T-P(O2) TOPOLOGIC ANALYSIS OF PHASE-RELATIONS IN THE SYSTEM CAO-CRO-CR2O3-SIO2

A temperature (T)- oxygen partial pressure (p(O2)) topologic analysis of phase relations in the system CaO-CrO-Cr2O3-SiO2, involving divalen t and trivalent chromium, was undertaken to determine the relative pos itions of isobaric invariant points and univariant lines in T-p(O2) sp ace. Liquidus phase relationships in equilibrium with metallic chromiu m, CO2-H-2 gas mixtures, and air were used to identify the univariant and invariant assemblages involving Cr2+ and Cr3+. The T-p(O2) grid wa s constructed using Schreine-makers rules for the disposition of univa riant lines about an invariant point. The topological relationship bet ween univariant lines and invariant points assists in the interpretati on of phase relationships over the T-p(O2) regions that were not exper imentally investigated. For example, the effect of p(O2) on the liquid us phase relationships involving CaSiO3 (wollastonite), Cr2O3 (eskolai te), and Ca3Cr23+Si3O12 (uvarovite) is clearly illustrated by the grid ; these details are not borne out by the experimentally determined pha se diagrams. The grid furthermore illustrates the stability fields of the Cr2+-hearing phases, (Ca0.4Cr0.62+)Cr23+O4 CaCr2+Si4O10 as a funct ion of temperature and p(O2) and demonstrates the observed increase in liquidus temperatures with increasing P(O2).