Is marcasite the same as pyrite?
Same formula, different species. Mohs cannot separate them — both 6 to 6.5 — but the Vickers number can: marcasite 915 to 1099 against pyrite 1505 to 1520.
No. They share the formula FeS2 and are separate species: pyrite is cubic, marcasite orthorhombic. The Mohs scale cannot tell them apart, since both are 6 to 6.5, but the instrumented Vickers hardness can — marcasite is 915 to 1099 against pyrite's 1505 to 1520, so marcasite is 28 to 39% softer on a scale that actually measures.
In short
- The Mohs scale fails here and the Vickers number does not. Both species are quoted at Mohs 6 to 6.5. The Vickers hardness numbers in the same reference are 915 to 1099 for marcasite and 1505 to 1520 for pyrite — a difference of 28 to 39%, invisible to a scratch test.
- Density is no help either: 4.887 against 5.018, a gap of 2.7%. That is measurable on a good balance with a clean fragment and it is nowhere near feelable in the hand.
- Habit is the practical field test. Pyrite forms cubes and pyritohedra. Marcasite is tabular on {010}, with curved faces, and forms cockscomb and spearhead groups from repeated twinning on {101}. A cockscomb is not a pyrite.
- Colour on a fresh break separates them for about a day. Marcasite is tin-white when fresh and darkens to pale bronze-yellow with an iridescent tarnish; pyrite is brass-yellow from the start. After tarnishing, the distinction is gone.
- Marcasite jewellery is a trade name, not a mineral determination, and the stones set in it are generally pyrite. We have found no regulatory definition of the jewellery term and say so rather than asserting one.
| Property | Pyrite | Marcasite | Does it separate them? |
|---|---|---|---|
| Formula | FeS2 | FeS2 | No — identical |
| Crystal system | Cubic | Orthorhombic | Yes, on a sharp crystal |
| Mohs hardness | 6 to 6.5 | 6 to 6.5 | No |
| Vickers hardness (VHN) | 1505 to 1520 (100 g) | 915 to 1099 (200 g) | Yes — but it needs a laboratory |
| Density | 5.018 | 4.887 | Marginally, with a clean fragment and a good balance |
| Cleavage | Indistinct on {001} | {101} rather distinct; {110} in traces | Weakly |
| Habit | Cubes, pyritohedra, octahedra | Tabular on {010}, curved faces, stalactitic, reniform | Yes, usually |
| Twinning | Iron cross on {110} | Common and repeated on {101}: cockscomb, spearhead | Yes — the cockscomb is decisive |
| Fresh colour | Pale brass-yellow | Tin-white, darkening to pale bronze | Yes, briefly |
| Streak | Greenish black to brownish black | Greyish or brownish black | No |
The one property that genuinely separates them, and why you cannot use it
Both species are printed at Mohs 6 to 6.5 in the Handbook of Mineralogy. Identical. A scratch test on an unknown FeS2 returns no information at all.
The same sheets also carry a Vickers hardness number, measured by pressing a diamond indenter into a polished section under a known load and measuring the impression. Pyrite: VHN 1505 to 1520 at 100 g. Marcasite: VHN 915 to 1099 at 200 g.
Take the extremes: 915 / 1505 = 0.61, and 1099 / 1520 = 0.72. Marcasite is between 28% and 39% softer than pyrite on an instrumented scale, while both read the same on the scale a collector owns.
That is the whole lesson, and it generalises well beyond these two. The Mohs scale is ordinal: it ranks, it does not measure. The intervals between its points are not equal and not even approximately so, and two species sharing a Mohs value may differ by half again in the property the scale is standing in for. The same argument, from the other end, is on the talc page, where hardness 1 turns out to be a definition rather than a measurement.
What a collector can actually do with this: nothing directly, since it needs a polished section and a microhardness tester. What it is worth is the negative conclusion — stop trying to separate marcasite from pyrite by scratching, and go and look at the crystal shape instead.
