Mica

Key takeaways

Mica 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 Mica to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mica from memory before moving on to harder problems.

Reference excerpt

Micas () are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as perfect basal cleavage. Mica is common in igneous and metamorphic rock and is occasionally found as small flakes in sedimentary rock. It is particularly prominent in many granites, pegmatites, and schists, and "books" (large individual crystals) of mica several feet across have been found in some pegmatites. Micas are used in products such as drywalls, paints, and fillers, especially in parts for automobiles, roofing, and in electronics. The mineral is used in cosmetics and food to add "shimmer" or "frost".

Properties and structure The mica group comprises 37 phyllosilicate minerals. All crystallize in the monoclinic system, with a tendency towards pseudohexagonal crystals, and are similar in structure but vary in chemical composition. Micas are translucent to opaque with a distinct vitreous or pearly luster, and different mica minerals display colors ranging from white to green or red to black. Deposits of mica tend to have a flaky or platy appearance. The crystal structure of mica is described as TOT-c, meaning that it is composed of parallel TOT layers weakly bonded to each other by cations (c). The TOT layers in turn consist of two tetrahedral sheets (T) strongly bonded to the two faces of a single octahedral sheet (O). The relatively weak ionic bonding between TOT layers gives mica its perfect basal cleavage. The tetrahedral sheets consist of silica tetrahedra, each silicon ion surrounded by four oxygen ions. In most micas, one in four silicon ions is replaced by an aluminium ion, while aluminium ions replace half the silicon ions in brittle micas. The tetrahedra share three of their four oxygen ions with neighbouring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet. The octahedral sheet can be dioctahedral or trioctahedral. A trioctahedral sheet has the structure of a sheet of the mineral brucite, with magnesium or ferrous iron being the most common cation. A dioctahedral sheet has the structure and (typically) the composition of a gibbsite sheet, with aluminium being the cation. Apical oxygens take the place of some of the hydroxyl ions that would be present in a brucite or gibbsite sheet, bonding the tetrahedral sheets tightly to the octahedral sheet. Tetrahedral sheets have a strong negative charge since their bulk composition is AlSi3O5−10. The octahedral sheet has a positive charge, since its bulk composition is Al(OH)2+ (for a dioctahedral sheet with the apical sites vacant) or M3(OH)4+2 (for a trioctahedral site with the apical sites vacant; M represents a divalent ion such as ferrous iron or magnesium) The combined TOT layer has a residual negative charge, since its bulk composition is Al2(AlSi3O10)(OH)−2 or M3(AlSi3O10)(OH)−2. The remaining negative charge of the TOT layer is neutralized by the interlayer cations (typically sodium, potassium, or calcium ions). Because the hexagons in the T and O sheets are slightly different in size, the sheets are slightly distorted when they bond into a TOT layer. This breaks the hexagonal symmetry and reduces it to monoclinic symmetry. However, the original hexahedral symmetry is discernible in the pseudohexagonal character of mica crystals. The short-range order of K+ ions on cleaved muscovite mica has been resolved.

Classification Chemically, micas can be given the general formula

X2Y4–6Z8O20(OH, F)4, in which

X is K, Na, or Ca or less commonly Ba, Rb, or Cs; Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.; Z is chiefly Si or Al, but also may include Fe3+ or Ti. Structurally, micas can be classed as dioctahedral (Y = 4) and trioctahedral (Y = 6). If the X ion is K or Na, the mica is a common mica, whereas if the X ion is Ca, the mica is classed as a brittle mica.

Dioctahedral micas Muscovite Paragonite Brittle micas:

Margarite

Trioctahedral micas Common micas:

Biotite Lepidolite Phlogopite Zinnwaldite Brittle micas:

Clintonite

Interlayer-deficient micas Very fine-grained micas, which typically show more variation in ion and water content, are informally termed "clay micas". They include:

Hydro-muscovite with H3O+ along with K in the X site; Illite with a K deficiency in the X site and correspondingly more Si in the Z site; Phengite with Mg or Fe2+ substituting for Al in the Y site and a corresponding increase in Si in the Z site. Sericite is the name given to very fine, ragged grains and aggregates of white (colorless) micas.

Occurrence and production

Mica is widely distributed and occurs in igneous, metamorphic and sedimentary regimes. Large crystals of mica used for various applications are typically mined from granitic pegmatites. The largest documented single crystal of mica (phlogopite) was found in Lacey Mine, Ontario, Canada; it measured 10 m × 4.3 m × 4.3 m (33 ft × 14 ft × 14 ft) and weighed about 330 tonnes (320 long tons; 360 short tons). Similar-sized crystals were also found in Karelia, Russia. Scrap and flake mica is produced all over the world. In 2010, the major producers were Russia (100,000 tonnes), Finland (68,000 t), the United States (53,000 t), South Korea (50,000 t), France (20,000 t) and Canada (15,000 t). The total global production was 350,000 t, although no reliable data were available for China. Most sheet mica was produced in India (3,500 t) and Russia (1,500 t). Flake mica comes from several sources: the metamorphic rock called schist as a byproduct of processing feldspar and kaolin resources, from placer deposits, and pegmatites. Sheet mica is considerably less abundant than flake and scrap mica, and is occasionally recovered from mining scrap and flake mica. The most important sources of sheet mica are pegmatite deposits. Sheet mica prices vary with grade and can range from less than $1 per kilogram for low-quality mica to more than $2,000 per kilogram for the highest quality. In Madagascar and India, it is also mined artisanally, in poor working conditions and with the help of child labour.

Uses The commercially important micas are muscovite and phlogopite, which are used in a variety of applications.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Mica

Start with the simplest possible case. Write down what Mica 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 Mica 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 Mica 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 Mica

In research: Mica 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 Mica 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: Mica is common in secondary-school and first-year university syllabi. It links to neighbouring topics Dielectrics, Industrial minerals, Mica group, so understanding it makes those chapters shorter.
In everyday life: Look for Mica 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.

How to study Mica in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mica 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 Mica out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mica in simple terms?

Micas () are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as perfect basal cleavage.

Why does Mica 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 Mica?

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 Mica.