Myrmekite

Myrmekit is a microscopically sized growth of wurmförmigem quartz in plagioclase. The diameter of the wormlike quartz stems or quartz tubes usually moves far less than a millimeter. This intergrowth of Myrmekits with plagioclase is usually in contact with alkali. Myrmekit formed under metasomatic conditions in combination with tectonic deformations. He should be on no account mixed with micrographic intergrowths as graphic granite or granophyrischen adhesions because they are of magmatic origin.

• 3.1 Kalziummetasomatose of deformed alkali feldspar in igneous rocks
• 3.2 Kalziummetasomatose of deformed alkali feldspar in charnockites
• 3.3 Kalziummetasomatose of deformed plagioclase in anorthosites

Etymology

The name derives from the Greek Myrmekit μὑρμηχἰα ( wart ) or from μὑρμηξ ( ant ). It was used in 1899 by Jakob John Sederholm in a scientific description of this structure for the first time.

Formation of Myrmekit during Kaliummetasomatose

Different Myrmekittypen can form during the Kaliummetasomatose:

• Randmyrmekit
• Warzenmyrmekit
• Geistermyrmekit

Randmyrmekit

Randmyrmekit, Eng. myrmekite rim, formed during the initial stage of Kaliummetasomatose in cataclastic relatively weakly deformed magmatic rocks. The fractures mainly put on at the grain boundaries. This allows for example the Kaliummetasomatose with hot potassium solutions penetrate the edges zoned plagioclase crystals. As a corollary to Randmyrmekit forms on the Plagioklaskristall and interstitial alkali feldspar. Due to the low anorthite content of the quartz Plagioklasränder the stems are very thin.

The chemical changes remain at this stage is limited to the sub-millimeter range and are therefore relatively low. However, there are transitions to the next stage of Warzenmyrmekits.

Examples of Randmyrmekit found in plutons of the Sierra Nevada.

Warzenmyrmekit

With increasing tectonic stresses also the Kataklasis is intensified. Fractures can spread to the crystal interior and crystals can be bent now. Consequently, the Kaliummetasomatose continue to move forward. It finally comes to an almost complete to complete displacement of plagioclase by alkali feldspar. At sites with incomplete displacement is Warzenmyrmekit (English wartlike myrmekite ) forms.

There are transitions between the rocks only Randmyrmekit lead to those with the same Randmyrmekit and Warzenmyrmekt and finally to such alone with Warzenmyrmekit. A very important observation is the correlation of quartz stem thickness (diameter of the worm-like tubes) and calcium content of the original plagioclase in unaltered igneous parent rock. The thickest stems occur here in rocks on the highest calcium content in plagioclase.

An example of the occurrence of the quartz monzonite is Warzenmyrmekit of Twentynine Palms in California.

Geistermyrmekit

This is the third type of quartz Plagioklasverwachsungen during Kaliummetasomatose in granitoids. This type is also dependent on tectonic deformation. For him, there is an unbalanced removal of calcium, sodium and aluminum from the deformed Plagioklasgitter. This in turn creates an imbalance in the relative ratio of the remaining aluminum and silicon. There is an excess of silicon, which can not be installed by the plagioclase displacing the alkali in the crystal structure. This silicon excess then in turn forms the Geistermyrmekit (English ghost myrmekite ) - it arise either tiny Quarzovoide in leftover Albitinseln in alkali feldspar or independent group arranged in the shape Quarzovoide (without Albitinseln ) in alkali feldspar ( see figure for clarity).

The Rubidoux Mountain leucogranite and granodiorite several in the Sierra Nevada have this structure.

Formation of Myrmekit during Kalziummetasomatose

Also during the Kalziummetasomatose it comes under different circumstances in the formation of Myrmekit:

• Kalziummetasomatose of deformed alkali feldspar in igneous rocks
• Kalziummetasomatose of deformed alkali feldspar in charnockites
• Kalziummetasomatose of deformed plagioclase in anorthosites

Kalziummetasomatose of deformed alkali feldspar in igneous rocks

In this Metasomatosetypus hot, calcium-containing solutions penetrate through tectonically caused cracks in the primary alkali feldspar and react with the crystal lattice. This central quartz and of marginal Myrmekit in the cracks to form. These displacement reactions, large parts of the primary Alkalifeldspats capture (up to about 60%). A characteristic feature of this type is the constant thickness of the quartz tubes; in Kaliummetasomatose however, the thickness is of the calcium content of the plagioclase depends, in addition, the tubes bend toward the alkali feldspar.

As an example of this Metasomatosetyp the mega crystal - leading granite at Alastaro can mention in Finland.

