Chalcocite
Copper(I) sulfide (Cu2S)
A dark gray copper sulfide that is one of the richest copper ores, prized by collectors when found as rare sharp crystals.
- Mohs hardness
- 2.5-3
- Color
- dark lead-gray to blackish, often bluish tarnish
- Type
- mineral
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Overview
Chalcocite is a copper sulfide mineral (Cu2S) and one of the most important and copper-rich ores of the metal, containing nearly 80% copper by weight. Its name comes from the Greek word for copper.
It is usually found massive, dark lead-gray to sooty black, often with a bluish or greenish tarnish from associated copper minerals. Well-formed crystals are rare and highly prized by collectors, especially the famous pseudohexagonal twins from Cornwall.
Chalcocite is a key component of secondary copper enrichment zones in many of the world's great porphyry copper deposits.
Formation & geology
Chalcocite forms most importantly through supergene enrichment: as primary copper sulfides like chalcopyrite and bornite weather near the surface, dissolved copper migrates downward and reprecipitates as chalcocite in a zone below the water table, greatly enriching the ore.
It also occurs as a primary mineral in some low-temperature hydrothermal veins and sedimentary copper deposits.
Celebrated localities include Cornwall in England (notably Redruth, source of the name redruthite), Bristol in Connecticut, Butte in Montana, and numerous copper districts in Arizona, Chile, and elsewhere.
How to identify it
Look for a dark lead-gray to blackish mineral with a metallic luster that often dulls or develops a bluish-green tarnish. It is soft (2.5-3), so it can be scratched by a coin or copper, and it is somewhat sectile, shaving rather than powdering under a knife.
The streak is dark gray to blackish, sometimes shining. It is heavy due to high copper content.
Look-alikes include galena (cubic cleavage, heavier, lead streak), bornite (iridescent purple-blue tarnish), and acanthite. The combination of softness, dark gray color, sectility, and association with other copper minerals helps confirm chalcocite.
Uses & significance
Chalcocite is one of the most valuable copper ores, mined extensively for the metal that goes into wiring, plumbing, electronics, and alloys such as brass and bronze. Its high copper content makes enriched chalcocite zones especially profitable.
Well-crystallized specimens, particularly antique pieces from Cornwall and Connecticut, are among the most sought-after mineral collectibles and can command high prices.
It has no role in jewelry due to its softness and tarnishing. Metaphysical sellers occasionally associate it with communication and transformation, but these claims are not scientific; its real importance is industrial and to collectors.
Frequently asked questions
Why is chalcocite an important copper ore?
It contains nearly 80% copper by weight, far more than most other copper sulfides, making it one of the richest copper ores.
How do I tell chalcocite from galena?
Galena has perfect cubic cleavage, is denser, and gives a lead-gray streak, while chalcocite is sectile, lacks cubic cleavage, and is often associated with greenish copper staining.
What causes the bluish tarnish on chalcocite?
Surface alteration to other copper minerals such as covellite produces blue to greenish tarnish.
Where are the best chalcocite crystals found?
Classic sharp crystals come from Cornwall, England, and Bristol, Connecticut; fine specimens also occur in Montana and Arizona.
Is chalcocite the same as copper glance?
Yes, copper glance is an old name for chalcocite, referring to its metallic gray appearance.
Chalcocite guides
In-depth guides for identifying, valuing, and understanding Chalcocite.
