Websterite
“Two pyroxenes from the deep crust”
Websterite is an ultramafic rock composed almost entirely of two pyroxenes — orthopyroxene and clinopyroxene — with little or no olivine or feldspar. It forms as a cumulate rock in the deep crust or upper mantle and frequently appears as layers within large mafic layered intrusions.
orthopyroxene (enstatite), clinopyroxene (diopside/augite), spinel (minor), garnet (at high pressure), olivine (minor)
Bushveld Complex (South Africa), Stillwater Complex (Montana, USA), Webster (North Carolina, USA — type locality), Beni Bousera (Morocco), Finero (Italy)
5.5-6.5
Websterite xenoliths brought up by volcanoes from the lower crust reveal that the deepest parts of continental crust are richer in pyroxene than surface rocks suggest — our view from the surface gives a biased picture of crustal composition.
Test Your Knowledge
In a layered mafic intrusion, where would you typically find websterite layers relative to dunite and anorthosite?
Related Specimens
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