How to Identify Pyrite and Its Importance in Chemistry
Pyrite is a common mineral (also among the frequent natural sulphides and the most common disulfide) found in a wide range of geological formations, including sedimentary layers, hydrothermal veins, and metamorphic rocks. Pyrite's brassy-yellow metallic colour has made many people mistake it for gold, earning it the nickname the Fool's gold. Pyrite stone and pyrite quartz are easy to differentiate from gold because it is considerably lighter than gold. Yet, it is much more complicated and cannot be scratched with a fingernail or pocket blade.
Member of: Pyrite Group
Name: When it was struck with another mineral or metal in antiquity, Sparks flew from it, earning it the name "pyr" for "fire".
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Pyrite is a nickel-bearing mineral that forms a series with Vaesite and Bravoite is a nickel-bearing variation of Pyrite.
Pyrite forms a series with Cattierite and usually contains minor cobalt. Metal nanoparticles are responsible for the majority of the foreign metal concentration in Pyrite. In a damp atmosphere, Pyrite will slowly oxidize and release sulfuric acid generated during the process. Pyrite developed as sedimentary concretions tends to dissolve quickly, although well-crystallized specimens are often very stable.
Pyrite Properties
Physical Properties
Lustre: Metallic
Transparency: Opaque
Colour: Pale brass-yellow
Streak: Greenish-black
Hardness level: 6 - 6½ on Mohs scale
Hardness Data: Measured
Tenacity: Brittle
Cleavage: Poor/Indistinct; Indistinct on {001}.
Fracture: Irregular/Uneven, Conchoidal
Density: 4.8 - 5 g/cubic centimeters (Measured); 5.01 g/cubic centimeters (Calculated)
Chemical Properties
Formula: FeS2
Elements listed: Fe, S
Common Impurities: Ni,Co,As,Cu,Zn,Ag,Au,Tl,Se,V
Crystallography of Pyrite
Crystal System: Isometric
Class (H-M): m3 (2/m 3) - Diploidal
Space Group: Pa3
Setting: Pa3
Cell Parameters: a = 5.417 Å
Unit Cell V: 158.96 ų (Calculated from Unit Cell)
Z: 4
Morphology: Typical faces are cubic or pyritohedral (pentagonal dodecahedral); however, octahedral combinations are also prevalent. Less typically octahedral, but more commonly granular, extensive, as well as occasionally radiating, discoidal, reniform, or globular.
Pyrite Types
Identifying Pyrite
Pyrite hand specimens are usually straightforward to recognise. The mineral is always brassy yellow in colour, has a metallic sheen, and has a high specific gravity. Its streak is black, frequently with a tinge of green, and it is more challenging than other yellow metallic minerals. It is most commonly found in well-formed crystals in the shapes of cubes, pyritohedron, or octahedrons, with striated faces.
Marcasite, a dimorph of Pyrite with the same chemical composition but an orthorhombic crystal structure, is the only common mineral with attributes similar to Pyrite. The brassy yellow tint of marcasite differs from that of Pyrite. Instead, it's a soft brass colour with a tiny green tint to it. Marcasite is more brittle than Pyrite and has a lower specific gravity (4.8) than Pyrite.
Pyrite With Hematite: Pyrite with hematite from Rio Marina on the Italian island of Elba. The specimen measures about 3 inches (7.6 cm) in diameter.
Fool's Gold
It's easy to tell the difference between Pyrite and gold. With pin pressure, gold is exceedingly soft and will bend or dent. Pyrite is fragile, and pin pressure will cause thin sections to break. The streak of gold is yellow, while the streak of Pyrite is greenish-black. Gold has a much higher specific gravity than silver. You can avoid the "Fool's Gold" problem with some careful testing.
Massive Pyrite: Massive Pyrite from Rico, Colorado. The specimen measures about 3 inches (7.6 cm) in diameter.
Pyrite: Pyrite with hematite from Italy, Rio Marina, Isle of Elba. The specimen measures about 3 inches (7.6 cm) in diameter.
Studying with a collection of tiny specimens that you can hold, analyse, and notice their qualities is the most excellent approach to learning about minerals. The Geology.com Store offers inexpensive mineral collections.
Pyrite Uses
Pyrite is made up of iron and sulphur, but neither of these elements is found in large quantities in the mineral. Oxide ores, such as hematite and magnetite, are the most common sources of iron. These ores are found in far more significant quantities, the iron is easier to extract, and the metal is not tainted with sulphur, weakening it.
