Myrmekite
Encyclopedia
Myrmekite describes a vermicular, or wormy, intergrowth of quartz
in plagioclase
. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The plagioclase
is sodium
-rich, usually albite
or oligoclase
. These quartz-plagioclase intergrowths are associated with and commonly in contact with potassium
feldspar
. Myrmekite is formed under metasomatic conditions
, usually in conjunction with tectonic deformations. It has to be clearly separated from micrographic
and granophyric
intergrowths, which are magma
tic.
μὑρμηχἰα (wart) or μὑρμηξ (ant) and was used by Jakob Sederholm
in 1899 for the first time to describe these structures.
-metasomatism
in cataclastically-deformed magma
tic plutonic rocks. The breakage happens primarily along grain-boundary seals and the K-metasomatism may locally replace rims of zoned plagioclase
crystals to form interstitial alkali feldspar
and rim myrmekite (see illustration).
Gradations occur from rocks containing exclusively rim myrmekite to those containing both rim myrmekite and wartlike myrmekite and finally to those containing exclusively wartlike myrmekite.
A very important observation is that the maximum coarseness (tubular diameter) of the quartz
vermicules shows a strong correlation with the Ca
content of the plagioclase in the original, unreplaced, non-myrmekite-bearing magmatic rock. The coarsest vermicules occur in the
metasomatized rock where the original plagioclase was the most calcic.
An example for the formation of wartlike myrmekite can be found at the Palms
quartz monzonite
which issued from the felsic Gold Park diorite
.
. Again this process depends on tectonically deformed crystals. In this particular case an irregular subtraction of Ca, Na and Al
from deformed plagioclase happens which causes an imbalance in the relative amounts of residual Al and Si. More Si remains than can fit into the lattice structure of the alkali feldspar that replaces the plagioclase. The result is ghost myrmekite – either as tiny quartz ovoids in remnant albite
islands in the alkali feldspar or as tiny quartz ovoids as clusters without albite hosts in the alkali feldspar (see illustration).
Examples for this structure are found in California in the Rubidoux Mountain leucogranite
and in granodiorite
s in the Sierra Nevada.
An example for this type of Ca-metasomatism is found in a megacrystal granite near Alastaro
in Finland
.
) and can also be of metamorphic
origin.
An example for this type of Ca-metasomatism is found in Sri Lanka
.
Examples are found in layered igneous complexes.
An example is the Velay granite in the northeastern Massif Central
in France
.
In the second variety Na- and Ca-bearing fluids truly act together. This leads via the replacement of primary K-feldspar (perthitic and non-perthitic microcline) to the formation of plagioclase (albite or oligoclase) and in certain places also to the formation of myrmekite. The myrmekite does not show wartlike tapering vermicules, but vermicules that are confined and scattered entirely within the interior of the plagioclase forming irregular spindles, arcuate patterns and ovals.
For this process to operate is important that Ca is sufficiently present so that a fairly calcic plagioclase can be formed which in turn releases enough silica for the myrmekite vermicules. If only Na is present then no myrmekite will form.
An example can be found in the Lyon Mountain granite gneiss north of Ausable Forks in New York.
s and similar igneous rock
s (granitoids, diorite
s, gabbro
s) and in metamorphic
gneiss
es similar to granite in composition. It can also occur in anorthosite
s and the orthopyroxene-bearing charnockite
s.
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
in 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...
. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter. The 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...
is sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
-rich, usually albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
or 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...
. These quartz-plagioclase intergrowths are associated with and commonly in contact with potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. Myrmekite is formed under metasomatic conditions
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
, usually in conjunction with tectonic deformations. It has to be clearly separated from micrographic
Micrographic texture
In petrology, micrographic texture is a fine-grained intergrowth of quartz and alkali feldspar, interpreted as the last product of crystallization in some igneous rocks which contain high or moderately high percentages of silica...
and granophyric
Granophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
intergrowths, which are magma
Magma
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...
tic.
Etymology
The word myrmekite is derived from the Ancient GreekAncient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
μὑρμηχἰα (wart) or μὑρμηξ (ant) and was used by Jakob Sederholm
Jakob Sederholm
Jakob Johannes Sederholm was a Finnish petrologist most associated with his studies of migmatites.Troubled by illness throughout his life, Sederholm originally chose to study geology to allow him to work outdoors...
in 1899 for the first time to describe these structures.
