6.7.2: Mafic and Intermediate Igneous Rocks (0% to 20% Quartz)

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Mafic rocks contain less that 5% quartz, if any. A dark color, due to an abundance of hornblende, clinopyroxene, or olivine, and sometimes Ca-rich plagioclase, characterizes these rocks. They include syenite, monzonite, and gabbro, and their volcanic equivalents trachyte, latite, and basalt. K-feldspar dominates syenitic rocks. Plagioclase is the only feldspar in most gabbros. Plagioclase composition varies from Na-rich in syenite to intermediate Ca-rich in gabbro. Hornblende, biotite, and pyroxene are common, but usually in small amounts.

Intermediate igneous rocks are those in which quartz accounts for 5% to 20% of the rock. They include quartz syenite, quartz monzonite, and diorite, and their volcanic equivalents quartz trachyte, quartz latite, and andesite. The minerals in these rocks are the same as listed in the previous paragraph for their more mafic cousins. As with silicic rocks, intermediate rocks with more plagioclase (diorite and some monzonite) tend to have more mafic minerals and thus a darker color.

While all intermediate and mafic igneous rocks may contain hornblende and clinopyroxene, hornblende is more common in diorite and andesite, and clinopyroxene is more common in gabbros and basalts. Gabbros and basalts may also contain olivine. Magnetite, ilmenite, and apatite are common as accessory minerals in mafic and intermediate rocks.

The photos below (Figures 6.109 to 6.116) show some of the most common intermediate and mafic rocks. They include four plutonic rocks:
• syenite from Brazil that contains the feldspathoid, nepheline
• monzonite that contains subequal amounts of plagioclase and K-feldspar with hornblende
• diorite that contains almost entirely plagioclase and hornblende
• gabbro that contains green olivine, light colored plagioclase, and darker clinopyroxene

The bottom row in the table contains photos of the volcanic equivalents of the four plutonic rocks above them:
• trachyte from the Massif Central of France (mostly K-feldspar and biotite)
• latite of unknown origin with subequal amounts of plagioclase and K-feldspar
• andesite (containing dark colored xenoliths) from New Mexico
• a view of basalt outcrops in Washington

plutonic rocks	6.109 Nepheline syenite in Rio de Janeiro	6.110 Monzonite	6.111 Diorite from Massachusetts	6.112 Olivine gabbro
volcanic rocks	6.113 Trachyte from Le Mont Dore, France	6.114 Latite	6.115 Rhyolite	6.116 Outcrop with basalt columns in Mt. Rainier National Park
	K-feldspar rich	both feldspars		plagioclase rich