Phase Relations in the System CaO‐Ta ₂ O ₅ ‐SiO ₂

Abstract

The system CaO‐Ta 2 O 3 ‐SiO 2 was studied using a combination of hot‐stage microscopy and the quenching technique. Primary crystallization fields were defined for the various calcium silicates, and for the calcium tantalates: CaO·2Ta 2 O 5 , CaO·Ta 2 O 5 , 2CaO·Ta 2 O 5 , and 4CaO·Ta 2 O 5 . A congruently melting ternary compound 10CaO·Ta 2 O 5 ·6SiO 2 , isostructural with the mineral niocalite, is the only ternary phase in the system. A large twoliquid region extends across the system from the CaO‐SiO 2 binary to within 1 wt% of the Ta 2 O 3 ‐SiO 2 binary but does not cut it, in marked contrast to the analogous CaO‐Nb 2 O 5 ‐SiO 2 system. Other somewhat unexpected differences were noted between these systems.