This metallic mineral smells like garlic and acts as a hidden gold indicator

Nature · Published 3 October 2026 Swipe more like this →

In short: Arsenopyrite is a striking, steel-grey mineral that hides a dangerous secret. Beyond its metallic sheen and high density, it serves as a primary source of arsenic and a vital geological marker for gold. When struck, it releases a distinct garlic odor, revealing the volatile elements trapped within its complex structure.

Arsenopyrite, chemically defined as FeAsS, is a hard, opaque mineral that crystallizes in the monoclinic system. While older literature sometimes incorrectly labeled it as orthorhombic, modern analysis confirms its monoclinic structure, where each iron center links to three arsenic and three sulfur atoms. Its atomic connectivity shares more in common with marcasite than with the more common pyrite. Because its iron-arsenic and iron-sulfur bonds are highly covalent, the material functions as a semiconductor rather than a simple ionic solid.

The mineral is a significant source of arsenic, containing 46% of the element by mass. When heated, it releases both sulfur and arsenic vapors, and when dissolved in nitric acid, it yields elemental sulfur. Unlike iron pyrite, which is notorious for causing acid mine drainage, arsenopyrite is generally an acid-consuming sulfide. When exposed to the atmosphere, it undergoes a slow conversion process, eventually transforming into iron arsenates. Its physical properties, including a hardness of 5.5 to 6 on the Mohs scale and a high relative density of 6.1, make it relatively easy to identify in the field.

Geologically, arsenopyrite is found in high-temperature hydrothermal veins, pegmatites, and zones of contact metamorphism. Its presence is highly valued by prospectors because it often associates with gold-bearing reefs. However, extracting this gold presents a challenge; many arsenopyrite-hosted ores are considered refractory, meaning the gold is not easily recovered through standard cyanide leaching processes. Historically, the mineral has been known by various names, including mispickel, of German origin, and mundic, a term from the Cornish dialect that also describes certain types of concrete deterioration.

The arsenopyrite group is diverse, with various transition metals capable of substituting for iron. Rare minerals within this group include glaucodot and alloclasite, which incorporate cobalt, as well as osarsite and ruarsite, which feature osmium and ruthenium. This chemical flexibility highlights the complex nature of this sulfide mineral, which remains both a critical industrial source of arsenic and a persistent puzzle for gold mining operations worldwide.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

Intellect Plus is a visual educational discovery platform for exploring fascinating facts and ideas. Brain Food, Not Brain Rot. What is Intellect Plus?

Related

• Arsine is a lethal, invisible gas that once revolutionized forensic science and criminal investigations.
• Arsenate minerals are far more than just toxic rocks hidden in the earth
• Fool's gold is a sophisticated semiconductor with a history of sparking human innovation
• Sulfide minerals are the hidden inorganic foundation of our global metal supply chain
• The hunt for rare blue opal beneath a desert floor
• Azurite is the deep blue mineral that once defined the color of ancient history.
• The Arabian Plate is a restless giant reshaping the map of the Middle East
• Life on the edge of a volcanic landscape
• Apatite is the deceptive mineral that builds your bones and hides water on the Moon.
• Why do deep-sea basins form behind volcanic arcs instead of at mid-ocean ridges?
• Why this radioactive mineral from Bavaria releases a pungent, ozone-rich scent when crushed
• These ancient, striped rocks hold the secret to Earth's oxygen-rich atmosphere

More in Geology · More in Nature · Latest · A–Z · All topics