The Golden Iron Mineral
Rock&Gem Magazine|June 2021
PYRITE’S MANY CRYSTAL FORMS KEEP COLLECTORS FASCINATED
BOB JONES

From simple cubes to complex twinned dodecahedrons, pyrite appears in over a half dozen different forms. And it is certainly found in countless deposits all over the world. Pyrite is so varied and common any collector could devote an entire collecting career to this one species. Chemically pyrite is not complicated. One molecule of pyrite is composed of one atom of iron and two atoms of sulfur, iron disulfide.

What is complex about pyrite is the many crystal forms it can take and still remain in the isometric or cubic crystal system! Some pyrite crystals differ enough to be given odd names like iron cross and oscillating cubes, pyrite suns, pyrite bars and pyritized fossils. Yet cubic pyrite’s most common and very recognizable crystal form is pyritohedron. Instead of having a square crystal face of a cube as you would expect, pyritohedron faces are five-sided. Luckily today, thanks to one important deposit in Spain, the most common form for pyrite available is simply a cube on a white marl matrix.

COMMON CUBES

Many of the pyrite cubes you see on the market today are from the Ampliación a Victoria mine, Navajun, Spain, where pyrite crystals are the only ore. Typically pyrite is almost always found as a by-product during mining for other minerals. But this Spanish mine is unique in that it is worked exclusively for pyrite crystals. It produces simple cubes found in an amazing range of sizes — from a fraction of an inch on an edge to as much as six or more inches. Specimens for sale may have just one sharp, lustrous cube on the gray-white marl matrix. Or, you may see specimen clusters of a dozen or more stacked pyrite cubes of varying sizes attached to and penetrating each other at odd but pleasing angles. The cubes are always lustrous with mirror-smooth reflective faces, sharp edges, and unblemished. They do not look like that when mined. Though pyrite is the most common sulfide mineral in the earth’s crust, a deposit of just pyrite crystals is exceptional.

The deposit at Navajun, Spain, has a long history. The Romans mined these cubes and used them in artwork by embedding them in tile mosaics. It’s hard to believe the very tiny cubes were believed to have some medicinal value, and people would swallow the little crystals whole! Imagine how that would feel if the crystals did not dissolve.

The Spanish cubes are loosely embedded in the marl, consisting of a mixture of clay and calcium carbonate. The deposit formed 350 million years ago in what was mostly a freshwater deposit. The mud environment was rich in iron with some form of algae in the mud, which broke down the organic material releasing sulfur. Water descending through the mud picked up the iron and sulfur, which combined to form pyrite cubes in the soft matrix. This quiet environment allowed crystals to grow in size throughout the deposit. And therein is a problem.

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