Which intrusive igneous rock is composed principally of plagioclase feldspar with possible biotite, hornblende, and/or pyroxene and lies between gabbro and granite in composition?

Which intrusive igneous rock is composed principally of plagioclase feldspar with possible biotite, hornblende, and/or pyroxene and lies between gabbro and granite in composition?

• A. Diorite
• B. Gabbro
• C. Granite
• D. Peridotite

Answer: (A)

Diorite is the intrusive igneous rock that fits this description. It crystallizes slowly underground, giving it a coarse, visible (phaneritic) texture, and its mineral makeup is dominated by plagioclase feldspar with smaller amounts of dark minerals such as biotite, hornblende, and/or pyroxene. This combination places it between gabbro (mafic, richer in pyroxene and calcium-rich plagioclase) and granite (felsic, richer in quartz and alkali feldspars) on the compositional spectrum. Peridotite is ultramafic and dominated by olivine and pyroxene, not plagioclase, so it doesn't match the description.

Diorite is the intrusive igneous rock that fits this description. It crystallizes slowly underground, giving it a coarse, visible (phaneritic) texture, and its mineral makeup is dominated by plagioclase feldspar with smaller amounts of dark minerals such as biotite, hornblende, and/or pyroxene. This combination places it between gabbro (mafic, richer in pyroxene and calcium-rich plagioclase) and granite (felsic, richer in quartz and alkali feldspars) on the compositional spectrum. Peridotite is ultramafic and dominated by olivine and pyroxene, not plagioclase, so it doesn't match the description.