Essexite
Encyclopedia
Essexite also called nepheline monzogabbro (ˈ), is a dark gray or black holocrystalline plutonic igneous rock
. Its name is derived from the type locality in Essex County, MA.
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.
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 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.
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.
, 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.
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
. Its name is derived from the type locality in Essex County, MA.
Modern petrology
Petrology
Petrology 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 diagram
QAPF diagram
A 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 nepheline
Nepheline
Nepheline, 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 orthoclase
Orthoclase
Orthoclase 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. Alternate names are alkali feldspar and potassium feldspar...
to plagioclase
Plagioclase
Plagioclase is an 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
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
composition suitable for forming essexite the partial melting
Partial melting
Partial melting occurs when only a portion of a solid is melted. For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the 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
Gabbro 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 nepheline
Nepheline
Nepheline, 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...
, plagioclase
Plagioclase
Plagioclase is an 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 feldspar
Feldspar
Feldspars 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; titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
(pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate 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...
), hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It 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....
and biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
.
Trace mineralogy may include magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, ilmenite
Ilmenite
Ilmenite 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 olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
(<5%).
Essexite grades into a nepheline monzogabbro with a decrease in potassium feldspar and an increase in the feldspathoid
Feldspathoid
The 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.
Geochemistry
Essexite is an alkaline igneous rock equivalent to an alkaline basalt. The presence of nephelineNepheline
Nepheline, 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 feldspathoid
Feldspathoid
The 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 microcline
Microcline
Microcline 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 oligoclase
Oligoclase
Oligoclase 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 . The albite:anorthite molar ratio ranges from 90:10 to 70:30.Oligoclase is a high sodium...
.
Essexites are generally rich in aluminium, alkalis (sodium and calcium), potassium (>3% K2O), LILE-enriched (strontium
Strontium
Strontium 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...
, caesium
Caesium
Caesium 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 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
and barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium 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 with...
), as compared to tholeiitic basalts and gabbros.