Which rock is most likely to contain plagioclase, quartz, biotite, and hornblende?

• A. Basalt
• B. Peridotite
• C. Obsidian
• D. Dacite

Answer: (D)

Mineral assemblages in igneous rocks reveal silica content and how the rock crystallized. Quartz and plagioclase signal a silica-rich melt, while biotite and hornblende are dark mafic minerals that appear in intermediate to felsic rocks.

Dacite is an intermediate volcanic rock that commonly contains plagioclase and quartz, with biotite and hornblende also present. This combination matches a silica-rich, but not rhyolitic, composition, where quartz is retained and some dark minerals crystallize as the melt evolves. That puts dacite as the rock most likely to host all four minerals.

Basalt tends to be more mafic and typically lacks quartz, and obsidian is glassy with no visible crystals, so they don’t fit the mineral set. Peridotite is ultramafic and rich in ferromagnesian minerals with little to no quartz, biotite, or hornblende in common settings.