We focus on the minerals that make up the groundmass, because we can be reasonably certain that they these crystallized from the magma in the lava flow or intrusion at the site sampled. Olivine may be present as phenocrysts, while quartz or tridymite may occur in the matrix of olivine-free rocks. The opaque mineral is magnetite. Groundmass pyroxene may include ferroan enstatite or pigeonite in addition to subcalcic augite . Quartz or tridymite may occur in groundmass in rocks with no olivine.
Nepheline occurs in undersaturated varieties and neither ferroan enstatite nor quartz are present.
Olivine may crystallize both phenocrysts and in groundmass. Titanian augite may crystallize both as phenocrysts and in the groundmass. Nepheline may occur in groundmass, never quartz or tridymite. Olivine is commonly altered to serpentine along its margins and along fractures. In plane light, augite and pigeonite are colorless or very pale green, but not pleochroic.