Abstract Gems & Gemology, Spring 2013, Vol. 49, No. 1

Peridot, Pyroxene, and Plagioclase: Mantle Megacrysts from Lunar Crater Volcanic Field in Nevada

Megacrysts within xenoliths transported by explosive volcanic eruptions to the surface are fingerprints of the earth’s upper mantle and possible collectors’ items.

Lunar Crater is located in Nye County, Nevada. It is a Quaternary alkali basaltic field with lava flows, cinder cones, and maars covering more than 100 square miles of Nevada desert. At numerous alkali basaltic volcanic fields in the western United States, ultramafic mantle xenoliths of peridotite, dunite, lherzolite, and monomineralic xenocrysts are found in lunar craters. Olivine, pyroxene, plagioclase, hornblende, and magnetite ranging from a few millimeters to centimeters are common megacrysts. These megacrysts are usually ejected from the volcanic vent as the cores of volcanic bombs. Bomb morphology can vary considerably due to different viscosities of lava. While these magacrysts can provide scientific information on the upper mantle composition, the authors are interested in their possible gemological significance. Peridot, the green variety of olivine, is reported to occur in nodules up to 50 cm. But the crystals are highly fractured, making them unfit for faceting. Plagioclase crystals from the lunar crater can be clear to translucent white and show typical feldspar twinning. Pyroxene megacrysts found in the lunar crater are usually the augite variety and apple green, bottle green, or nearly black.

Minerals in the Lunar Crater volcanic field are abundant and easily accessed. Volcanic bombs contain exotic xenocrysts of possible gem significance, making this a worthy destination for rock hunters.

Abstracted by Tao Hsu