THE MINERAL CHALCOCITE
Chemistry: Cu2S, Copper Sulfide
- Uses: As a minor ore of copper and as mineral specimens.
Chalcocite is an important copper mineral ore.
It has been mined for centuries and is one of the most profitable copper ores.
The reasons for this is its high copper content (67% atomic ratio and nearly 80% by weight) and the ease at which copper can be separated from sulfur.
It is not however the primary ore of copper due to its scarcity.
Although the richest chalcocite deposits have probably been mined out, it is still being mined and will almost certainly always be mined in the future.
Chalcocite occurs as a secondary mineral in many ore bodies in a zone called the supergene enrichment zone.
Called a secondary enrichment mineral, although also a primary mineral as well, chalcocite commonly forms from the alteration of primary copper minerals that
are attacked above the water table by oxygen.
The oxygenated copper fluids descend to the water table where a reaction with primary ores results in the copper being reduced back to a sulfide,
most commonly chalcocite.
Ore bodies will have a layer of chalcocite which corresponds to the present or a past water table level and this layer is called a "chalcocite blanket".
The chalcocite blanket is richer in copper than the upper oxidized portion of the ore body and usually richer than the primary unaltered ores below.
The chalcocite blanket represents a real gold mine, or should that be copper mine, to the copper prospectors.
Fine crystals of chalcocite are quite uncommon and are much sought after.
The now depleted mines at Cornwall, England and Bristol, Connecticut produced the
most famous clusters of wonderfully formed chalcocite crystals.
Some new localities with well formed crystals are promising, but so
far the specimens from those old mines are the only good chalcocite
crystals available on the market.
The heavily striated pseudohexagonal tabular crystals are real classics
for the mineral collector and often command an equally classic price.
Since chalcocite is a secondary mineral that forms from the alteration of
other minerals, it has been known to form pseudomorphs of many different
A pseudomorph is a mineral that has replaced another mineral atom by atom,
but it leaves the original mineral's crystal shape intact.
Chalcocite has been known to form pseudomorphs of the minerals
Pseudo means false and morph means shape or form, thus pseudomorph means false shape since the mineral is chalcocite but the shape is that of a
covellite crystal, for example.
Color is dark gray to black.
Luster is metallic.
Transparency: Crystals are opaque.
Crystal System is orthorhombic; 2/m 2/m 2/m below 105 degrees celsius and hexagonal; 6/m 2/m 2/m above 105 degrees celsius.
Crystal Habits include pseudohexagonal tabular to prismatic crystals often with a shallow pyramidal truncation with a flat point.
Also found massive and compact.
Twinning is common and sometimes results in a six pointed star shaped cyclic twin called a trilling or in elbow shaped twins.
Cleavage is imperfect in two directions, prismatically.
Fracture is conchoidal.
Hardness is 2.5 - 3
Specific Gravity is approximately 5.5 - 5.8 (above average for metallic minerals)
Streak is shiny black to lead gray.
Other Characteristics: Crystals are usually deeply grooved
with lateral striations and a tarnish will tend to dull the luster of
crystals over time.
Associated Minerals are
pyrite and other
Notable Occurrences include Bristol, Connecticut, Butte, Montana, Morenci and Bisbee, Arizona and Bingham Canyon, Utah, Ducktown, Tennessee, USA; Cornwall, England; Tsumeb, Namibia; Tuscany, Italy and Rio Tinto, Spain.
Best Field Indicators are crystal habit, streak, striations, color and associations.