Pyrite

Pyrite is iron disulphide, FeS₂, and one of the most widespread minerals in the Earth's crust. It occurs in nearly every geological setting: hydrothermal veins, sedimentary rocks, coals, black shales, and in fossils it has replaced atom by atom.

Its name comes from the Greek pyr, fire: struck against flint it throws sparks. That is how fire was lit before steel, and it is also why it turns up on so many archaeological sites.

Its identity card

Property	Value
Composition	iron disulphide, FeS₂
Hardness	6 to 6.5
Density	5.0
Lustre	metallic
Colour	brass yellow, sometimes tarnished to iridescent brown
Streak	greenish black
Cleavage	very indistinct
Fracture	uneven to conchoidal, brittle
System	cubic

What gives it away at first glance

The striations on the faces. This is the most characteristic detail, and the most overlooked. The faces of a pyrite cube carry fine parallel grooves — and on two adjacent faces those striations run perpendicular to each other. No imitation, and no other yellow mineral, does that.

The geometric shapes. Pyrite crystallises as perfect cubes, as octahedra, or as pyritohedra — a solid with twelve pentagonal faces. Finding a metallic cube with right angles inside a rock is nearly a certainty on its own.

The brittleness. Pyrite is brittle. A blow chips it and breaks it into angular fragments. That is the exact opposite of gold.

Pyrite or gold: the three tests

Test	Pyrite	Gold
Hardness	6 to 6.5, scratches glass	2.5 to 3, scratched by a copper coin
Under a hammer	chips, shatters	flattens, cuts with a knife
Streak on porcelain	greenish black	bright yellow
Density	5.0	19.3
Face appearance	striated, geometric	smooth, rounded, without edges

The streak test is the quickest and the most decisive: one stroke on the back of a ceramic tile and the question is closed. The density difference is enormous too — gold is nearly four times heavier — which is obvious in the hand as soon as the samples are of comparable size, and it is what panning has always exploited.

The other confusions

Chalcopyrite. A copper-iron sulphide, more golden than pyrite, often covered in a blue-green iridescent tarnish. Markedly softer: hardness 3.5 to 4, a knife scratches it easily. It rarely forms good cubes and tends to occur as irregular masses.

Marcasite. Exactly the same chemical formula, a different crystal structure. Paler, greener, in needles or radiating nodules rather than cubes. It decays even faster than pyrite.

Yellow mica. In river sand, glittering flakes are almost always mica: they are flexible, they bend under a fingernail, and they float for a moment. Pyrite sinks at once.

Pyrite disease

This is the most important practical point for anyone keeping specimens, and the one nobody mentions before the sale.

With moisture and oxygen both present, pyrite oxidises. The iron and sulphur become sulphates, and sulphuric acid is released. The specimen grows a powdery whitish or yellowish coating, takes on a sulphurous smell, swells, then crumbles. In the end it reduces to dust.

The process is irreversible: it can be slowed, not reversed.

• Keep it dry. Air below 50% relative humidity is enough in most cases.
• Keep it apart. The acid attacks labels, cardboard boxes and neighbouring minerals — carbonates especially.
• Do not shut it in with paper. Cardboard and paper absorb moisture and give it back.
• Do not wash it. Water accelerates everything.

The most stable pieces are large, compact, well-formed crystals; the most fragile are finely crystallised pyrite, pyritised fossils, and nodules from black shales.

Where it comes from

Everywhere. In metal-bearing veins, in spoil heaps and mine dumps, in shales and marls, in nodules within clay cliffs, and as golden pyritised ammonites — magnificent, and particularly prone to the disease described above.