Abstract: Soda-rich pyroxenes in felsic rocks from the Warrumbungle Volcano, central New South Wales, contain up to 14.5 wt. % ZrO₂, which is more than double the previously reported maximum ZrO₂ in pyroxene. Zr is believed to enter aegirine as the component Na(Fe²⁺,Mn,Mg)_0.5Zr_0.5Si₂O₆ via the coupled substitution: (Fe²⁺,Mn,Mg)^VI+Zr^VI = 2(Fe³⁺)^VI. This component exceeds 50 mol. % in some analyses.

Pronounced pyroxene Zr-enrichment is restricted to rocks in which sodic amphibole is the major ferromagnesian mineral, with pyroxene only a minor late-stage phase. The Zr-rich pyroxenes resulted from a combination of host lava peralkalinity, low oxygen fugacity, rapid disequilibrium crystallization and low mobility of the Zr ion. These factors collectively led to the development of interstitial Zr-enriched microdomains in the felsic hosts during their final stages of crystallization.