The System CaO–“CaCr ₂ O ₄ ”–CaAl ₂ O ₄ in Air and under Mildly Reducing Conditions

Abstract

The system CaO–chromium oxide in air is reinvestigated and the existence of intermediate phases with chromium in oxidation states >3+ (Ca 5 Cr 3 O 12 , Ca 3 (CrO 4 ) 2 , and Ca 5 (CrO 4 ) 3 ) confirmed. Under reducing conditions these phases are unstable. A metastable, polymorphic form of calcium chromite, δ ‐CaCr 2 O 4 , is observed. In the CaO‐rich section of the CaO–Al 2 O 3 –Cr 2 O 3 system a ternary intermediate phase, chrome‐haüyne, Ca 4 [(Al,Cr 3+ ) 6 O 12 ](Cr 6+ O 4 ), coexists with calcium chromate and calcium aluminate phases. In air, low melting temperatures are preserved in all assemblages containing calcium chromate phases. Under reducing conditions a new ternary phase, Ca 6 Al 4 Cr 2 O 15 , coexists with CaO, CaCr 2 O 4 , chrome‐haüyne, and calcium aluminate phases. The influence of chromium oxide additions on the solidus temperatures of the CaO–Al 2 O 3 system is insignificant.