essexites

Essexites are a group of igneous rocks that are classified as a type of teschenite. They are characterized by the presence of plagioclase feldspar, augite, and olivine, with nepheline and sometimes analcite being essential constituents. These rocks typically occur as minor intrusions, such as dikes, sills, and plugs, and are often found in association with basaltic lavas. The name "essexite" originates from Essex County, Massachusetts, where these rocks were first described.

The mineralogy of essexites can vary, but the defining feature is the presence of feldspathoids like nepheline or analcite, alongside typical mafic minerals. The texture is often granular to porphyritic, with larger crystals of augite and plagioclase set in a finer-grained groundmass. The color is usually dark, ranging from grey to black, depending on the abundance of dark minerals.

Essexites are considered to be relatively undersaturated rocks, meaning they contain less silica than would be required to form feldspar. This undersaturation leads to the crystallization of feldspathoids. Their formation is generally attributed to magmas that are enriched in alkali metals and relatively poor in silica. These magmas often originate from deep within the Earth's mantle.

Geographically, essexite occurrences are not widespread but are found in several locations globally, often in regions with a history of volcanic or intrusive igneous activity. Examples besides the type locality in Massachusetts include occurrences in Scotland, Ireland, Canada, and Brazil. The study of essexites provides insights into the complex processes of magma generation and differentiation in the Earth's crust and upper mantle.