Lanthanum Diffusion in Fluorapatite at 400 °C, 50 MPa and 4–16 wt %

Xiqiang Liu,Yunlong Liu,Hui Zhang

doi:10.3390/separations8060072

Abstract

Apatite is an important carrier of rare earth elements (REEs) and phosphorite is a potential REEs resource. However, the influence of hydrothermal fluids on the migration and enrichment of REE in apatite remains controversial. The experimental study of the interaction between REE-bearing fluid and apatite is one of the essential ways to understand the chemical behavior of rare earth elements in apatite. In this study, we conducted the fluid–mineral reaction experimental study (at 400 °C, 50 MPa and 4–16 wt %) between high lanthanum (La) content hydrothermal solution and low REE content to reveal the influence of different salinities on the diffusion of rare earth elements in fluorapatite. Based on in situ geochemical analyses of experimental products, we show that the diffusion coefficients of La in fluorapatite are between 3.24 × 10−15 and 5.88 × 10−15 m2/s. The salinity of the fluid has a great influence on the diffusion coefficient, with the increase of salinity, the diffusion coefficient increase.

Highlights

Rare earth elements (REEs, include yttrium) in natural solutions have attracted much attention due to the importance of chemical tracers for natural fluid–rock reactions [1]
The distribution of rare earth elements in minerals can be directly obtained by in-situ surface scanning of rare earth elements in mineral particles
The results show that in a specific range of salinity, the rise of salinity was conducive to the diffusion of La in apatite, which may be related to the increase of mineral solubility caused by the growth of salinity

Summary

Introduction

Rare earth elements (REEs, include yttrium) in natural solutions have attracted much attention due to the importance of chemical tracers for natural fluid–rock (mineral) reactions [1]. The composition of rare earth elements in geological fluids is affected by ion complexation, recrystallization and selective adsorption of minerals [2]. The distribution pattern of rare earth elements in minerals or rocks have been applied to the dating of minerals, the correlation of sedimentary basin rocks, the inference of marine paleochemistry, and the distinction of the biological and abiotic genesis of minerals [3,4,5,6]. Apatite is widely distributed in all kinds of igneous rocks, metamorphic rocks and sedimentary rocks, and it contains more or less rare earth elements, up to 11.14% (RE2 O3 ) [7]. Benefit from its concentrations equal to or even higher than any known resource and fewer environmental concerns, REEs resource in apatite may provide a new solution to solve the global rare earth crisis [9,12]

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Results

Conclusion