They are darker than their host rock and display a porphyritic texture, with plagioclase, orthopyroxene, amphibole, and quartz phenocrysts in a microlitic
groundmass mainly composed of acicular plagioclase and accessory opaque minerals.
For most samples, only the fine-grained matrix (composed mainly of microlitic
plagioclase) was analyzed, because its lack of phenocrysts suggest that it was formed at the time of eruption.
Accessory minerals (fine-grained microlitic
zircon, apatite, monazite, and allanite) were identified and confirmed using a scanning electron microscope including backscattered electron imaging, energy-dispersive X-ray spectroscopy and X-ray element mapping.
Rare monocrystalline sand grains of clinopyroxene are also present, as are microlitic
lithic fragments (plagioclase microlites set in nearly opaque tachylitic glass) probably derived from chilled lava surfaces or basaltic tephra.