What Is the Chemical Formula for Gold? Au, Plus Muscovite

The chemical formula for gold is simply Au. Gold is a chemical element, not a compound, so a pure sample is made of one kind of atom and needs no subscripts. NIST lists the formula as Au, with atomic number 79 on its atomic data table.

Muscovite, the pale mica often found in the same rocks, has a far longer formula: KAl₂(AlSi₃O₁₀)(OH)₂. This page explains both, shows the gold minerals that do have longer formulas, and sets out why a geologist looking for Au spends so much time looking at mica.

What is the chemical formula for gold?

Au is the whole answer. The two letters are a chemical symbol, and in a formula the symbol stands for one atom of that element. Because metallic gold is a lattice of identical atoms, chemists write it as Au rather than as a molecule such as Au₂.

The letters come from Latin. The Handbook of Mineralogy traces the symbol to aurum, “shining dawn”, and the English word to Old English.

Gold formula facts at a glance

Gold is also unusual in having a single natural isotope. The Commission on Isotopic Abundances and Atomic Weights calls it monoisotopic, so its atomic weight rests on gold-197 alone. In practice, one mole of gold weighs about 197 grams, which is about 6.33 troy ounces (one troy ounce is 31.1035 grams).

Does the formula change for jewelry gold?

No formula covers jewelry gold, because it is an alloy, a metal mixture with no fixed ratio of atoms. The karat figure tells you the gold share by weight in 24ths. US Federal Trade Commission guides treat 24 karat as fine gold and anything less as an alloy. By simple arithmetic, 18 karat is 75% gold and 14 karat is about 58.3%.

Does natural gold have the same formula?

Mineralogists still write native gold as Au, but natural grains are rarely perfectly pure. One analysis in the Handbook of Mineralogy reports 99.6% gold with small amounts of silver and copper. The same entry says native gold commonly contains silver, copper and iron, and that it forms a series with silver.

Gold minerals with longer formulas

Native gold is the plain metal, but gold also bonds with a few other elements. These are the formulas a consultant will meet in a mineralogy report. The gold share matters because it changes how the ore is treated and how much metal each grain holds.

Figures for the four compounds are from the Webmineral entries for aurostibite, calaverite, sylvanite and petzite. Brackets with a comma, as in (Au,Ag), mean either metal can sit at that position, so the gold share of sylvanite varies from sample to sample.

Gold in cyanide leaching: the formula that matters at the plant

At a mill, the formula a metallurgist cares about is often not Au but the dissolved form, Au(CN)₂⁻. Elsner’s equation, as set out in a metallurgical conference paper, describes the leach:

4Au + 8CN⁻ + O₂ + 2H₂O → 4Au(CN)₂⁻ + 4OH⁻

In words, gold metal, cyanide, oxygen and water produce a soluble gold cyanide complex and hydroxide. The equation implies an ideal 8:1 molar ratio of cyanide to oxygen, although the same paper notes that oxygen is not fully reduced to hydroxide at the gold surface and that sulfide minerals can shift the ratio. Gold is then recovered from the solution; our guide to oxidized gold ore covers why ore type changes that route.

▶ How gold moves from rock to ring, including dissolving and catching it on carbon. · 5:27

Muscovite chemical formula explained

The muscovite formula is KAl₂(AlSi₃O₁₀)(OH)₂, the generalized composition given by Geology.com. You will sometimes see it as KAl₂(Si₃Al)O₁₀(OH,F)₂, as Webmineral writes it. That version shows that fluorine can take the place of some hydroxyl.

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What ideal muscovite is made of

Percent by weight, calculated from KAl2(AlSi3O10)(OH)2

Reading the formula piece by piece

• K: one potassium ion sits between the mineral’s stacked layers and holds them together.
• Al₂: two aluminum ions sit in the middle (octahedral) sheet of each layer.
• (AlSi₃O₁₀): the outer (tetrahedral) framework, where one aluminum takes the place of one silicon in every four sites.
• (OH)₂: two hydroxyl groups, which is why muscovite shows water-related features in lab and satellite spectra.

Checking that the charges balance

A correct mineral formula is electrically neutral, and this one can be checked by hand. On the positive side, one K (+1), three Al (+9) and three Si (+12) add up to +22. On the negative side, ten oxygens (−20) and two hydroxyls (−2) add up to −22. The sum is zero.

What muscovite is made of by weight

Using standard atomic weights, the ideal formula has a molar mass of about 398.3 g/mol. Oxygen is close to half of that weight, and potassium is just under 10%. As an oxide, that potassium equals about 11.8% K₂O, which is how assay labs usually report it. Keep the two numbers apart: 9.8% K and 11.8% K₂O describe the same mineral.

Real samples differ from the ideal. Geology.com notes that sodium, rubidium or cesium can replace some potassium, and that other ions can replace some aluminum.

Muscovite, biotite and gold compared

Readers often arrive here comparing the two common micas with the metal they are hunting. They have little in common chemically. The table gives ranges from the Webmineral and Handbook of Mineralogy entries.

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Gold is far denser than the minerals around it

g/cm³; gold is the Handbook’s measured value, the others are listed average values

Density is the difference that counts in the field. Gold is roughly seven times as dense as muscovite, which is why a gold pan or gravity circuit can separate them. It also explains a common beginner’s error: flakes of weathered mica can glitter like gold, but they float off in the pan. Webmineral lists similar hardness for biotite and gold, so a scratch test alone will not separate them. Pyrite, FeS₂, known as fool’s gold, is far harder at 6.5.

