Essexite

Essexite (

plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States

Modern

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

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 (

tholeiitic basalts

and gabbros.

References

^ "nepheline". Dictionary.com Unabridged (Online). n.d. Retrieved 2016-01-23.
^ "nepheline". Merriam-Webster.com Dictionary. Retrieved 2016-01-23.
^ 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.

External links

Russian alkaline igneous rock occurrences Archived 2008-08-28 at the Wayback Machine

[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.

[3]

v t e Types of rocks
Igneous rock	Adakite Andesite Alkali feldspar granite Anorthosite Aplite Basalt Basaltic trachyandesite Mugearite Shoshonite Basanite Blairmorite Boninite Carbonatite Charnockite Enderbite Dacite Diabase Diorite Napoleonite Dunite Essexite Foidolite Gabbro Granite Granodiorite Granophyre Harzburgite Hornblendite Hyaloclastite Icelandite Ignimbrite Ijolite Kimberlite Komatiite Lamproite Lamprophyre Latite Lherzolite Monzogranite Monzonite Nepheline syenite Nephelinite Norite Obsidian Pegmatite Peridotite Phonolite Phonotephrite Picrite Porphyry Pumice Pyroxenite Quartz diorite Quartz monzonite Quartzolite Rhyodacite Rhyolite Comendite Pantellerite Scoria Shonkinite Sovite Syenite Tachylyte Tephriphonolite Tephrite Tonalite Trachyandesite Benmoreite Trachybasalt Hawaiite Trachyte Troctolite Trondhjemite Tuff Websterite Wehrlite
Sedimentary rock	Argillite Arkose Banded iron formation Breccia Calcarenite Chalk Chert Claystone Coal Conglomerate Coquina Diamictite Diatomite Dolomite Evaporite Flint Geyserite Greywacke Gritstone Itacolumite Jaspillite Laterite Lignite Limestone Lumachelle Marl Mudstone Oil shale Oolite Phosphorite Sandstone Shale Siltstone Sylvinite Tillite Travertine Tufa Turbidite Varve Wackestone
Metamorphic rock	Anthracite Amphibolite Blueschist Cataclasite Eclogite Gneiss Granulite Greenschist Hornfels Calcflinta Itabirite Litchfieldite Marble Migmatite Mylonite Metapelite Metapsammite Phyllite Pseudotachylite Quartzite Schist Serpentinite Skarn Slate Suevite Talc carbonate Soapstone Tectonite Whiteschist
Specific varieties	Adamellite Appinite Aphanite Borolanite Blue Granite Epidosite Felsite Flint Ganister Gossan Hyaloclastite Ijolite Jadeitite Jasperoid Kenyte Lapis lazuli Larvikite Litchfieldite Llanite Luxullianite Mangerite Novaculite Pietersite Pyrolite Rapakivi granite Rhomb porphyry Rodingite Shonkinite Taconite Tachylite Teschenite Theralite Unakite Variolite Wad