Why did my pyrite break
A bright, curved break on clean pyrite is a knock: pyrite is brittle, hardness 6 to 6.5, with no good cleavage. Powder, smell and cracks from inside mean decay.
Why did my pyrite break? Read the broken surface. Pyrite, FeS2, hardness 6 to 6.5, is brittle with indistinct cleavage, so a knock leaves a bright, curved, conchoidal-to-uneven surface. Powder, a rotten-egg smell or cracks opening from inside mean oxidation, which BGS records destroying 56 of 65 pyritic specimens in one tray.
In short
- The fresh-face rule: a clean, bright, brass-yellow break with no powder is mechanical; a dusty break with efflorescence is chemical. The Handbook of Mineralogy sheet records pyrite as brittle, with cleavage Indistinct on {001} and fracture Conchoidal to uneven, so a knocked crystal breaks across itself on a curved surface rather than splitting on a flat plane.
- Decay announces itself in three ways. The Natural Sciences Collections Association lists white, yellow or green crystalline efflorescence, a rotten egg smell and the occasional collapse of specimens. A break with none of these on a solid crystal is, in our judgement, a knock.
- The kind of pyrite predicts which. A British Geological Survey report summarises Howie's finding that oxidation in collections generally occurs only at 60% relative humidity or more, and only if the pyrite is microcrystalline or framboidal. The pyrite sheet lists framboidal among its common habits, beside sharp cubes to 25 cm or more.
- Why decay splits a specimen, worked. BGS says the reaction entails a large solid volume increase and that specimens can be destroyed by the stresses. Worked from the sheets' densities, melanterite, the first product Howie names, takes 6.1 times the room of the pyrite it replaces, per iron atom.
- Storage numbers are on the crumbling page. Humidity bands, isolation and what not to do are set out on why is my pyrite crumbling and are not repeated. A broken specimen worth keeping is a conservator's question. This is not health, safety or legal advice.
| What you see | Likely cause | The number or record behind it | Check that separates it | Next step |
|---|---|---|---|---|
| Bright brass-yellow surface, curved or uneven, on a solid crystal; no powder | A knock or drop: mechanical fracture | Brittle; fracture conchoidal to uneven; hardness 6–6.5 (Handbook) | Lens: fresh metallic lustre, no efflorescence, no smell | Keep the pieces; see the repair page |
| A crystal lifted off cleanly at its base, matrix intact | The contact with the matrix failed, not the pyrite | The sheet records no good cleavage to explain a flat break | Is the underside a growth contact rather than a fracture? | Record it; repairs must be disclosed |
| Shale or mudstone matrix flaking or splitting, pyrite intact | The matrix, not the pyrite | NatSCA: shale samples can dry out and become brittle in low RH | Does the crack follow the rock's bedding? | A matrix question; ask a conservator before treating |
| White, yellow or green crust; rotten egg smell; cracks opening from inside | Oxidation (pyrite decay) | NatSCA signs; BGS: large solid volume increase; melanterite 6.1 times the volume per iron, worked | Powder in the crack; acid-browned label or tray | Isolate it today; read the crumbling page |
| Fine-grained, dull, nodular or fossil pyrite falling apart | Oxidation of microcrystalline or framboidal pyrite | Howie, via BGS: oxidation generally only at 60% RH or more, and only in that kind of pyrite | Habit: granular, nodular or replacing a fossil, not sharp crystals | As above; BGS records heavy loss in a tray that read close to 40% RH over a week |
| Tin-white or pale bronze on the fresh break, cockscomb or spear habit | Not pyrite: marcasite | Marcasite sheet: tin-white on fresh surface; cleavage {101}, rather distinct | Colour of a fresh break; habit | See is marcasite the same as pyrite |
| Beads split on a bracelet, or a bracelet came apart | A bead: one of the causes above. Apart at the cord: the stringing, not the mineral | As above | Where did it part: through a bead, or between beads? | Treat a split bead like any pyrite |
Why did my pyrite break? Read the broken surface first
The fresh-face rule: if the new surface is bright, brass-yellow, metallic and clean, the break was mechanical; if it carries powder or crust, smells, or keeps opening, it is chemical. Everything else on this page follows from that one look under a 10x lens and a good lamp. The Handbook of Mineralogy sheet for pyrite gives the facts the rule rests on: pale brass-yellow, tarnishing darker and iridescent; lustre metallic, splendent; tenacity brittle; hardness 6–6.5.
Pyrite does not cleave the way fluorite and galena do. Its sheet records cleavage Indistinct on {001}; partings on {011} and {111}, indistinct, and fracture Conchoidal to uneven; the indices were read from the rendered sheet. So a knocked pyrite crystal breaks across itself on a curved or irregular surface. So a flat, mirror-like surface on a pyrite cube is unlikely to be a cleavage break; it may be a growth face, a contact with matrix, or one of the indistinct partings. Compare why did my fluorite break, where the answer is a perfect cleavage, and cleavage or fracture for the general distinction.
