希土類元素リン酸塩鉱物の加熱変化

抄録

Thermal changes of synthesized rare earth phosphates were studied at 50°C to 1800°C in air under atmospheric pressure or the pressure of 7.5 to 12.5 GPa. 1) Rhabdophane-type RPO₄·nH₂O (n=0.5 to 1.0, R=La-Gd, Tb, Dy, Y, Er, or R=Ce+Y) were synthesized by precipitation from the mixed solution of R-salt and orthophosphoric acid at 50°C or 90°C and pH=1, or by grinding weinschenkite-type RPO₄·2H₂O (R=Dy, Y, or Er) at room temperature for 10 days. Rhabdophane-type phosphates were dehydrated at 190°C to 250°C. The water corresponding to nH₂O was zeolitic water. Thermal changes of rhabdophane-type anhydrous RPO₄, when heated in air under atmospheric pressure, were as follows : (1) rhabdophane-type RPO₄ (R=La-Gd, or R=Ce+Y)→monazite, (2) rhabdophane-type RPO₄ (R=Tb or Dy)→monazite→xenotime, (3) rhabdophane-type RPO₄ (R=Y or Er)→xenotime. The transformation from monazite into xenotime (2) was monotropic when heated in air under atmospheric pressure. Xenotime-type DyPO₄, however, transformed into monazite-type DyPO₄ when heated at 650°C to 1000°C in air under the pressure of 7.5 to 12.5 GPa. 2) Weinschenkite-type RPO₄·2H₂O (R=Dy, Y, Er, or Yb) were synthesized by precipitation from the mixed solution of R-salt and orthophosphoric acid at 50°C and pH=1. Weinschenkitetype RPO₄·2H₂O gradually converted to xenotime when heated in air under atmosheric pressure.