Myrmekite formed during K-metasomatism
During K-metasomatism of plagioclase several different types of myrmekite can appear:- rim myrmekite
- wartlike myrmekite
- ghost myrmekite
Rim myrmekite
This is the initial stage of KPotassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
-metasomatism
Metasomatism
Metasomatism is the chemical alteration of a rock by hydrothermal and other fluids.Metasomatism can occur via the action of hydrothermal fluids from an igneous or metamorphic source. In the igneous environment, metasomatism creates skarns, greisen, and may affect hornfels in the contact...
in cataclastically-deformed magma
Magma
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...
tic plutonic rocks. The breakage happens primarily along grain-boundary seals and the K-metasomatism may locally replace rims of zoned 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...
crystals to form interstitial alkali feldspar
Alkali feldspar
The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
and rim myrmekite (see illustration).
Wartlike myrmekite
When tectonic strains increase and the cataclasis becomes more intense interior breakage in the crystals ensues and albite-twinned plagioclase crystals are bent. The K-metasomatism therefore can reach deeper into the crystals and increase its effects. Nearly complete to complete replacement of plagioclase takes place and leads to the formation of wartlike myrmekite in places where the replacement was incomplete. The illustration shows tartan-twinned microcline having completely replaced plagioclase. The places with incomplete replacement are taken up by wartlike myrmekite.Gradations occur from rocks containing exclusively rim myrmekite to those containing both rim myrmekite and wartlike myrmekite and finally to those containing exclusively wartlike myrmekite.
A very important observation is that the maximum coarseness (tubular diameter) of the quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
vermicules shows a strong correlation with the Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
content of the plagioclase in the original, unreplaced, non-myrmekite-bearing magmatic rock. The coarsest vermicules occur in the
metasomatized rock where the original plagioclase was the most calcic.
An example for the formation of wartlike myrmekite can be found at the Palms
Twentynine Palms, California
Twentynine Palms is a city in San Bernardino County, California, United States. It was previously called Twenty-Nine Palms...
quartz monzonite
Quartz monzonite
Quartz monzonite is an intrusive igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. The plagioclase is typically intermediate to sodic in composition, andesine to oligoclase. Quartz is present in significant amounts. Biotite and/or hornblende...
which issued from the felsic Gold Park diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
.
Ghost myrmekite
This is the third type of quartz-feldspar intergrowth in metasomatic granitoidsGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
. Again this process depends on tectonically deformed crystals. In this particular case an irregular subtraction of Ca, Na and Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
from deformed plagioclase happens which causes an imbalance in the relative amounts of residual Al and Si. More Si remains than can fit into the lattice structure of the alkali feldspar that replaces the plagioclase. The result is ghost myrmekite – either as tiny quartz ovoids in remnant albite
Albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
islands in the alkali feldspar or as tiny quartz ovoids as clusters without albite hosts in the alkali feldspar (see illustration).
Examples for this structure are found in California in the Rubidoux Mountain leucogranite
Leucogranite
In geology, leucogranites are amongst the youngest intrusions related to anatexis of continental crust anywhere in the world. Leucogranites are commonly found in deformed metapelitic/metagraywacke sequences that have been thrusted over basements during crustal thickening associated with continental...
and in granodiorite
Granodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
s in the Sierra Nevada.
Myrmekite formed during Ca-metasomatism
During Ca- metasomatism myrmekite can be formed under different circumstances:- Ca-metasomatism of deformed K-feldspar in magmatic rocks
- Ca-metasomatism of deformed K-feldspar in charnockiteCharnockiteCharnockite is applied to any orthopyroxene-bearing granite, composed mainly of quartz, perthite or antiperthite and orthopyroxene , as an end-member of the charnockite series.-Charnockite series:...
s - Ca-metasomatism of deformed plagioclase in anorthositeAnorthositeAnorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
s
Ca-metasomatism of deformed K-feldspar in magmatic rocks
Here Ca-bearing fluids enter primary alkali feldspar through cracks and react with the alkali feldspar. Through this reaction cracks are filled with quartz and myrmekite. The replacement reactions can affect large portions (> 60 %) of the primary alkali feldspar. An important distinctive feature of this type of myrmekite formation is the constant thickness of the vermicules, whereas in the K-metasomatism their thickness changes as a function of the Ca-content of the plagioclase and they also taper towards the alkali feldspar.An example for this type of Ca-metasomatism is found in a megacrystal granite near Alastaro
Alastaro
Alastaro is a former municipality of Finland. It was merged to town of Loimaa on 1 January 2009.It is located in the province of Southern Finland and is part of the Finland Proper region. The municipality had a population of 2,910 and covered an area of 258.12 km² of which 1.32 km² is...
in Finland
Finland
Finland , officially the Republic of Finland, is a Nordic country situated in the Fennoscandian region of Northern Europe. It is bordered by Sweden in the west, Norway in the north and Russia in the east, while Estonia lies to its south across the Gulf of Finland.Around 5.4 million people reside...
.