Hardness does not prevent breakage. At 6–6.5, with a Vickers hardness of 1505–1520 on the sheet, pyrite resists scratching well; brittleness is a separate property, and the sheet records both. The order of causes in the table is our judgement of what is common, not a count. The damage and conservation pages are indexed at the collecting guides.
Is it pyrite decay? The signs NatSCA and the Field Museum list
Decay leaves products you can see and smell. The Natural Sciences Collections Association leaflet Care and Conservation of Geological Specimens lists white, yellow or green crystalline efflorescence, a rotten egg smell and the occasional collapse of specimens, and says that above 60% relative humidity iron pyrite reacts with atmospheric water to form hydrated iron sulphate and sulphur trioxide, which can form a weak sulphuric acid. It notes that reconstruction of the original specimen may not be possible.
The Field Museum poster gives the reaction. Shinya and Bergwall write that pyrite oxidation, also called pyrite disease, rot or decay, is caused and accelerated by the presence of oxygen and water, and print it as 4FeS2 + 13O2 + 2H2O giving 4FeSO4 + 2H2SO4 + 2SO2: ferrous sulphate, sulphuric acid and sulphur dioxide. They list the signs as a sulphuric acid odour, white crystalline powder, yellow sulphide powder and a gray to yellowish microcrystalline mass in and out of specimens.
Which pyrite is at risk. The British Geological Survey Internal Report IR/04/037 (Hodgkinson and Martin) summarises Howie's work: pyrite oxidation in collections generally occurs only at a relative humidity of at least 60%, and only if the pyrite is microcrystalline or framboidal, a guideline it calls still generally accepted. The pyrite sheet's own habit line separates the two kinds: cubes, pyritohedra and octahedra to 25 cm or more, and Commonly granular, globular, framboidal, stalactitic. A crumbling nodule or pyritised fossil and a broken sharp cube are different problems.
Why decay breaks pyrite apart: the volume arithmetic
The products of decay need far more room than pyrite, so they push the specimen apart from inside. BGS states the mechanism directly: the reaction entails a large solid volume increase and specimens can be destroyed by the stresses that this produces. Howie, as BGS summarises him, finds that collection decay tends to produce melanterite first, then copiapite, fibroferrite and other hydrates.
Worked, from the Handbook sheets. Volume per mole is formula mass divided by density. Pyrite, FeS2, 119.965 g/mol at 5.018 g/cm³, takes 23.91 cm³ per mole of iron. Melanterite, FeSO4·7H2O, 278.006 g/mol at the Handbook of Mineralogy sheet for melanterite's 1.895–1.898, takes 146.5 to 146.7 cm³: 6.1 times the volume, per iron atom. Römerite, the main product BGS identified in its tray, works out at 5.2 times (D 2.174, three iron atoms per formula), and coquimbite at 5.6 times (D 2.11). Standard atomic weights; the water and oxygen come from the air.
What the arithmetic does not say: how fast, or at what humidity a given specimen will fail. BGS reports Morth and Smith's calculation that above 31% RH the oxidation rate doubles for every 26% increase in RH, so on that rule a rise from 45% to 71% doubles the rate. It is a published estimate, not a threshold, and this page draws no storage recipe from it; the humidity guidance is on why is my pyrite crumbling.
A documented failure: 56 of 65 specimens lost in one BGS tray
The British Geological Survey found a tray of pyritic fossils in which 56 of 65 specimens had undergone total destruction and been replaced by a large volume of grey dust; the other nine were extensively damaged. That is 86.2% lost outright. The specimens, mainly goniatites from the Westphalian Coal Measures at Stubben Edge Hall, Derbyshire, came from a single highly pyritic unit; specimens from less pyritic units stored nearby had not degraded.
The humidity reading is the uncomfortable part. A week of monitoring inside an adjacent tray read generally close to 40% RH, although 25% to 75% was recorded elsewhere in the store. BGS concludes that catastrophic damage can happen even when RH can be kept close to 40%, while cautioning that a week in each location is not an annual record. Its review also records lacquered specimens that were observed to explode as decay products built up under the coating.
What it means for a broken pyrite at home: a decayed specimen from a pyritic shale or a fossil bed is not evidence that its keeper did something wrong, and a sharp crystal is not the same risk. Separating pyrite from its dimorph is on is marcasite the same as pyrite; the species is on pyrite. Keep every fragment and the label; disclosure of a repair is on how to tell if a specimen is repaired. Nothing on this site is offered for sale; the wanted list is the only commercial route.