Zircons of Summit Rock, Oregon

Abstract

Crystals of a variety of minerals, including feldspars, pyroxenes, ilmenite, apatites and tridymite project into cavities in a basaltic andesite body at Summit Rock. Oregon. One widely distributed minor mineral occurs as individuals and clusters of pale pink crystals ranging up to 0.3 mm in length, and is notable for the diverse habits in which it is found. These crystals had previously been identified as rutile on the basis of morphology (Kleck, 1970) and are labeled as such in many micromount collections. However, our studies show that the mineral is actually zircon. This identification was obtained by energy dispersive X-ray analysis, which showed that zirconium and silicon were the only cations present as major constituents. Titanium in particular was absent. The infrared absorption pattern of a typical crystal confirmed that it is a nonmetamict zircon.