earth science

Myrmekite

Myrmekite is a earth science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Myrmekite rather than just read about it. In short: Myrmekite is a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter.

Key takeaways

Myrmekite belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Myrmekite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Myrmekite from memory before moving on to harder problems.

Reference excerpt

Myrmekite is 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 magmatic.

Etymology The word myrmekite is derived from the Ancient Greek μὑρμηχἰα (wart) or μὑρμηξ (ant) and was used by Jakob Sederholm 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 K-metasomatism in cataclastically-deformed magmatic 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).

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 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 Twentynine Palms, California quartz monzonite which issued from an older, yet undated diorite.

Ghost myrmekite This is the third type of quartz-feldspar intergrowth in metasomatic granitoids. 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 Mount Rubidoux leucogranite and in granodiorites 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 charnockites Ca-metasomatism of deformed plagioclase in anorthosites

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 in Finland.

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 (hypersthene) and can also be of metamorphic origin. An example for this type of Ca-metasomatism is found in Sri Lanka.

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

… excerpt ends here. Continue reading the full article.

Illustrations

Myrmekite: Rim myrmekite on zoned plagioclase against interstitial microcline (gray and black) — Rim myrmekite on zoned plagioclase against interstitial microcline (gray and black)

Myrmekite: Fractured alkali feldspar filled with central quartz and myrmekite during Ca-metasomatism — Fractured alkali feldspar filled with central quartz and myrmekite during Ca-metasomatism

Worked examples

Example 1 — a first encounter with Myrmekite

Start with the simplest possible case. Write down what Myrmekite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Myrmekite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Myrmekite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Myrmekite

In research: Myrmekite appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Myrmekite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Myrmekite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Minerals, Petrology, so understanding it makes those chapters shorter.
In everyday life: Look for Myrmekite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Myrmekite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Myrmekite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Myrmekite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Myrmekite in simple terms?

Myrmekite is a vermicular, or wormy, intergrowth of quartz in plagioclase. The intergrowths are microscopic in scale, typically with maximum dimensions less than 1 millimeter.

Why does Myrmekite matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Myrmekite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Myrmekite.