Essexite
“The dark alkaline intruder”
Essexite is a coarse-grained, dark-colored alkaline igneous rock related to gabbro but enriched in alkali elements and containing nepheline. It's relatively rare and forms in association with other alkaline rocks in continental rift settings and oceanic island volcanoes.
plagioclase, augite, nepheline, olivine, biotite, hornblende, titanite
Essex County (Massachusetts, USA), Kola Peninsula (Russia), Mont Royal (Montreal, Canada), Oslo Rift (Norway)
5.5-6
Mount Royal in Montreal — the mountain that gave the city its name — is an eroded essexite and related alkaline rock intrusion that was never actually a volcano.
Test Your Knowledge
What type of tectonic setting produces the alkaline magmas that form essexite?
Photo Attribution
Title: Essexite (Early Cretaceous, ~111 Ma; Mount Johnson, Quebec, Canada) (27556294638).jpg
Author: James St. John
License: CC BY 2.0
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