composition with U and Th contents over 10—100 ppm, (2) sufficient crystal size of >100 µm, and (3) minimal impurities, voids, cracks. Regarding condition (1), doping with specific elements/isotopes is an international standard practice, but we employed natural monazite from Sri Lanka as the starting material, possibly containing U and Th.
In this study, we report the results of monazite synthesis under three cooling conditions. First, the starting material was crushed, and impurities and altered portions were removed by panning. The samples were then dried and mixed with the Na2MoO4 flux. Subsequently, they were exposed to a nitrogen atmosphere at 1300°C for 30 hours. The crystal growth was performed under three distinct cooling patterns: (a) 15°C/h to 900°C, (b) 5°C/h to 1100°C, and (c) a first cooling at 5°C/h to 1100°C, followed by a second cooling at 15°C/h to 900°C. After removing residual flux, we identified authigenic, colorless, and transparent monazite crystals of several hundred µm to several mm from conditions (b) and (c), based on energy dispersive X-ray spectroscopy at Shinshu University. Subsequent elemental mapping by EPMA at Tono Geoscience Center revealed homogeneous compositions of LREEs (i.e., La, Ce, Nd, Sm, Gd) and P as major elements in 10—100 µm scales. Homogeneous U and Th distributions, possibly lower than several hundred ppm, were also identified. Some crystals yielded the heterogeneous Si and Th. Additionally, we observed residual flux components and incorporation of Th oxides, silicate minerals of LREEs, and huttonite (ThSiO4) inside a few crystals, likely derived from the starting material, as well as these new precipitations. In each crystal, a sharp phosphate peak was detected by laser Raman spectrophotometry at the University of Toyama, inferring little structural disorder, namely, a well-crystalized structure. Integrated with these results, the synthetic monazite is expectable to show potential as a reference material. In the future, we plan to measure trace elements and isotopes including U and Th using LA-ICP-MS, and to study better recipes for higher contents and more homogeneous U and Th distributions, also to conduct flux synthesis with minerals such as apatite and zircon.
Flux synthesis is a method of crystal growth from the liquid phase. The starting material and solvent (flux) are mixed and heated, and the target single crystal is grown by cooling the solution or evaporating the flux (e.g., Hasegawa, 1968, Jour. Mineral.). This method can control the crystal morphology (e.g., euhedral crystal, size, amount of crystal strain) and the number of crystals produced by manipulating the growing conditions. Such conditions include starting materials, atmosphere, flux type, temperature, heating hold time, and cooling patterns. Given the prevalent use of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) or electron probe microanalyzer (EPMA) for dating, the synthetic crystal should meet the following conditions : (1) homogeneous major and trace composition with U and Th contents over 10—100 ppm, (2) sufficient crystal size of >100 µm, and (3) minimal impurities, voids, cracks. Regarding condition (1), doping with specific elements/isotopes is an international standard practice, but we employed natural monazite from Sri Lanka as the starting material, possibly containing U and Th.
In this study, we report the results of monazite synthesis under three cooling conditions. First, the starting material was crushed, and impurities and altered portions were removed by panning. The samples were then dried and mixed with the Na2MoO4 flux. Subsequently, they were exposed to a nitrogen atmosphere at 1300°C for 30 hours. The crystal growth was performed under three distinct cooling patterns: (a) 15°C/h to 900°C, (b) 5°C/h to 1100°C, and (c) a first cooling at 5°C/h to 1100°C, followed by a second cooling at 15°C/h to 900°C. After removing residual flux, we identified authigenic, colorless, and transparent monazite crystals of several hundred µm to several mm from conditions (b) and (c), based on energy dispersive X-ray spectroscopy at Shinshu University. Subsequent elemental mapping by EPMA at Tono Geoscience Center revealed homogeneous compositions of LREEs (i.e., La, Ce, Nd, Sm, Gd) and P as major elements in 10—100 µm scales. Homogeneous U and Th distributions, possibly lower than several hundred ppm, were also identified. Some crystals yielded the heterogeneous Si and Th. Additionally, we observed residual flux components and incorporation of Th oxides, silicate minerals of LREEs, and huttonite (ThSiO4) inside a few crystals, likely derived from the starting material, as well as these new precipitations. In each crystal, a sharp phosphate peak was detected by laser Raman spectrophotometry at the University of Toyama, inferring little structural disorder, namely, a well-crystalized structure. Integrated with these results, the synthetic monazite is expectable to show potential as a reference material. In the future, we plan to measure trace elements and isotopes including U and Th using LA-ICP-MS, and to study better recipes for higher contents and more homogeneous U and Th distributions, also to conduct flux synthesis with minerals such as apatite and zircon.