Plagioclase, quartz, biotite, and hornblende would most likely be found in which rock type?

Plagioclase, quartz, biotite, and hornblende would most likely be found in which rock type?

• A. Dacite
• B. Basalt
• C. Granite
• D. Gabbro

Answer: (A)

Mineral assemblages reveal the silica content and rock type in igneous rocks. The mix of quartz with plagioclase, plus dark minerals biotite and hornblende, points to a quartz-bearing rock that is intermediate in composition rather than strongly felsic or mafic. Basalt and gabbro are typically mafic and lack quartz. Granite, while quartz-bearing, usually has abundant alkali feldspar (potassium feldspar) and a mineral suite that isn’t as dominated by plagioclase together with hornblende and biotite. The combination here fits dacite, an intermediate, quartz-bearing volcanic rock where plagioclase is common, quartz is present, and hornblende and biotite can occur.

Mineral assemblages reveal the silica content and rock type in igneous rocks. The mix of quartz with plagioclase, plus dark minerals biotite and hornblende, points to a quartz-bearing rock that is intermediate in composition rather than strongly felsic or mafic. Basalt and gabbro are typically mafic and lack quartz. Granite, while quartz-bearing, usually has abundant alkali feldspar (potassium feldspar) and a mineral suite that isn’t as dominated by plagioclase together with hornblende and biotite. The combination here fits dacite, an intermediate, quartz-bearing volcanic rock where plagioclase is common, quartz is present, and hornblende and biotite can occur.