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Crystal chemistry of REEXO₄ compounds (X = P, As, V). II. Review of REEXO₄ compounds and their stability fields

¹ Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstr. 14, A-1090 Wien, Austria
² Department of Mineralogy, Research Division, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden

^* Corresponding author, e-mail: uwe.kolitsch{at}univie.ac.at

A comprehensive critical review of the phase fields, metastable modifications, solid solution ranges and phase transitions of monazite- and zircon-type REEXO₄ (X = P, As, V) compounds is given. Monazite-type REEPO₄ compounds are stable for REE = La to Gd and metastable for Tb to Ho; zircon-type members exist for REE = Gd to Lu, and Y, Sc. REEAsO₄ compounds with monazite-type structure exist for REE = La to Nd, while zircon-type compounds are known for REE = Pm to Lu, and Y, Sc; no metastable arsenate members are known. The only stable monazite-type REEVO₄ is LaVO₄, but metastable members are known for REE = Ce to Nd. Zircon-type REEVO₄ compounds are stable for REE = Ce to Lu, and Y, Sc, and metastable for REE = La. Solid solution series are complete only if minor size differences exist between REE³⁺ or X⁵⁺ cations in respective end-members. Phase transitions occur under pressure (zircon (monazite ) scheelite) and at very low temperatures. The evaluation of the metastable phase fields and of naturally occurring members suggests that metastable modifications of REEXO₄ compounds can occur in nature under certain conditions (formation at temperatures < 200–300°C; formation via hydrated precursor phases; stabilisation by various impurity cations).

Key-words: REEXO₄ compounds, review, monazite, xenotime, zircon, stability, phase transition.

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