Essexite (Nepheline Monzogabbro)
“Alkaline gabbro with nepheline”
Essexite is reclassified in modern nomenclature as nepheline monzogabbro — a coarse-grained alkaline plutonic rock with roughly equal plagioclase and alkali feldspar, plus nepheline and clinopyroxene. It forms in alkaline ring complexes and is the plutonic equivalent of tephrite or phonotephrite.
plagioclase, alkali feldspar, nepheline, titanaugite, olivine, biotite, analcime, apatite
Salem (Massachusetts, USA — type locality), Mont Royal (Montreal, Canada), Alnö (Sweden), Kola Peninsula (Russia), Pilansberg (South Africa)
5.5-6
The type locality for essexite is Essex County, Massachusetts — the same region whose diverse igneous rocks gave geology a remarkable number of rock names including essexite, beverlyite, and Salem gabbro.
Test Your Knowledge
What does the coexistence of nepheline and alkali feldspar alongside plagioclase in essexite tell a petrologist about the parent magma?
Photo Attribution
Title: Essexite (Early Cretaceous, ~111 Ma; Mount Johnson, Quebec, Canada) (27556294638).jpg
Author: James St. John
License: CC BY 2.0
Related Specimens
Want to identify rocks yourself?