How to tell them apart in practice
Look at the habit first; it does most of the work. Marcasite is tabular on {010} with commonly curved faces, and twins repeatedly on {101} to make the cockscomb and spearhead groups it is famous for. It also forms stalactitic and reniform masses. Pyrite forms cubes, pyritohedra and octahedra with flat striated faces. A curved, ridged, feathery group is marcasite; a striated cube is pyrite.
Then the colour, if the break is fresh. Marcasite is tin-white on a fresh surface and darkens to pale bronze-yellow with an iridescent tarnish. Pyrite is brass-yellow immediately. This is a real difference that disappears within days of exposure, so it is only available on material you have just broken.
Then the setting. Marcasite forms under low-temperature, highly acidic conditions — in shales, limestones and low-rank coals, and in hydrothermal veins from ascending solutions. The Handbook's classic localities are the Tri-State district around Joplin, Missouri; Cap Blanc-Nez in the Pas-de-Calais; and, in England, in the chalk of Kent and between Folkestone and Dover, and at Tavistock in Devon. Nodules out of chalk or shale are marcasite far more often than pyrite.
Density is the last resort and it is marginal. 5.018 against 4.887 is 2.7%, and the method on how to measure specific gravity at home gives ±0.05 as a realistic home accuracy — about 1% at this density. It is therefore just possible with a clean single-species fragment and a 0.01 g balance, and impossible with a nodule in matrix.
What marcasite jewellery is, and what we can and cannot say about it
The commonest reason people ask this question is not a specimen in a drawer; it is a piece of silver jewellery set with small faceted grey stones and sold as marcasite.
Two things about that are checkable. First, marcasite as a species does not occur in the equant, robust crystals that faceting and setting require — the Handbook gives its habit as tabular with curved faces, capillary, stalactitic, reniform and fine-granular massive, with cockscomb and spearhead twin groups. Pyrite's cubes and pyritohedra are exactly what a lapidary wants. Second, marcasite is the less stable of the two: it forms under low-temperature acidic conditions and is the species most associated with decay in museum collections.
The trade name is therefore describing a look, not a determination. The stones in marcasite jewellery are generally pyrite, and the name survives from a period when both species were called by the same word — the Handbook notes that marcasite is a word of Arabic or Moorish origin early applied to pyrite and other substances.
What we deliberately do not do here is cite a regulation, and it is worth saying what we checked. The United States FTC Guides for the Jewelry, Precious Metals, and Pewter Industries are the most detailed published rules on gem and metal naming in the English-speaking world: the current text runs to eight separate misuse of the word sections and mentions pearls 71 times and diamonds 47 times. It does not mention marcasite once. We found no UK equivalent that does either, so this page states the mineralogy and stops rather than inventing a legal position. The IMA-CNMNC Master List lists marcasite and pyrite as two valid species with the same formula; that is the part that is settled.
Nothing on this site is offered for sale and there is no catalogue here — we do not deal in jewellery at all. The rest of this section is indexed at collecting guides, and the only commercial route is the wanted list.
Why the distinction matters for keeping the specimen
If the question were purely taxonomic it would be a curiosity. It is not, because the two species behave differently on a shelf.
Marcasite is the classic decaying sulphide. Its formation environment — low temperature, high acidity, sedimentary — is the same environment that produces pyritised fossils, and pyritised fossils are the standard illustration of specimen decay. The Care and Conservation of Geological Specimens leaflet puts the mechanism generally: above 60% relative humidity iron sulphides react with atmospheric water to form hydrated iron sulphate and sulphur trioxide, producing a weak sulphuric acid.
So a cockscomb group out of a shale wants the same treatment a decaying pyrite does, and it wants it sooner: 40 to 45% relative humidity, its own sealed box, no salt, no acid, no water. The method is on how to clean pyrite safely, and the diagnosis of decay already under way is on why is my pyrite crumbling.
One more practical consequence. If a drawer contains an unlabelled FeS2 that you cannot place, treat it as marcasite. The precautions are identical and the cost of being wrong runs only one way.