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Solubility of Na2SO4 in silica-saturated solutions: Implications for REE mineralization

  • Huan Chen , Hao Cui , Richen Zhong , Yuling Xie EMAIL logo , Chang Yu , Zimeng Li and Yifan Ling
Published/Copyright: October 28, 2020
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American Mineralogist
From the journal American Mineralogist Volume 105 Issue 11

Abstract

Sulfate is traditionally considered to have retrograde solubility in aqueous solutions. However, our recent hydrothermal diamond-anvil cell (HDAC) experiments have shown that the solubility of Na2SO4 changes from retrograde to prograde in the presence of silica, leading to the formation of sulfate-rich solutions at high temperatures, in line with observations on natural geofluids. In this study, we use synthetic inclusions of fused silica capillary capsules containing saturated Na2SO4 solutions and Na2SO4 crystals to quantitatively investigate the solubility of Na2SO4 at different temperatures in the Na2SO4-SiO2-H2O system. Sulfate concentrations were measured using Raman spectroscopy and calibrated using Cs2SO4 solutions with known concentrations. The solubility of crystalline Na2SO4 dropped slightly when heated from 50 to 225 °C and dramatically from 225 to 313 °C. At 313 °C, the Na2SO4 crystals began to melt, forming immiscible sulfate melt coexisting with the aqueous solution, with or without solid Na2SO4. With the formation of sulfate melt, the solubility of Na2SO4 was reversed to prograde (i.e., solubility increased considerably with increasing temperatures). The solubility of Na2SO4 in the measured solution was significantly higher than that predicted in the absence of SiO2 over the entire temperature range (except for temperatures around 313 °C). This indicates that the presence of SiO2 greatly changes the dissolution behavior of Na2SO4, which may be caused by the formation of a sulfate–silicate intermediates such as Si(OH)4SO42−.Considering that most crustal fluids are silica-saturated, the solubility curve of Na2SO4 obtained in this study can better reflect the characteristics of geofluids when compared to that of Na2SO4-H2O binary system. At temperatures of 313–425 °C, the solubility of Na2SO4 increases with temperature following the function Csulfate = –3173.7/T + 5.9301, where Csulfate and T represent the solubility of Na2SO4 in mol/kg H2O and temperature in Kelvin, respectively. As an application, this temperature-solubility relationship can be used to evaluate the sulfate contents in fluid inclusions that contain sulfate daughter minerals, based on the temperature of sulfate disappearance obtained from microthermometric analysis. The sulfate concentrations of the ore-forming fluids of the giant Maoniuping carbonatite-related rare earth element (REE) deposit (southwest China) were calculated to be 4.67–4.81 m (mol/kg H2O). These sulfate concentrations were then used as internal standards to calibrate the previously reported semi-quantitative results of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of REE-forming stage fluid inclusions at this deposit. The calculated Ce concentrations in the REE-mineralizing fluid range from 0.42 to 0.49 wt%. The high fluid REE contents suggest that the sulfate-rich fluids are ideal solvents for REE transport. A mass-balance calculation was carried out to evaluate the minimal volume of carbonatite melt that was required for the formation of the giant Maoniuping REE deposit. The result indicates that the carbonatite dikes in the mining area are enough to provide the required fluids and metals, and thus a deep-seated magma chamber is not necessary for ore formation.

Keywords: Na2SO4 solubility; silica saturation; rare earth element; mineralizing fluid; FSCC

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: 41472072

Award Identifier / Grant number: 41872078

Award Identifier / Grant number: 41502069

Funding source: Australian Research Council

Award Identifier / Grant number: DP190100216

Funding statement: This work was supported by the National Natural Science Foundation of China (grant nos. 41472072, 41872078, and 41502069), the State Key Research Plan (grant no. 2017YFC0601302), the Australian Research Council (grant no. DP190100216), the Young Elite Scientists Sponsorship Program by CAST (grant no. YESS), and the Fundamental Research Funds for the Central Universities (grant no. FRF-TP-18-017A3).

Acknowledgments

We thank Ying Cui and the School of Earth and Space Sciences of Peking University for assistance in conducting Raman spectrum analysis. We also thank two anonymous reviewers for their suggestions in improving the manuscript.

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Received: 2020-02-10
Accepted: 2020-04-25
Published Online: 2020-10-28
Published in Print: 2020-11-25

© 2020 Walter de Gruyter GmbH, Berlin/Boston

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https://doi.org/10.2138/am-2020-7470
Keywords for this article
Na2SO4 solubility; silica saturation; rare earth element; mineralizing fluid; FSCC

Articles in the same Issue

  1. Parageneses of TiB2 in corundum xenoliths from Mt. Carmel, Israel: Siderophile behavior of boron under reducing conditions
  2. Crystal structure and Raman spectroscopic studies of OH stretching vibrations in Zn-rich fluor-elbaite
  3. Crystal structure of Ag-exchanged levyne intergrown with erionite: Single-crystal X-ray diffraction and Molecular Dynamics simulations
  4. Br diffusion in phonolitic melts: Comparison with fluorine and chlorine diffusion
  5. Crystal chemistry and microfeatures of gadolinite imprinted by pegmatite formation and alteration evolution
  6. A new occurrence of corundum in eucrite and its significance
  7. Zircon survival in shallow asthenosphere and deep lithosphere
  8. Reconsidering initial Pb in titanite in the context of in situ dating
  9. Solubility of Na2SO4 in silica-saturated solutions: Implications for REE mineralization
  10. Vanadium micro-XANES determination of oxygen fugacity in olivine-hosted glass inclusion and groundmass glasses of martian primitive shergottite Yamato 980459
  11. Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments
  12. Magnetite texture and trace-element geochemistry fingerprint of pulsed mineralization in the Xinqiao Cu-Fe-Au deposit, Eastern China
  13. Magmatic haggertyite in olivine lamproites of the West Kimberley region, Western Australia
  14. Trace elements in sulfides from the Maozu Pb-Zn deposit, Yunnan Province, China: Implications for trace-element incorporation mechanisms and ore genesis
  15. Letter
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  17. Effects of small crystallite size on the thermal infrared (vibrational) spectra of minerals
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