Pyrite
Pyrite is iron disulphide, FeS2: cubic, hardness 6 to 6.5, density 5.018, greenish-black streak. How it differs from gold, chalcopyrite and marcasite.
Pyrite is iron disulphide, FeS2: cubic, pale brass-yellow, hardness 6 to 6.5, density 5.018, with a greenish-black to brownish-black streak. It is the mineral called fool's gold. Gold is hardness 2.5 to 3 and density 19.3; chalcopyrite is 3.5 to 4. Hardness first, streak second and the scale third separate them.
In short
- The identity card, from the Handbook of Mineralogy sheet: FeS2, cubic, hardness 6 to 6.5, measured density 5.018, cleavage indistinct, brittle, pale brass-yellow, tarnishing darker and iridescent. Worked on standard atomic weights, FeS2 is 46.55% iron and 53.45% sulphur, the sheet’s own ideal column. The September 2026 IMA-CNMNC Master List lists it as grandfathered, with no year and country unknown.
- The three-step rule: hardness, streak, then water. Of the five brass-yellow minerals in the table, read by script from their sheets, only pyrite and marcasite are harder than 5. Gold is 2.5 to 3, very malleable and ductile; chalcopyrite 3.5 to 4; pyrrhotite 3.5 to 4.5.
- The scale separates gold, not the sulphides. A 20.00 g piece weighs 18.96 g in water as gold (19.3) and 16.01 g as pyrite (5.018). Chalcopyrite gives 15.12 to 15.35 g and pyrrhotite 15.63 to 15.70 g, but marcasite gives 15.91 g, only 0.11 g from pyrite.
- Not always pure iron. The sheet’s analysis from the Mill Close mine, Derbyshire, has 16.69% nickel: worked, 35.1% of the metal atoms are nickel. Its Gladhammar, Sweden, analysis has 13.90% cobalt, 28.2% of the metal. Iron still dominates both, so both are pyrite; the sheet records a series with cattierite.
- Crystal habits are named on the sheet: cubes, pyritohedra and octahedra to 25 cm or more; suns from Sparta, Illinois; bars from the Buick mine, Missouri; and sculptural groups of crystals from Navajún, Spain.
| Mineral | Formula | Crystal system | Hardness | Density (measured) | Streak on the sheet | Tenacity on the sheet | 20.00 g piece in water |
|---|---|---|---|---|---|---|---|
| Pyrite | FeS2 | Cubic | 6 to 6.5 | 5.018 | Greenish black to brownish black | Brittle | 16.01 g |
| Marcasite | FeS2 | Orthorhombic | 6 to 6.5 | 4.887 | Greyish or brownish black | Brittle | 15.91 g |
| Pyrrhotite | Fe1-xS (x = 0 to 0.17) | Monoclinic, pseudohexagonal | 3.5 to 4.5 | 4.58 to 4.65 | Dark greyish black | Not printed | 15.63 to 15.70 g |
| Chalcopyrite | CuFeS2 | Tetragonal | 3.5 to 4 | 4.1 to 4.3 | Greenish black | Not printed | 15.12 to 15.35 g |
| Gold | Au | Cubic | 2.5 to 3 | 19.3 | Not printed | Very malleable and ductile | 18.96 g |
What pyrite is: iron disulphide, named for fire
FeS2, with a definite ideal composition. On standard atomic weights the formula mass is 119.97, iron 46.55% and sulphur 53.45%. That reproduces column (4) of the sheet, and the Elba analysis, Fe 46.49 and S 53.49, is within 0.06 of it. The name is from the Greek for fire, as sparks may be struck from it. The sheet puts pyrite in the pyrite group, dimorphous with marcasite, and in a series with cattierite.
Worked: when is nickel-rich pyrite still pyrite? Divide each metal percentage by its atomic weight:
- Mill Close mine, Derbyshire: Fe 29.30, Ni 16.69. Iron is 64.9% of the metal atoms and nickel 35.1%: pyrite, nickel-rich.
- Gladhammar, Sweden: Fe 33.32, Co 13.90, Ni 0.19. Iron 71.4%, cobalt 28.2%: pyrite, cobalt-rich.
- Elba, Italy: Fe 46.49 with no other metal reported: pyrite, near the ideal.
