Essexite , also called
nepheline monzogabbro is a dark gray or black holocrystalline plutonic
igneous rockIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
. Its name is derived from the type locality in Essex County, MA.
Modern
petrologyPetrology is the branch of geology that studies rocks, and the conditions in which rocks form...
identifies rocks according to mineralogical criteria. Utilising the IUGS
QAPF diagramA QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...
of Streckeisen (1974) "essexite" is more formally known as
nephelineNepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
monzodiorite or nepheline monzogabbro depending on the ratio of
orthoclaseOrthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. An alternate name is alkali feldspar...
to
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
and the abundance of nepheline.
In order to produce a
magmaMagma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano...
composition suitable for forming essexite the
partial meltingPartial melting occurs when only a portion of a solid is melted. When that solid contains many different crystalline phases, some phases will melt while others will stay solid....
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 K, Ba, Rb, Cs, Sr.
The source melts of essexites contain more aluminium and alkali ions than available silica tetrahedra, which is why essexites crystallise nepheline instead of plagicolase.
Essexite , also called
nepheline monzogabbro is a dark gray or black holocrystalline plutonic
igneous rockIgneous rock is one of the three main rock types . Igneous rock is formed by magma being cooled and becoming solid. They may form with or without crystallization, either below the surface as intrusive rocks or on the surface as extrusive rocks...
. Its name is derived from the type locality in Essex County, MA.
Modern
petrologyPetrology is the branch of geology that studies rocks, and the conditions in which rocks form...
identifies rocks according to mineralogical criteria. Utilising the IUGS
QAPF diagramA QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...
of Streckeisen (1974) "essexite" is more formally known as
nephelineNepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
monzodiorite or nepheline monzogabbro depending on the ratio of
orthoclaseOrthoclase is an important tectosilicate mineral which forms igneous rock. The name is from the Greek for "straight fracture," because its two cleavage planes are at right angles to each other. An alternate name is alkali feldspar...
to
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
and the abundance of nepheline.
Petrology
In order to produce a
magmaMagma [from Greek μάγμα, paste] is molten rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles. Magma often collects in a magma chamber inside a volcano...
composition suitable for forming essexite the
partial meltingPartial melting occurs when only a portion of a solid is melted. When that solid contains many different crystalline phases, some phases will melt while others will stay solid....
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 K, Ba, Rb, Cs, Sr.
The source melts of essexites contain more aluminium and alkali ions than available silica tetrahedra, which is why essexites crystallise nepheline instead of plagicolase. 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
gabbroGabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
or monzodiorite primarily composed of
nephelineNepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
,
plagioclasePlagioclase is a very important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, with lesser amounts of alkali
feldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
, with mafic minerals composed of any of the following;
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
augiteAugite is a single chain inosilicate mineral described chemically as SiO
3 or calcium magnesium iron silicate. The crystals are monoclinic and prismatic. Augite has two prominent prismatic cleavages, meeting at angles near 90°....
(
pyroxeneThe pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
),
hornblendeHornblende is a complex inosilicate series of minerals . Hornblende is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. It is an isomorphous mixture of three molecules; a calcium-iron-magnesium silicate, an...
and
biotiteBiotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula K3AlSi3O102. More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the...
.
Trace mineralogy may include
magnetiteMagnetite is a ferrimagnetic mineral with chemical formula Fe
3O
4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name ferrous-ferric oxide...
,
ilmeniteIlmenite is a weakly magnetic titanium-iron oxide mineral which is iron-black or steel-gray. It is a crystalline iron titanium oxide . It crystallizes in the trigonal system, and it has the same crystal structure as corundum and hematite....
and accessory
olivineThe mineral olivine is a magnesium iron silicate with the formula
2SiO
4...
(<5%).
Essexite grades into a nepheline monzogabbro with a decrease in potassium feldspar and an increase in the
feldspathoidThe feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
minerals.
Geochemstry
Essexite is an alkaline igneous rock equivalent to an alkaline basalt. The presence of
nephelineNepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
, a
feldspathoidThe feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
mineral, indicates that essexites are silica undersaturated. The presence of orthoclase indicates that it contains sufficient potassium to favor production of orthoclase over
microclineMicrocline is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase; it is more stable at lower temperatures...
or potassic
oligoclaseOligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite...
.
Essexites are generally rich in aluminium, alkalis (sodium and calcium), potassium (>3% K
2O), LILE-enriched (
strontiumStrontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
,
caesiumCaesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of , which makes it one of only five metals that are liquid at or near room temperature...
and
bariumBarium is a chemical element. It has the symbol Ba, atomic number 56, and is the fifth element in Group 2. Barium is a soft silvery metallic alkaline earth metal. It is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts...
), as compared to tholeiitic basalts and gabbros.
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