Why muscovite and gold occur together

Muscovite does not contain gold, and neither mineral forms from the other. They meet because the hot fluids that carry gold through fractures also change the surrounding rock. Fine-grained muscovite formed in that way is called sericite.

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From a mica clue to a gold result in 4 steps

1. 1Map alterationMark zones of fine white mica from imagery or field work.
2. 2Add structureLook for faults and quartz veins near those zones.
3. 3SampleTake soil, rock or drill samples from the best ground.
4. 4AssayMeasure gold in g/t. Only this confirms the metal.

The USGS low-sulfide quartz gold deposit model, which covers what are also called Mother Lode type deposits, lists sericite among the main minerals of the gold-bearing zones, along with quartz, carbonates, albite and pyrite. The same model calls potassic alteration characteristic of these deposits but usually restricted to the deposit areas. Our overview of types of ore deposits and the guide to gold deposit and rock types put this class in context.

This is a clue, not proof. Muscovite is also a common mineral of ordinary schist and granite-related rocks with no gold at all. Sericite tells a geologist that fluids altered the rock; only sampling and assays show whether those fluids left metal behind.

What sericite means for a licence holder or consultant

1. Find it: record where fine white mica appears in mapping, core logging and thin sections.
2. Separate it from background: compare it with the muscovite of the regional rock, which formed without any ore fluid.
3. Look for the rest of the pattern: quartz veins, carbonate, pyrite and a controlling fault strengthen the case.
4. Test it: take soil, rock or drill samples and assay them for gold.

How satellites map muscovite when they cannot see gold

A satellite cannot read the Au formula from orbit, since gold grains are far too small and scarce to detect. It can pick up the OH in the muscovite formula. The bond between aluminum and hydroxyl absorbs shortwave infrared light at about 2.2 micrometers.

A USGS mapping study using ASTER satellite data applied a band ratio to this feature and noted that rocks with phyllic alteration (a quartz, sericite and pyrite type of change) typically contain sericite, or muscovite. In that work vegetation and dark pixels had to be masked out first, and detrital clays were a separate source of confusion. An alteration map therefore needs checking on the ground.

For a consultant, this is the practical link between the two formulas: the mica is mapped and the gold is then tested. Farmonaut’s satellite-based mineral detection screens licence areas for alteration and structure ahead of field work, and results are exploration targets, not a resource estimate. You can send an area through the mining query form or contact the team.

▶ What a satellite gold map shows, including clay and mica clues. · 4:18

Muscovite in mining and processing

In a gold mine, muscovite is normally gangue, the worthless mineral that accompanies the ore. Its formula holds no gold position, so it adds nothing to the metal count. Each plant has to measure how it behaves through testwork on its own ore, since mica content and grain size vary between deposits.

Elsewhere mica itself is the product. USGS Mineral Commodity Summaries 2026 put estimated world scrap and flake mica output at about 350,000 tons in 2025. US output was an estimated 26,000 tons, mined in Georgia and North Carolina, and uses included joint compound, paint, roofing and rubber products. US scrap and flake mica averaged about US$130 per metric ton in 2025. Our mica mineral and uses guide goes further into that market.

From a formula to a value: gold grade and price

Because Au is a single element, an assay converts directly into metal. A grade of 1 g/t means one gram of gold in each metric ton of rock. Multiply tons by grade to get grams, then divide by 31.1035 to get troy ounces.

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Average US gold price

US$ per troy ounce, annual; 2025 is an estimate

Gold traded near US$4,120 per ounce on 5 November 2026, according to Trading Economics, well above the US$3,300 estimated 2025 average in the USGS gold summary. The USGS also estimates world mine output at 3,300 tons in 2025. Contained gold is not the same as profit: recovery, mining cost, processing cost and royalties all come off first.

Frequently asked questions

What is the chemical formula for gold?

It is Au. Gold is an element, so its formula is the same as its chemical symbol.

Why is the symbol for gold Au and not Go?

The symbol comes from the Latin name aurum, while the English word gold comes from Old English.

Is gold an element or a compound?

Gold is an element with atomic number 79. A compound such as calaverite, AuTe₂, contains gold and at least one other element.

Is Au2 the formula for gold?

No. Solid gold is written as Au because it is a continuous metal lattice, not a collection of two-atom molecules.

What is the molar mass of gold?

About 196.97 g/mol, based on the CIAAW atomic weight of 196.966 570.

What is the chemical formula for muscovite?

KAl₂(AlSi₃O₁₀)(OH)₂, a hydrous potassium aluminum silicate. Some sources write it with (OH,F) to show that fluorine can replace hydroxyl.

How much potassium is in muscovite?

The ideal formula holds about 9.8% potassium by weight, equal to about 11.8% K₂O.

Does muscovite contain gold?

Its formula does not include gold. Gold can occur beside muscovite in altered rock, but as separate grains in the ore rather than as part of the mica.

Does finding muscovite mean there is gold nearby?

Not by itself. Muscovite is common in rocks with no gold. Fine-grained sericite alongside quartz veins, pyrite and faults is a stronger clue, and assays are still needed.

What is the difference between muscovite and sericite?

Sericite is a name for fine-grained white mica, usually muscovite, often formed when hot fluids alter a rock.

What is the formula of gold in cyanide solution?

Gold dissolves as the complex ion Au(CN)₂⁻, as shown in Elsner’s equation.

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