For faster navigation, this Iframe is preloading the Wikiwand page for Essexite.

Essexite

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.Find sources: "Essexite" – news · newspapers · books · scholar · JSTOR (April 2010) (Learn how and when to remove this template message)
Craigleith, an island in Scotland, composed of essexite

Essexite ( /ˈɛsəkˌst/), also called nepheline monzogabbro (/ˈnɛfəˌlɪn ˌmɒnzˈɡæbr, -ˌln-/[1][2]), is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States.

Modern petrology identifies rocks according to mineralogical criteria. Utilising the IUGS QAPF diagram of Streckeisen (1974) "essexite" is more formally known as nepheline monzodiorite or nepheline monzogabbro depending on the ratio of orthoclase to plagioclase and the abundance of nepheline.[3]

Petrology

In order to produce a magma composition suitable for forming essexite the partial melting of the source rocks must be restricted, generally to less than 10% partial melting. This favors producing a melt rich in large-ion lithophile elements (LILE) such as potassium, barium, rubidium, cesium, and strontium.

The source melts of essexites contain more aluminium and alkali ions than available silica tetrahedra, which is why essexites crystallise nepheline instead of plagioclase. Higher than normal potassium favors the production of orthoclase, which is usually absent from most mafic igneous rocks.

Mineralogy

Essexite can be considered as an alkali gabbro or monzodiorite primarily composed of nepheline, plagioclase, with lesser amounts of alkali feldspar, with mafic minerals composed of any of the following; titanium augite (pyroxene), hornblende and biotite.

Trace mineralogy may include magnetite, ilmenite and accessory olivine (<5%).

Essexite grades into a nepheline monzogabbro with a decrease in potassium feldspar and an increase in the feldspathoid minerals.

Geochemistry

Essexite is an alkaline igneous rock equivalent to an alkaline basalt. The presence of nepheline, a feldspathoid mineral, indicates that essexites are silica undersaturated. The presence of orthoclase indicates that it contains sufficient potassium to favor production of orthoclase over microcline or potassic oligoclase.

Essexites are generally rich in aluminium, alkalis (sodium and calcium), potassium (>3% K2O), LILE-enriched (strontium, caesium and barium), as compared to tholeiitic basalts and gabbros.

References

  1. ^ "nepheline". Dictionary.com Unabridged (Online). n.d. Retrieved 2016-01-23.
  2. ^ "nepheline". Merriam-Webster.com Dictionary. Retrieved 2016-01-23.
  3. ^ Henry, Darrell. "Classification of Igneous Rocks - Flow Chart". Geology 3041: Igneous and Metamorphic Petrology Lectures. Louisiana State University. Archived from the original on 10 May 2008. Retrieved 1 March 2006.
{{bottomLinkPreText}} {{bottomLinkText}}
Essexite
Listen to this article

This browser is not supported by Wikiwand :(
Wikiwand requires a browser with modern capabilities in order to provide you with the best reading experience.
Please download and use one of the following browsers:

This article was just edited, click to reload
This article has been deleted on Wikipedia (Why?)

Back to homepage

Please click Add in the dialog above
Please click Allow in the top-left corner,
then click Install Now in the dialog
Please click Open in the download dialog,
then click Install
Please click the "Downloads" icon in the Safari toolbar, open the first download in the list,
then click Install
{{::$root.activation.text}}

Install Wikiwand

Install on Chrome Install on Firefox
Don't forget to rate us

Tell your friends about Wikiwand!

Gmail Facebook Twitter Link

Enjoying Wikiwand?

Tell your friends and spread the love:
Share on Gmail Share on Facebook Share on Twitter Share on Buffer

Our magic isn't perfect

You can help our automatic cover photo selection by reporting an unsuitable photo.

This photo is visually disturbing This photo is not a good choice

Thank you for helping!


Your input will affect cover photo selection, along with input from other users.

X

Get ready for Wikiwand 2.0 🎉! the new version arrives on September 1st! Don't want to wait?