Amphibolite is regionally metamorphosed, nonfoliated metamorphic rock consisting principally of which minerals?

• A. Hornblende and plagioclase
• B. Quartz and feldspar
• C. Biotite and garnet
• D. Olivine and pyroxene

Answer: (A)

Amphibolite is a high‑grade metamorphic rock that forms from mafic protoliths, and its defining mineralogy is dominated by hornblende (an amphibole) and plagioclase feldspar. This pairing reflects the stable minerals at the temperatures and pressures of the amphibolite facies, where hornblende grows and plagioclase remains stable with it. The rock is typically dark, coarse-grained, and often nonfoliated, since the mineral grains are relatively evenly distributed and not preferentially aligned. Other mineral combinations don’t fit because quartz and feldspar suggest a felsic source rather than a mafic one, biotite and garnet point to different metamorphic environments (often pelitic or higher-pressure/metasomatic contexts), and olivine with pyroxene characterize ultramafic rocks. So the principal minerals in amphibolite are hornblende and plagioclase.