Solubility of niobium in the system CaCO3-Ca(OH)2-NaNbO3 at 0.1 GPa pressure

Experimental data are presented for the solubility of NaNbO3 in the ternary system CaCO3-Ca(OH)(2)-NaNbO3 (or calcite-portlandite-lueshite) over the temperature range 500-950 degreesC at 0.1 GPa pressure. Near-liquidus phase relationships are given for the pseudo-binary join ([CaCO3](45)[Ca(OH)(2)](55))(100)-(-)(x)(NaNbO3)(x); (0 < x < 70 wt%). These data show the presence of a large field of calcite plus liquid and the absence of NaNbO3 as a primary liquidus phase until the melts contain greater than 60 wt% NaNbO3. The primary crystallization fields of calcite and NaNbO3 are separated by a steep thermal valley located at about 55 wt% NaNbO3. Quenched liquids contain calcite, portlandite, sodium carbonates and perovskite-structured calcium niobates. The maximum solubility of Nb2O5 in this system is estimated to be on the order of 48 wt% at 650 degreesC at 0.1 GPa pressure. The experiments show that perovskite-structured compounds rather than pyrochlore crystallize from these fluorine-free, water-rich melts.