Kalziummetasomatose of deformed alkali feldspar in charnockites

The process is the same as before, the difference lies in the parent rocks, which act on the calcium solutions. Charnockite differ from ordinary granitoids by the appearance of orthopyroxene ( hypersthene ) and are often metamorphic origin or metamorphic overprints.

Examples can be found in Sri Lanka.

Kalziummetasomatose of deformed plagioclase in anorthosites

Anorthosites have practically almost no alkali feldspar, therefore, is attacked in this type of plagioclase instead of Alkalifeldspats of the calcium solutions. The resulting Myrmekit has also stems from constant thickness. Unlike the first type, however, the quartz stems may tend to the primary, quartz- free plagioclase. This behavior finds an explanation in the simultaneous incorporation of sodium, which means increased SiO2 content in Feldspatgitter by itself.

Examples can be found in anorthosites of the so -called " layered igneous complexes ' ( layered intrusive bodies ).

Formation of Myrmekit during the sodium-calcium metasomatism

In her first variant, this metasomatism affects only rock inclusions in granitoids. Hot sodium- rich, coming from the host rock liquids in the temperature range from 450 ° C to 650 ° C react with the alkali in the inclusions to form Myrmekit. During this process, there is a balance reaction ( re-equilibration ) with the poorer sodium plagioclase in the inclusions. As a consequence of free calcium is in the plagioclase, which in turn can act now to form Myrmekit on the alkali feldspar. Basically, this process with the above-described deformed Kalziummetasomatose of alkali feldspars comparable function except that in this case, the sodium-containing solution as a reaction initiator.

As an example of this Metasomatosetypus the Velay granite in the north-eastern Massif Central serves.

In the second variant of sodium and calcium react in the composite. The primary alkali feldspar ( microcline perthitischer and ordinary ) is replaced it, it creates plagioclase ( albite or oligoclase ) and in some places also Myrmekit. The Myrmekit is not this time vermiform and not bent, it remains rather limited to the interior of the plagioclase and consists of irregular spindles, ovals and curved structures.

Calcium must be present in sufficient quantities, so that this process can proceed. Just as the emergence of a relatively calcium-rich plagioclase is made possible, which then in turn can provide sufficient SiO2 for Myrmekitbildung available. If only sodium is present, there is no Myrmekitbildung.

An example of this process can be found in the Lyon Mountain granite gneiss at Ausable Forks, New York.

Formation of Myrmekit with progressive deformation

In the course of progressive deformation in mylonitic ductile shear zones Myrmekit usually concentrated in the peripheral region of the sigmoidal Alkalifeldspatkristallen. It occurs more frequently in the two voltage (ie shortening ) related sectors. Simpson and Wintsch (1989 ) explain this asymmetric arrangement of Myrmekit with a resolution reaction of alkali feldspar, which preferably takes place at locations with high differential voltage during retrograde metamorphism. But not limited to the Alkalifeldspataugen but also Myrmekit internally observed a monoclinic symmetry. The latter can be used as an indicator of the shear sense independently of the Alkalifeldspataugen then also. The asymmetrically arranged Myrmekit is therefore a so-called quadrant structure (so called quarterback structure ).

However, for Lorence G. Collins of the alkali feldspar is not primary, magmatic origin, but by a metasomatic replacement reaction of the primary plagioclase originated ( potassium metasomatism in contrast to the discussion in the field of Simpson and Wintsch sodium-calcium metasomatism ). For him, therefore, the actual deformation event reaches much further back than is immediately apparent. Another conclusion is therefore that the mylonites of the shear zone are the result of original igneous rocks.

Occurrence

Myrmekit can occur in many rocks of different origins. It is usually found in granitoids and related igneous rocks such as diorite and gabbro. Also in metamorphic rocks it occurs, for example, in gneisses of granitic composition, in anorthosites and charnockites in the orthopyroxenreichen.

Formation

Myrmekitische adhesions see petrologists different explanations:

• Castle & Lindsley try with a so-called " exsolution silica -pump model " to interpret. This model is based on chemical segregation during cooling. As a result, plagioclase has segregated from the alkali feldspar, quartz as was mobile. The presence of magma in the formation of Myrmekit is not necessary in their view.
• LG Collins explains Myrmekit purely metasomatic, ie the formation was below the eutectic instead: Through an exchange of tectonically deformed, primary plagioclase with secondary K-feldspar during a potassium- stressed Metasomatosevorgangs.
• Through several Ca - and Na -Ca- Metasomatosetypen which act mainly on tectonically deformed, primary alkali feldspar; an exception in this case represents the metasomatic alteration of anorthosites in which primary plagioclase is replaced.

Swell

• Website of L.G. Dedicated exclusively to the phenomenon Collins Myrmekit