This rock has a mafic composition, with a total silica content of less than 55 percent; the quartz content is usually lower than 10 percent. Dolerite consists of calcium-rich plagioclase feldspar and pyroxene—often augite—with some quartz and sometimes magnetite and olivine. (If olivine is present, it is known as olivine dolerite; if the rock contains quartz, it is called quartz dolerite.)

TEXTURE A medium-grained rock with crystals between 1⁄64–3⁄16 in (0.5–5 mm) in diameter. Euhedral or anhedral crystals of plagioclase are embedded in pyroxene crystals.

ORIGIN This rock usually forms as dykes and sills in basaltic provinces. It may also occur as dyke swarms—hundreds of individual intrusions associated with a single igneous center.

Group: IGNEOUS

Origin: Intrusive

Grain size: Medium

Crystal shape: Anhedral, Euhedral

Classification: Mafic

Occurrence: Dyke, Sill

Color: Dark