Tetrahedrite Identification Guide

What Tetrahedrite Looks Like

Tetrahedrite is a copper antimony sulfosalt (a copper-iron-zinc-silver antimony sulfide) and an important ore of copper and silver. It forms a series with arsenic-rich tennantite.

• Color: steel-gray to iron-black, sometimes with a dull bluish tarnish.
• Luster: metallic to submetallic (dulls on weathering).
• Transparency: opaque.
• Crystal habit: distinctive tetrahedral crystals (four-sided pyramids); also massive or granular.

Step-by-Step Field ID Checklist

1. Look for tetrahedra. Well-formed four-faced tetrahedral crystals are nearly diagnostic and give the mineral its name.
2. Note the gray-black metallic color.
3. Streak test. Streak is black to brownish-black to dark cherry-red (reddish in thin streak)—useful versus galena's gray-black.
4. Hardness. Mohs 3-4.5; scratched by a knife, harder than galena.
5. No good cleavage. Tetrahedrite lacks the perfect cubic cleavage of galena; it fractures unevenly/subconchoidally.
6. Associations. Found with chalcopyrite, galena, sphalerite, pyrite, and silver minerals in hydrothermal veins.

Key Diagnostic Tests

• Hardness: 3-4.5.
• Streak: black, brownish, to dark red.
• Cleavage: none/indistinct; brittle, uneven fracture (contrast galena's perfect cubic cleavage).
• Density: ~4.6-5.1.
• Magnetism: none.
• Acid: decomposed by nitric acid (not a quick field test).

Common Look-Alikes

• Galena: brighter silvery, much softer (2.5), with perfect cubic cleavage and cubic crystals, plus a lead-gray streak and higher density—tetrahedrite has no cubic cleavage and is harder.
• Sphalerite: resinous luster, lighter-colored streak (yellow-brown), and dodecahedral/tetrahedral cleavage; often translucent on edges.
• Chalcocite: dark gray, softer (2.5-3), and slightly sectile; lacks tetrahedral crystals.
• Bournonite/other sulfosalts: can resemble it; crystal form (tetrahedra) and reddish streak favor tetrahedrite.
• Magnetite: strongly magnetic—tetrahedrite is not.

Where It Is Found

Tetrahedrite occurs in hydrothermal sulfide veins and replacement deposits worldwide, often as a silver-bearing ore. Notable localities include the Harz (Germany), Cornwall (England), Peru, Bolivia, Romania, and many silver-mining districts in the western United States.