What is Myrmekite in Geology?

Myrmekite (myr’-me-kite) is an intergrowth of plagioclase feldspar (generally oligoclase) and vermicular quartz, generally replacing potassium feldspar, formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity (Barker, 1970). The quartz occurs as blobs, drops, or vermicular shapes within the feldspar. The word myrmekite is derived from the Ancient Greek “myrmekia” (wart) and was used by Jakob Sederholm in 1899 for the first time to describe these structures.

Becke (1908) proposed the reaction of Na- and Ca-bearing fluids with K-feldspar:

KAlSi₃O₈ + Na⁺ = NaAlSi₃O₈ + K⁺

2KAlSi₃O₈ + Ca²⁺ = CaAl₂Si₂O₈ + 4SiO₂ + 2K⁺

SiO₂ is released because the Al/Si ratio is different in K-feldspar and in Ca-rich feldspar and because of the immobility of Al and Si, quartz forms a microscopic intergrowth with feldspar in quantities that are directly in proportion to the An% of plagioclase.

Occurrence of Myrmekite

Myrmekite can appear in many different rock types and different geologic settings. Typically it occurs in granites and similar igneous rocks (granitoids, diorites, gabbros) and in metamorphic gneisses similar to granite in composition. It can also occur in mylonites, in anorthosites and the orthopyroxene-bearing charnockites.

Formation

These characteristic intergrowths have been explained in a variety of ways:

1. 1. One explanation is Castle & Lindsley’s exsolution silica-pump model. Accordingly, the texture was created as plagioclase formed by exsolution from alkali feldspar during cooling, under conditions in which silicon was mobile in the rock. The process does not require that magma be present during myrmekite formation.
   2. 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.

Myrmekite (myr’-me-kite) is an intergrowth of plagioclase feldspar (generally oligoclase) and vermicular quartz, generally replacing potassium feldspar, formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity (Barker, 1970). The quartz occurs as blobs, drops, or vermicular shapes within the feldspar. The word myrmekite is derived from the Ancient Greek “myrmekia” (wart) and was used by Jakob Sederholm in 1899 for the first time to describe these structures.

Becke (1908) proposed the reaction of Na- and Ca-bearing fluids with K-feldspar:

KAlSi₃O₈ + Na⁺ = NaAlSi₃O₈ + K⁺

2KAlSi₃O₈ + Ca²⁺ = CaAl₂Si₂O₈ + 4SiO₂ + 2K⁺

SiO₂ is released because the Al/Si ratio is different in K-feldspar and in Ca-rich feldspar and because of the immobility of Al and Si, quartz forms a microscopic intergrowth with feldspar in quantities that are directly in proportion to the An% of plagioclase.

Occurrence of Myrmekite

Myrmekite can appear in many different rock types and different geologic settings. Typically it occurs in granites and similar igneous rocks (granitoids, diorites, gabbros) and in metamorphic gneisses similar to granite in composition. It can also occur in mylonites, in anorthosites and the orthopyroxene-bearing charnockites.

Formation

These characteristic intergrowths have been explained in a variety of ways:

1. 1. One explanation is Castle & Lindsley’s exsolution silica-pump model. Accordingly, the texture was created as plagioclase formed by exsolution from alkali feldspar during cooling, under conditions in which silicon was mobile in the rock. The process does not require that magma be present during myrmekite formation.
   2. 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.