Arsenopyrite

FeAsS

Mineral species Serious None Apy

Also known as

mispickel

Crystal system

monoclinic

Hardness

5.5–6 on the Mohs scale

Lustre

metallic

Colour

Silver-white, tarnishing to grey

Where it forms

In high-temperature hydrothermal veins, very often with gold.

Chiefly from

Cornwall, EnglandBoliviaOntario, CanadaPortugal

Why it is that colour

Metallic lustre; the colour is that of the iron arsenic sulfide.

Safety

Iron arsenic sulfide. Harder and less friable than realgar or orpiment, so it sheds less, but it is still arsenic and it should be handled with the same caution — not powdered, not heated, not put in water, hands washed. Struck with steel it smells of garlic, which is arsenic being released, and that is a reason not to do it.

Where it comes from

A common gangue mineral in gold deposits. Arsenic released by processing arsenopyrite-bearing gold ore is a major source of contamination at mine sites worldwide, and the Giant Mine in Canada holds a quarter of a million tonnes of arsenic trioxide dust underground that will require management indefinitely.

Historical use

Arsenopyrite is the reason a great deal of gold is difficult to extract. Gold locked inside its structure is called refractory and needs roasting or bio-oxidation to release, and roasting arsenopyrite produces arsenic trioxide.

Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.

Worth knowing

The Giant Mine at Yellowknife is one of the more sobering pieces of mining legacy anywhere. Roasting arsenopyrite ore to free the gold produced arsenic trioxide dust, and 237,000 tonnes of it were pumped into underground chambers. It is water-soluble and acutely toxic. The current plan is to freeze the ground around it permanently and monitor it, and the remediation is expected to require active management for centuries.

The mine produced gold for fifty years. The arsenic will need looking after for considerably longer than that.

Identity checked against IMA-CNMNC master list, June 2025.