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Essexite (/ˈɛsəkˌsaɪt/), also callednepheline monzogabbro (/ˈnɛfəˌlɪnˌmɒnzoʊˈɡæbroʊ,-ˌliːn-/[1][2]), is a dark gray or blackholocrystallineplutonicigneous rock. Its name is derived from the type locality inEssex County,Massachusetts, in theUnited States.
Modernpetrology identifies rocks according to mineralogical criteria. Utilising theIUGSQAPF diagram of Streckeisen (1974) "essexite" is more formally known asnephelinemonzodiorite or nepheline monzogabbro depending on the ratio oforthoclase toplagioclase and the abundance of nepheline.[3]
In order to produce amagma composition suitable for forming essexite thepartial melting of the sourcerocks must be restricted, generally to less than 10% partial melting. This favors producing a melt rich in large-ion lithophile elements (LILE) such aspotassium,barium,rubidium,cesium, andstrontium.
The source melts of essexites contain morealuminium 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.
Essexite can be considered as an alkaligabbro or monzodiorite primarily composed ofnepheline,plagioclase, with lesser amounts of alkalifeldspar, with mafic minerals composed of any of the following;titaniumaugite (pyroxene),hornblende andbiotite.
Trace mineralogy may includemagnetite,ilmenite and accessoryolivine (<5%).
Essexite grades into a nepheline monzogabbro with a decrease inpotassium feldspar and an increase in thefeldspathoid minerals.
Essexite is analkaline igneous rock equivalent to an alkaline basalt. The presence ofnepheline, afeldspathoid mineral, indicates that essexites are silica undersaturated. The presence of orthoclase indicates that it contains sufficient potassium to favor production of orthoclase overmicrocline or potassicoligoclase.
Essexites are generally rich in aluminium, alkalis (sodium and calcium), potassium (>3% K2O), LILE-enriched (strontium,caesium andbarium), as compared totholeiitic basalts and gabbros.