chemistry

Gold cycle

Gold cycle is a chemistry 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 Gold cycle rather than just read about it. In short: The gold cycle is the biogeochemical cycling of gold through the lithosphere, hydrosphere, atmosphere, and biosphere. Gold is a noble transition metal that is highly mobile in the environment and subject to biogeochemical cycling, driven largely by microorganisms.

Key takeaways

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

Reference excerpt

The gold cycle is the biogeochemical cycling of gold through the lithosphere, hydrosphere, atmosphere, and biosphere. Gold is a noble transition metal that is highly mobile in the environment and subject to biogeochemical cycling, driven largely by microorganisms. Gold undergoes processes of solubilization, stabilization, bioreduction, biomineralization, aggregation, and ligand utilization throughout its cycle. These processes are influenced by various microbial populations and cycling of other elements such as carbon, nitrogen, and sulfur. Gold exists in several forms in the Earth's surface environment including Au(I/III)-complexes, nanoparticles, and placer gold particles (nuggets and grains). The gold biogeochemical cycle is highly complex and strongly intertwined with cycling of other metals including silver, copper, iron, manganese, arsenic, and mercury. Gold is important in the biotech field for applications such as mineral exploration, processing and remediation, development of biosensors and drug delivery systems, industrial catalysts, and for recovery of gold from electronic waste.

Lithosphere

The lithosphere is the dominant reservoir of gold, containing an estimated 2.6 × 1013 Mg. Today, gold exists primarily as electrum, in hard rock deposits like tellurides, and as particles in placers in Earth's crust. Gold cycling starts with the microbial weathering of gold-bearing rocks and minerals which mobilizes gold in the environment via release of elemental gold and solubilization. The Witwatersrand gold deposits host approximately 30% of the world's gold resources, a large proportion of which is directly associated with organic carbon derived from microbial mats. Gold ore has been mined in many countries, including Japan, India, Spain, Yugoslavia, South Africa, Australia, the United States of America, Canada, Colombia, Mexico, and Brazil.

Ocean The ocean reservoir contains an estimated 5.6 × 109 Mg of gold and oceanic gold concentration is about 4 ng Au/L with higher values in some coastal waters. Au(I/III)-ions and Au(0)-colloids are unstable under surface conditions in aqueous solutions and commonly form ligand complexes with substances excreted by microorganisms. Similar to silver and mercury, these mobile Au(I/III)-complexes are toxic in nature. Some bacteria that live in biofilms on placer gold particle surfaces deal with this toxicity by precipitating Au(I/III)-complexes which leads to the biomineralization of gold. Other archaea, iron-reducing bacteria, and some sulfate-reducing bacteria have developed methods to regulate and detoxify their immediate environment when Au(III)-ions are present at toxic levels. Iron- and sulfur-oxidizing litho-autotrophic bacteria break down gold-hosting sulfide minerals, releasing gold as alloy particles or Au(I)-thiosulfate complexes. Eventually, gold nanoparticles released by these processes undergo transformation, are dispersed in oceans, or accumulate in sediments.

Atmosphere The atmosphere is the smallest reservoir of gold, containing an estimated 370 Mg. The most volatile gold compounds are Au2Cl6, which may occur in volcanic gases, and AuF3.

Influences and interactions of other biogeochemical cycles The biogeochemical cycle of gold is affected by the carbon, nitrogen, sulfur, and iron cycles. Decomposition of organic carbon under anoxic conditions creates a wide range of organic intermediates, e.g., organic acids, that are important determinants of gold mobility. Key microbial processes in the nitrogen cycle can be influenced by gold and vice versa; for example autotrophic denitrifying bacteria can destabilize Au-complexes and may play a role in gold cycling. Overall, it is likely that gold mobility, biomineralization, and ore forming processes are impacted by the reactive nitrogen-containing compounds. Gold is commonly incorporated in iron-sulfides and adsorbed by Fe(III)-oxyhydroxide precipitates; oxidation of gold-bearing pyrite can lead to the mobilization of soluble gold complexes.

Ancient Earth Throughout Earth's history, the interplay of gold, microorganisms, and physicochemical conditions such as pH and redox potential have led to the aggregation of gold particles to form grains and nuggets. Cyanobacteria in shallow surface waters on early anoxic Earth accumulated gold complexes dissolved in the water and geochemical modeling indicates that gold solubility in ancient waterbodies was much higher than today. Experimental evidence suggests that on early Earth, Fe(III)-reducing extremophiles and sulfate-reducing bacteria may have been contributed to formation of gold-bearing deposits.

See also California Gold Rush Biogeochemical cycle

References

Illustrations

Worked examples

Example 1 — a first encounter with Gold cycle

Start with the simplest possible case. Write down what Gold cycle claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, 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 Gold cycle 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 Gold cycle 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 Gold cycle

In research: Gold cycle appears in chemistry 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 Gold cycle 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: Gold cycle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biogeochemical cycle, Gold, so understanding it makes those chapters shorter.
In everyday life: Look for Gold cycle 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.

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Gold cycle in 20 minutes

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

Frequently asked questions

What is Gold cycle in simple terms?

Why does Gold cycle matter?

Because it connects several chemistry 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 Gold cycle?

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 Gold cycle.