The sheet calls it the most abundant and widespread sulfide, formed under a wide variety of conditions. Those range from hydrothermal veins and magmatic segregations to diagenetic replacements in sedimentary rocks. Its associates include pyrrhotite, marcasite, galena, sphalerite, arsenopyrite and chalcopyrite.
Is it pyrite or gold? The three-step rule, with numbers
Step one, hardness. Pyrite is 6 to 6.5. Gold’s sheet gives 2.5 to 3 and a tenacity of very malleable and ductile, with a hackly fracture; pyrite is brittle. A grain that dents rather than powders is not pyrite. The method is on how to test mineral hardness properly.
Step two, streak. Pyrite’s sheet prints greenish black to brownish black. The gold sheet prints no streak at all, so this page quotes none for gold. A black streak from a brass-yellow mineral rules gold out on the sulphide sheets’ own evidence. The technique is on how to use a streak plate.
Step three, the scale. Gold’s measured density of 19.3 is 3.85 times pyrite’s 5.018. A 20.00 g piece reads 18.96 g in water as gold and 16.01 g as pyrite, 2.95 g apart. The gold sheet adds that gold is silver-white to copper-red when impure and forms a series with silver, so natural gold need not read exactly 19.3. The method is on how to measure specific gravity at home. The broader question, with the cube arithmetic, is on what is fool’s gold.
Pyrite, chalcopyrite or marcasite? What separates them
Chalcopyrite, CuFeS2, is tetragonal, 3.5 to 4 in hardness and 4.1 to 4.3 in density, brass-yellow, often tarnished and iridescent. Its crystals are tetrahedral-shaped, where pyrite’s are cubes, pyritohedra and octahedra. Worked, it is 34.63% copper. Hardness separates the two by at least 2 Mohs steps. See chalcopyrite and is chalcopyrite the same as peacock ore.
Marcasite is the same FeS2 in an orthorhombic structure, the same 6 to 6.5, with streak grayish or brownish black. Its measured density, 4.887, is 2.61% below pyrite’s, which is 0.11 g on a 20.00 g piece and not a home test. Habit is the separating feature: marcasite is tabular, with curved faces, cockscomb and spearhead twin groups. The full comparison is on is marcasite the same as pyrite and marcasite.
Pyrrhotite, Fe1-xS, is bronze-yellow to pinchbeck-brown, 3.5 to 4.5, with a dark greyish-black streak. It is softer than pyrite: 3.5 to 4.5 against 6 to 6.5.
Pyrite crystals: cubes, pyritohedra and striations
The habit field reads typically cubic, pyritohedral, octahedral, and combinations of these and other forms, to 25 cm or more. It continues: striated conforming to pyritohedral symmetry; may be elongated to acicular, and commonly granular, globular, framboidal, stalactitic.
Twinning: twin axis [001] and twin plane {011}, penetration and contact twins, as read from the rendered sheet. Cleavage is indistinct on {001}, with indistinct partings on {011} and {111}, so pyrite breaks with a conchoidal to uneven fracture. Twins in general are on mineral twinning explained.
Tarnish and decay are separate questions. The sheet records only that pyrite tarnishes darker and iridescent. Crumbling, cracking and silvery surfaces are on why is my pyrite crumbling, why did my pyrite break and why is my pyrite silver. This page gives no storage, cleaning or handling advice; for health questions see HSE EH40, COSHH 2002 or a clinician. None of this is advice.
Where fine pyrite crystals come from
The Distribution field says only a few localities for large or fine crystals can be mentioned. In Europe it names Rio Marina on Elba and Traversella, Piedmont, Italy; Ambasaguas and Navajún, Logroño Province, Spain, for sculptural groups of crystals; and Aktchitao, Kazakhstan. In Peru it names many districts, with exceptional crystals from the Quiruvilca mine, La Libertad, and Huanzala.
In the USA: the Ibex mine, Leadville, Colorado; suns at Sparta, Illinois; very large crystals from the Santo Niño mine, Arizona; the French Creek mines, Pennsylvania; the Carleton talc mine, Vermont; Butte, Montana; the Spruce claim, Washington; and bars from the Buick mine, Missouri. The only British name on the sheet is the Mill Close analysis from Derbyshire.
Pyrite and gold together. The gold sheet lists pyrite first among gold’s associates and describes gold in epithermal veins typically in quartz with pyrite. Its analysed specimen is from Porthcurnick Beach, Cornwall. Finding the two together is expected; it does not make the pyrite gold-bearing. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.