Aegirine in Rhyolite (Volcanic Rock)

This specimen consists of aegirine crystals embedded within rhyolitic volcanic rock, collected from Mayor Island (Tūhua) — one of New Zealand’s most geologically distinctive volcanic centres. Aegirine is a sodium-iron clinopyroxene that typically forms in peralkaline volcanic systems, where sodium and iron are enriched relative to aluminium. On Mayor Island, aegirine commonly occurs as dark green to black prismatic crystals or elongated grains within a light grey to pale volcanic matrix, as seen in this specimen. The host rock, rhyolite, is a high-silica volcanic rock formed from viscous lava or explosive eruptions. Mayor Island rhyolites are notable for their unusual chemistry and mineralogy, hosting minerals such as aegirine, riebeckite, and arfvedsonite that are rare in most New Zealand volcanic settings. These specimens illustrate the crystallisation of alkaline pyroxenes within silica-rich magma, providing valuable insight into the evolution of peralkaline volcanic systems. They are of interest both as educational examples of mineral–host relationships and as representative material from one of Aotearoa New Zealand’s most significant volcanic islands.