Is arsenopyrite dangerous?
Arsenopyrite is FeAsS — 46.01% arsenic by mass. It is stable on a shelf and the exposure route is preparation: sawing, grinding and brushing, not display.
Arsenopyrite is 46.01% arsenic by mass, calculated from FeAsS at Fe 55.845, As 74.9216 and S 32.06. A sound crystal in a box is inert and safe to keep. The hazard is dust, and the route to dust is preparation — sawing, grinding, dry brushing and breaking — not the specimen sitting on a shelf.
In short
- The mass fraction is 46.01% arsenic. FeAsS has a formula mass of 162.827; arsenic contributes 74.9216 of it. For comparison, cobaltite, CoAsS, works out at 45.16% — the two arsenides a collector is likeliest to own are within one percentage point of each other.
- Display is not the exposure route; preparation is. Arsenopyrite is a hard, dense, stable sulpharsenide. It does not outgas, it is not soluble in water and it does not shed material on its own. Cutting, grinding, tumbling and vigorous dry brushing all make respirable dust, and that is a different proposition.
- The garlic smell is real, and it is the reason to stop rather than to continue. Striking or heating arsenopyrite liberates arsenic compounds, which is exactly why you can smell them. A test whose positive result is inhalation is a test worth retiring.
- Hardness 5.5 to 6 and density 6.07 make it hard to mistake for much. Silver-white to steel-grey with a metallic lustre, monoclinic pseudo-orthorhombic, flat tabular to prismatic crystals to 30 cm, striated parallel to [001], commonly twinned into cruciform or star-shaped trillings.
- Nothing on this page is health, safety or legal advice. It states one calculated number and the published sources for the rest. Exposure limits are in HSE guidance note EH40 on workplace exposure limits; duties at work are set by the Control of Substances Hazardous to Health Regulations 2002.
| Species | Formula | Arsenic by mass | Hardness | Stability on a shelf |
|---|---|---|---|---|
| Arsenopyrite | FeAsS | 46.01% | 5.5 to 6 | Stable; the commonest arsenic mineral there is |
| Cobaltite | CoAsS | 45.16% | 5.5 | Stable |
| Realgar | AsS | About 70% | 1.5 to 2 | Unstable in light — it alters and disintegrates |
| Olivenite | Cu2(AsO4)(OH) | About 25% | 3 | Stable; an arsenate, not an arsenide |
| Pharmacosiderite | KFe3+4(AsO4)3(OH)4·6-7H2O | About 26% | 2.5 | Stable, but variably hydrated |
| Mimetite | Pb5(AsO4)3Cl | About 15% | 3.5 to 4 | Stable; also a lead mineral |
The calculation, and what a mass fraction does and does not tell you
Arsenopyrite is FeAsS. Iron 55.845, arsenic 74.9216, sulphur 32.06, giving a formula mass of 162.827. Arsenic is therefore 74.9216 / 162.827 = 46.01% by mass. Iron is 34.30% and sulphur 19.69%.
That is a large number, and it is worth being precise about what it means. It tells you what the crystal is made of. It tells you nothing about what leaves the crystal. Arsenopyrite is a hard, dense, insoluble sulpharsenide with a hardness of 5.5 to 6 and a density of 6.07. Arsenic locked in a sulpharsenide lattice is not arsenic in the air.
The same distinction was drawn on is autunite dangerous, where the figure was 49.2% uranium by mass and the conclusion was likewise that the composition sets the ceiling and the physical form sets the actual risk. A mass fraction is an upper bound on what a specimen could release if it were entirely converted to dust. It is not an estimate of what it does release, which for an intact crystal in a box is nothing measurable.
Run the same arithmetic on cobaltite, CoAsS: 58.9332 + 74.9216 + 32.06 = 165.915, so arsenic is 45.16%. The cobaltite page sets out the arsenide handling position in general; this page is about arsenopyrite specifically, which matters because arsenopyrite is, in the Handbook's words, the most abundant and widespread arsenic mineral. It is the one you will actually meet.
The garlic test, and why it is the wrong test
Every older mineralogy text gives the same field test for arsenopyrite: strike it with a hammer and smell the garlic. It works. That is the problem.
The smell is produced because striking the mineral liberates volatile arsenic compounds. A positive result on that test is, by construction, a lungful of the thing being tested for. It also produces fresh dust and a fractured specimen, which are the two states in which an inert mineral stops being inert.
There is no need for it. Arsenopyrite is identifiable without destroying anything: silver-white to steel-gray metallic, hardness 5.5 to 6 — it will scratch glass and resist a steel point — density 6.07, distinct cleavage on {101} with {010} in traces, and a habit of flat tabular to prismatic crystals striated parallel to [001], commonly twinned on {100} and {001} and forming star-shaped trillings or cruciform twins on {012}. Its associates are pyrrhotite, pyrite, chalcopyrite, galena, gold, scheelite and cassiterite.
If the density and the crystal shape leave you unsure, the answer is not to hit it. The non-destructive alternatives are on how to identify a mineral specimen.
What actually creates risk, in the order it matters
1. Preparation. Sawing, grinding, tumbling, wire-brushing and drilling make respirable dust from a mineral that is 46% arsenic. This is the one that matters, and it is entirely avoidable: do not prepare arsenopyrite by any method that makes dust.
2. Breaking. Trimming a specimen makes chips and fine material. Do it wet if at all, collect everything, and do not sweep dry.
3. Hands to mouth. The ordinary route for any toxic mineral. Handle it, then wash your hands; do not eat, drink or smoke around it; keep it out of kitchens. This applies to the whole arsenate and arsenide suite — see toxic minerals in a collection.
4. Deterioration. Arsenopyrite is a sulphide-family mineral and can oxidise under damp conditions, producing secondary arsenates and a friable surface. A specimen that is powdering is one that can shed material, so keep it at the same humidity you would keep a pyrite: 45% RH or lower, per the Care and Conservation of Geological Specimens leaflet. The method is on how to clean pyrite safely.
What does not create meaningful risk: a sound crystal in a closed box on a shelf; looking at it; photographing it; picking it up and putting it down again.
Where this page stops, and who to ask instead
This page states one number it calculated and reports what published sources say. It gives no exposure figure, no dose, no threshold and no disposal instruction, because those are not a mineral reference site's to give.
For workplace exposure limits, the published UK source is HSE guidance note EH40 on workplace exposure limits. For duties around substances hazardous to health, the Control of Substances Hazardous to Health Regulations 2002. For a specimen in an institutional or teaching collection, a conservator — the Care and Conservation of Geological Specimens leaflet exists for exactly that. For disposal, your local authority's hazardous waste route, not the household bin.
Two things this site will not do. It will not tell you a quantity of arsenopyrite that is safe to own, because that depends on form, handling and setting rather than on mass. And it will not tell you the specimen is or is not worth keeping — we hold no stock, quote no prices and there is no catalogue here; the only commercial route on this site is the wanted list, and the rest of this section is indexed at collecting guides.
The honest summary is short. Arsenopyrite is 46% arsenic, it is stable, and the collector who never grinds it and always washes their hands has addressed essentially all of the risk there is.