Pyrite was once a valuable resource for producing sulphur and sulfuric acid. The majority of sulphur is now obtained as a byproduct of the oil and gas industry. As a byproduct of gold production, some sulphur is still created from Pyrite.
Sometimes, this pyrite mineral is also used as a gemstone. It's made into beads, cabochons, faceted, and carved into different shapes. In the mid-to-late-1800s, this kind of jewellery was popular in the United States and Europe. Most of the jewellery stones were mistakenly labelled as "marcasite," but they are Pyrite. (Marcasite is a poor choice for jewellery because it oxidises quickly, and the oxidation products destroy anything it comes into contact with.) Pyrite is a poor choice for jewellery because it tarnishes quickly.)
FAQs on Pyrite: Key Properties and Identification
1. What is pyrite and why is it famously known as "Fool's Gold"?
Pyrite is an iron sulfide mineral with the chemical formula FeS₂. It is nicknamed "Fool's Gold" because its metallic lustre and pale brass-yellow hue can be easily mistaken for real gold by those with an untrained eye. However, chemically and physically, it is very different, being much harder and more brittle than actual gold.
2. What are the ideal conditions for the natural formation of pyrite?
Pyrite typically forms in environments that meet three key criteria: a source of iron, a supply of sulphur, and an oxygen-poor (anoxic) setting. This is why it is commonly found in sedimentary rocks rich in organic matter, like black shale and coal beds, where decaying organic material consumes oxygen and releases the necessary sulphur.
3. How can you distinguish between pyrite and real gold using simple physical tests?
Several simple tests can differentiate pyrite from gold without complex equipment:
- Hardness: Pyrite is much harder than gold. On the Mohs scale, pyrite is 6-6.5, while gold is only 2.5-3. Pyrite can scratch glass, whereas gold is too soft to do so.
- Streak Test: When scraped across an unglazed ceramic plate, pyrite leaves a characteristic greenish-black or brownish-black streak. Real gold leaves a true yellow streak.
- Brittleness: Striking pyrite with a hard object will cause it to shatter, as it is brittle. Gold is highly malleable and will simply flatten or bend under pressure.
4. What are the main industrial applications of pyrite?
While not valuable as a gem, pyrite is an important industrial mineral. Its primary applications include:
- Production of Sulfuric Acid: The main use of pyrite is to produce sulfur dioxide (SO₂) gas by roasting the ore. This gas is the key raw material for manufacturing sulfuric acid (H₂SO₄) via the Contact Process.
- Source of Gold: In some geological deposits, pyrite contains small, but economically recoverable, amounts of gold as inclusions or substitutions within its crystal lattice, making it a valuable ore of gold.
- Iron Ore: After the sulphur is driven off during roasting, the remaining iron oxide (Fe₂O₃) can sometimes be used as an iron ore in steel production.
5. Why is the presence of pyrite in coal and rock formations an environmental concern?
Pyrite is an environmental concern for two main reasons. Firstly, when coal containing pyrite is burned, the sulphur oxidizes to form sulfur dioxide (SO₂), a major air pollutant that leads to acid rain. Secondly, when pyrite in rock is exposed to air and water, it oxidizes to form sulfuric acid, leading to a phenomenon known as acid mine drainage, which can contaminate soil and water sources with acid and heavy metals.
6. What is the chemical reaction that occurs when pyrite is roasted, and why is this step crucial in metallurgy?
Roasting pyrite involves heating it in the presence of excess oxygen. This exothermic reaction converts the iron sulfide into iron(III) oxide and sulfur dioxide gas. The balanced chemical equation is:
4 FeS₂(s) + 11 O₂(g) → 2 Fe₂O₃(s) + 8 SO₂(g)
This step is crucial because it effectively separates the valuable sulphur (as SO₂ gas for acid production) from the iron, which can then be further processed.
7. How does the crystal structure of pyrite (FeS₂) account for its cubic shape?
Pyrite crystallises in the cubic crystal system. Its structure consists of iron(II) ions (Fe²⁺) and dumbbell-shaped disulfide ions (S₂²⁻) arranged in a highly ordered, repeating lattice. The Fe²⁺ ions form a face-centred cubic arrangement, and the disulfide ions are positioned within this framework. This internal symmetrical arrangement is expressed externally as the mineral's characteristic, often perfectly formed, cubic crystals.
