Rare-earth perovskites along the CaTiO3-Na0.5La0.5TiO3 join: Phase transitions, formation enthalpies, and implications for loparite minerals

Dawei Feng, Radha Shivaramaiah, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

4 Scopus citations

Abstract

The mineral loparite is the principal ore of the light rare earth elements (LREE) with solid solution compositions in the quaternary system perovskite-lueshite-tausonite-loparite (CaTiO3-NaNbO3-SrTiO3-Na0.5Ln0.5TiO3) (Ln = La, Ce, Pr, Nd). In this study, perovskite solid solutions Ca1-2xNaxLaxTiO3 (0 ≤ x ≤ 0.50) along the perovskite-loparite join were synthesized using solid-state sintering methods. XRD analysis indicates that as the Na+La content increases, the structure changes from orthorhombic to tetragonal. The enthalpies of formation at 298 K from the constituent oxides (ΔH°f, ox) and from the elements (ΔH°f,el) have been determined using high-temperature oxide melt solution calorimetry in molten 3Na2O.4MoO3 at 973 K. Enthalpy of formation from oxides ΔH°f,ox becomes more exothermic with increasing Na+La content, suggesting a stabilization effect of the substitution Ca2+ → 0.5Na+ + 0.5La3+. The observed trend of increasing thermodynamic stability with decreasing structural distortion is in agreement with that seen in many other ABO3 perovskites. The thermodynamic stability of perovskite solid solutions Ca1-2xNaxLaxTiO3 (0 ≤ x ≤ 0.50) along the CaTiO3-Na0.5La0.5TiO3 join provides insights into the natural occurrence of loparite minerals (La, Na, Sr, Ca)(Ti, Nb, Ta, Fe3+)O3.

Original languageEnglish (US)
Pages (from-to)2051-2056
Number of pages6
JournalAmerican Mineralogist
Volume101
Issue number9
DOIs
StatePublished - Sep 1 2016
Externally publishedYes

Keywords

  • Rare earth perovskites
  • calorimetry
  • enthalpy of formation
  • loparite

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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