Which metamorphic rock is nonfoliated and dominantly composed of hornblende and plagioclase?

• A. Amphibolite
• B. Marble
• C. Quartzite
• D. Schist

Answer: (A)

In metamorphic rocks, the mineral makeup is the quickest way to identify the rock type. A rock dominated by hornblende (an amphibole) and plagioclase feldspar points to amphibolite. This assemblage is characteristic of metamorphosed mafic rocks (like basalt or gabbro) under moderate to high-grade conditions, and the minerals are typically aligned to give a foliated or schistose texture. Among the options, marble is rich in calcite, quartzite is mostly quartz, and schist is dominated by micas and other platy minerals; none of those are defined by hornblende and plagioclase in such abundance. So the rock that best fits the mineral composition described is amphibolite.