Crystal Chemistry and Stability of Hydrated Rare-Earth Phosphates Formed at Room Temperature

Asumi Ochiai,Satoshi Utsunomiya

doi:10.3390/min7050084

Abstract

In order to understand the crystal chemical properties of hydrous rare-earth (RE) phosphates, REPO4,hyd, that form at ambient temperature, we have synthesized REPO4,hyd through the interaction of aqueous RE elements (REEs) with aqueous P at room temperature at pH < 6, where the precipitation of RE hydroxides does not occur, and performed rigorous solid characterization. The second experiment was designed identically except for using hydroxyapatite (HAP) crystals as the P source at pH constrained by the dissolved P. Hydrated RE phosphate that precipitated at pH 3 after 3 days was classified into three groups: LREPO4,hyd (La → Gd) containing each REE from La-Gd, MREPO4,hyd (Tb → Ho), and HREPO4,hyd (Er → Lu). The latter two groups included increasing fractions of an amorphous component with increasing ionic radius, which was associated with non-coordinated water. REallPO4,hyd that contains all lanthanides except Pm transformed to rhabdophane structure over 30 days of aging. In the experiments using HAP, light REEs were preferentially distributed into nano-crystals, which can potentially constrain initial RE distributions in aqueous phase. Consequently, the mineralogical properties of hydrous RE phosphates forming at ambient temperature depend on the aging, the pH of the solution, and the average ionic radii of REE, similarly to the well-crystalline RE phosphates.

Highlights

Rare-earth (RE) elements (REEs), such as the lanthanides (La-Lu), are important rare metals in geological resources, and commonly used for technological application owing to their fluorescent, catalytic, and magnetic properties [1]
Properties of REPO4,hyd Formed at Room Temperature
The LREPO4,hyd that formed at room temperature exhibited a rhabdophane structure similar to [40]

Summary

Introduction

Rare-earth (RE) elements (REEs), such as the lanthanides (La-Lu), are important rare metals in geological resources, and commonly used for technological application owing to their fluorescent, catalytic, and magnetic properties [1]. Crystalline RE phosphates occur in four different structures: monazite (light REPO4 ; P21 /n), rhabdophane (light REPO4 ·nH2 O; P62 22 or C2), xenotime (heavy REPO4 ; I41 /amd), and churchite (heavy REPO4 ·nH2 O; I2/a) [5,6,7,8]. The previous studies have reported on the mineralogical properties of all four types of RE phosphates and their application to accommodate foreign cations [17,18], these studies were mainly concerned with the well-crystalline phase formed at elevated temperatures at acidic pH ~1, e.g., [16,19]. The crystal chemical property of the hydrous RE phosphates that formed at ambient temperature has not Minerals 2017, 7, 84; doi:10.3390/min7050084 www.mdpi.com/journal/minerals

Methods

Results

Conclusion