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.
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.
Minor grain types include chalcedonic microcrystalline quartz of indeterminate origin but possibly from amygdules in volcanic rock, polycrystalline microlitic
and felsitic volcanic rock fragments (in addition to more abundant glassy volcanic rock fragments), equant opaque iron oxide grains that are probably magnetite, internally foliated fragments of metamorphic slate, and rare aggregate epidote grains.