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The Mineral chalcocite

Classic Chalcocite from Bristol

Chalcocite is a coveted and iconic mineral among collectors. Specimens from classic and extinct localities, such as Cornwall, England and Bristol, Connecticut, will command extremely high prices, especially when in good crystals.


Chalcocite forms from the alteration of other minerals, especially other copper sulfides such as Bornite, Covellite, and Chalcopyrite, and may also form pseudomorphs after these minerals. It often forms in association with Chrysocolla, with the Chrysocolla forming as an alteration around the Chalcocite. Chalcocite is named after the Greek word chalcos, which means "copper" in Greek, in allusion to the copper content in this mineral.

Chemical Formula

Cu2S

Color

Gray to black, often with a bluish tinge. Sometimes iridescent with a bluish or purplish tinge.

Crystal System

Orthorhombic

Properties

Streak
Dark gray. Streak may be shiny.
Hardness
2.5 - 3
Transparency
Opaque
Specific Gravity
5.5 - 5.8
Luster
Metallic
Cleavage
3,1
Fracture
Conchoidal
Tenacity
Brittle
Other ID Marks
May develop a thin film layer on crystal faces.

Crystal Habits

Crystals are tabular, and may be pseudohexagonal in shape. They may be in dense, random aggregates of thin crystals or almost snowflake in nature. Also in bent twins of prismatic crystals and in sixlings. Elongated crystals and groups of elongated crystals do occur, but this is rare. Also occurs platy, grainy, and massive. Crystals are commonly striated.

3D Crystal Atlas

Additional Information

Composition
Copper sulfide
In Group
Sulfides; Simple Sulfides
Striking Features
Heaviness and crystal forms
Environment
Formed in copper deposits, usually as a secondary mineral in the oxidized zone, and often as an alteration of primary copper minerals in ore veins. Also found in volcanic basalt deposits.
Rock Type
Igneous, Metamorphic

Varieties

 -   Chalcocite coating or pseudomorph over Pyrite.
 -   Chalcocite from the Redruth area in Cornwall, England.

Uses

Chalcocite is an important copper ore. It has a high copper content and the process of extracting the copper from the sulfur in Chalcocite is relatively easily. Chalcocite is also an important collector's mineral, and good crystals command very high prices.

Noteworthy Localities

In Europe, a classic Chalcocite locality is Cornwall, England. Several well-known Cornish occurrences include the Carn Brea area; the Geevor Mine; the Levant Mine; and St Ives. Two African Chalcocite localities of note are Tsumeb, Namibia; and Shaba, Congo (Zaire). A relatively new Asian occurrence is the copper deposit of Dzhezkazgan, Kazakhstan. An odd new form of Chalcocite with iridescent root-like growths has been recently described from the Tongshan Mine, Nenjiang, China. Large, well-formed crystals come from the Mammoth Mine, Mount Isa, Queensland, Australia.

In the U.S., the best Chalcocite crystals once came from the old mine operations at Bristol, Hartford County, Connecticut. Another classic locality is Butte, Silver Bow Co., Montana. A new Chalcocite finding in the 1990's in the Flambeau Mine, Ladysmith, Rusk Co., Wisconsin, had yielded some excellent crystals. Other notable U.S. finds are the Santa Rita Mine, Grant Co., New Mexico; Bisbee, Cochise Co., Arizona; and the Chimney Rock Quarry, Bound Brook, Somerset Co., New Jersey.

Common Mineral Associations

Chrysocolla, Malachite, Bornite, Calcite, Galena, Quartz

Distingushing Similar Minerals

Galena - Different crystals and cleavage.
Acanthite - Is more sectile.


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