Marcasite
Marcasite is FeS2, hardness 6 to 6.5, density 4.887, dimorphous with pyrite. Hardness and streak do not separate the two species — only habit and twinning do.
Marcasite is iron sulphide, FeS2, orthorhombic and dimorphous with pyrite, hardness 6 to 6.5 with a measured density of 4.887. Fresh surfaces are tin-white, darkening on exposure to a pale bronze-yellow. Cockscomb and spearhead groups come from repeated twinning on {101}. It forms under low-temperature, highly acidic conditions.
In short
- Hardness does not separate marcasite from pyrite. Both are 6 to 6.5. Neither does the streak, which is greyish to brownish black on marcasite and greenish to brownish black on pyrite. The density difference — 4.887 against 5.018 — is under 3% and is not judgeable in the hand.
- Habit is the whole test. Marcasite is orthorhombic and makes tabular crystals, cockscomb ridges and spearhead twins on {101}, plus stalactitic and reniform masses. Pyrite is cubic and makes cubes, pyritohedra and octahedra. A cube is pyrite; a cockscomb is marcasite.
- Fresh marcasite is tin-white, fresh pyrite is pale brass-yellow. This works only on a genuinely fresh surface, because marcasite darkens on exposure to a bronze-yellow that looks a great deal like pyrite.
- It forms in exactly the settings where sulphide decay is worst — shales, limestones and low-rank coals — and conservation guidance puts the threshold for iron sulphide deterioration above 60% relative humidity.
- Marcasite jewellery is not marcasite. The faceted stones set in Victorian and Art Deco silver are pyrite. The name has been applied loosely since long before the two species were distinguished.
| Test | Marcasite | Pyrite | Does it separate them? |
|---|---|---|---|
| Hardness | 6–6.5 | 6–6.5 | No. Identical |
| Streak | Greyish or brownish black | Greenish black to brownish black | No. Not reliably distinguishable |
| Density | 4.887 | 5.018 | No. A 2.6% difference, below what a hand can judge |
| Crystal system | Orthorhombic | Cubic | Yes, if crystals are present |
| Habit | Tabular on {010}, cockscomb, spearhead, stalactitic, reniform | Cubes, pyritohedra, octahedra | Yes. This is the working test |
| Colour on a fresh break | Tin-white | Pale brass-yellow | Yes, but only on a genuinely fresh surface |
| Cleavage | {101} rather distinct | Indistinct {001}, partings on {011} and {111} | Sometimes, under a loupe |
Why the standard tests fail here
A collector handed an unlabelled brassy sulphide reaches for hardness, streak and heft, and on this pair all three are wasted effort. Marcasite and pyrite are both FeS2 at hardness 6 to 6.5. The streaks overlap. The densities, 4.887 and 5.018, differ by 2.6%, which is less than the variation introduced by a bit of quartz matrix.
The separation is structural: marcasite is orthorhombic and pyrite is cubic, and that difference is expressed in the crystals. Marcasite makes tabular crystals on {010}, with curved faces common, and its repeated twinning on {101} builds the cockscomb ridges and spearhead groups that give the species away instantly. Pyrite makes cubes, pyritohedra and octahedra. Massive, granular or botryoidal material with no crystal form on it cannot be determined by eye at all, and an honest label says so.
There is one more usable clue: cleavage. Marcasite has a rather distinct cleavage on {101}; pyrite’s is indistinct. Under a loupe on a broken edge, a flat reflective plane on a brassy sulphide argues for marcasite. See cleavage, parting or fracture for how to read that without over-interpreting it.
Marcasite, pyrite and the decay problem
Iron sulphide decay — the white, yellow or green efflorescence, the smell, and eventually the collapse of the specimen into powder — is a real risk to a collection, and marcasite is the species most collectors associate with it.
Here is what can be said accurately. The chemistry is the same for both dimorphs: iron sulphide reacts with atmospheric water and oxygen to form hydrated iron sulphates and sulphuric acid. The NatSCA and Icon leaflet Care and Conservation of Geological Specimens sets the thresholds explicitly: above 60% relative humidity iron pyrite will react with atmospheric water, and it adds that other sulphide mineral species can also deteriorate via similar reactions at high relative humidities. Its storage recommendation is not above 60% RH and preferably 45% or lower.
What we are not going to assert is a clean rule that marcasite always decays faster than pyrite. What is documented is the setting: marcasite typically forms under low-temperature, highly acidic conditions in shales, limestones and low-rank coals, and that sedimentary material is where decay is most often reported. Fine-grained, framboidal and sedimentary iron sulphide of either species is the material at risk; coarse hydrothermal crystals of either species are not, particularly.
Our page on why pyrite crumbles covers the diagnosis and what can and cannot be done about it, and storing a collection in a damp house covers the humidity side.
Marcasite jewellery, and what is actually in it
The faceted, foiled-looking grey stones pavé-set in Victorian mourning jewellery and Art Deco silver are almost always pyrite, not marcasite. The reason is practical: marcasite is unstable enough that nobody would choose it for something worn against skin, and pyrite cuts and holds a polish.
The name carries the confusion honestly. The Handbook of Mineralogy records that marcasite is a word of Arabic or Moorish origin, early applied to pyrite and other substances — that is, the word was in use for pyrite long before the two species were separated in the nineteenth century. The trade kept the older usage; mineralogy moved on.
This matters when a jewellery piece arrives in a collection, or when an inherited box mixes the two. The label should record what it is, not what the trade called it. The three-state approach on labelling and cataloguing a collection handles exactly this case.
Where marcasite comes from
The classic American localities are the Tri-State district — Joplin in Jasper County, Missouri; Picher and Cardin in Ottawa County, Oklahoma; Baxter Springs and Galena in Cherokee County, Kansas — with more from Mineral Point and Racine in Wisconsin. In Europe: Vintířov, Most and Osek in the Czech Republic; Clausthal in the Harz; Freiberg and Annaberg in Saxony; Cap Blanc-Nez in the Pas-de-Calais.
Britain’s marcasite is chalk marcasite. The Handbook names the chalk at Kent and the coast between Folkestone and Dover, and also Tavistock in Devon. The nodular, radiating masses that weather out of the Kent and Sussex chalk are the material most British collectors meet first, and they are also the material most likely to disintegrate in a drawer, because they are fine-grained sedimentary sulphide of exactly the kind conservation guidance is written about.
Associations are galena, sphalerite, pyrrhotite, fluorite, dolomite and calcite. Our Derbyshire notes cover a British orefield where several of those occur together, and the wider species reference covers the sulphides. If you are looking for well-crystallised cockscomb material with a firm locality, the wanted list is the place to say so.