Quartz Monzonite

Photographed by Michael P. Klimetz

NORTH CAROLINA

Quartz monzonite (or adamellite) is an intrusive igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende constitute the dark minerals. Quartz monzonite porphyry is often associated with copper mineralization in porphyry copper ore deposits. Because of its coloring it is often confused with granite, but whereas the mass of granite is 20% quartz, quartz monzonite is only 5-20% quartz. A massive outcrop of this igneous rock comprises the summit of New Hampshire's Croydon Mountain.