Ca-metasomatism of deformed K-feldspar in charnockites
The process stays the same, the only difference being the country rocks the Ca-bearing fluids act upon. Charnockites distinguish themselves from ordinary granitoids by the appearance of orthopyroxene (hyperstheneHypersthene
Hypersthene is a common rock-forming inosilicate mineral belonging to the group of orthorhombic pyroxenes. Many references have formally abandoned this term, preferring to categorise this mineral as enstatite or ferrosilite. It is found in igneous and some metamorphic rocks as well as in stony and...
) and can also be of metamorphic
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
origin.
An example for this type of Ca-metasomatism is found in Sri Lanka
Sri Lanka
Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka is a country off the southern coast of the Indian subcontinent. Known until 1972 as Ceylon , Sri Lanka is an island surrounded by the Indian Ocean, the Gulf of Mannar and the Palk Strait, and lies in the vicinity of India and the...
.
Ca-metasomatism of deformed plagioclase in anorthosites
In this type of Ca-metasomatism instead of the alkali feldspar it is the ubiquitous plagioclase that gets attacked by the Ca-bearing fluids. The resulting myrmekite also shows vermicules with constant thickness but unlike in the first case the vermicules formed in anorthosites can taper locally to the primary, non-quartz-bearing plagioclase. This behaviour can be explained by the incorporation of Na demanding more silica in the feldspar lattice.Examples are found in layered igneous complexes.
Myrmekite formed during Na-Ca-metasomatism
A first variety of this type of metasomatism affects only enclaves within a granitoid. Here the influx of Na-rich fluids in the temperature range 450 °C to 650 °C from the host leads to the replacement of alkali feldspar by myrmekite within the enclaves. During this process a reequilibration with the Na-poorer feldspars (plagioclase) in the enclaves takes place. As a consequence Ca gets released in plagioclase which in turn can now react upon K-feldspar to form myrmekite. Basically this process is very similar to the Ca-metasomatism on K-feldspar described above except for the Na-fluids acting as a trigger.An example is the Velay granite in the northeastern Massif Central
Massif Central
The Massif Central is an elevated region in south-central France, consisting of mountains and plateaux....
in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
.
In the second variety Na- and Ca-bearing fluids truly act together. This leads via the replacement of primary K-feldspar (perthitic and non-perthitic microcline) to the formation of plagioclase (albite or oligoclase) and in certain places also to the formation of myrmekite. The myrmekite does not show wartlike tapering vermicules, but vermicules that are confined and scattered entirely within the interior of the plagioclase forming irregular spindles, arcuate patterns and ovals.
For this process to operate is important that Ca is sufficiently present so that a fairly calcic plagioclase can be formed which in turn releases enough silica for the myrmekite vermicules. If only Na is present then no myrmekite will form.
An example can be found in the Lyon Mountain granite gneiss north of Ausable Forks in New York.
Occurrence
Myrmekite can appear in many different rock types and different geologic settings. Typically it occurs in graniteGranite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
s and similar igneous rock
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...
s (granitoids, diorite
Diorite
Diorite is a grey to dark grey intermediate intrusive igneous rock composed principally of plagioclase feldspar , biotite, hornblende, and/or pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite, sphene, magnetite, ilmenite and sulfides occur as accessory...
s, gabbro
Gabbro
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....
s) and in metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es similar to granite in composition. It can also occur in anorthosite
Anorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
s and the orthopyroxene-bearing charnockite
Charnockite
Charnockite is applied to any orthopyroxene-bearing granite, composed mainly of quartz, perthite or antiperthite and orthopyroxene , as an end-member of the charnockite series.-Charnockite series:...
s.
Formation
These characteristic intergrowths have been explained in a variety of ways:- One explanation is Castle & Lindsley's exsolution silica-pump model. Accordingly the textureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
was created as plagioclase formed by exsolution from alkali feldsparFeldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
during cooling, under conditions in which siliconSiliconSilicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
was mobile in the rock. The process does not require that magmaMagmaMagma 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...
be present during myrmekite formation. - L.G. Collins explains the formation of myrmekite by:
- the K-metasomatic replacement of deformed primary plagioclase by secondary K-feldspar
- different varieties of Ca- and Na-Ca-metasomatism acting mainly upon deformed primary alkali feldspar, an exception being the replacement of deformed plagioclase in anorthosite.
See also
- GranophyreGranophyreGranophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
, - Rock microstructureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
- GraniteGraniteGranite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
- Eutectic
- SolidusSolidus (chemistry)In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...
- SymplectiteSymplectiteIn mineralogy, a symplectite is a very fine intergrowth of two or more distinct mineral phases. Symplectites are largely considered to result from the breakdown of unstable minerals....