COPPER

Copper – Cu – is a relatively common native element, which was locally a very important copper ore, as in Keeweenaw (Michigan, USA). Aggregates with crystalline forms reach prices in the market of collect minerals much higher than the price of the contained metal.

Copper typically forms dendritic aggregates, lamellae, plates, vanes, wires and nuggets. Crystals are rare and present much distorted forms, difficult to interpret: cubes, dodecahedrons and tetrahexahedrons, more rarely octahedrons. The largest crystal ever found was just 4.4 x 3.2 x 3.2 cm, but copper can form masses hundreds of tons in weight. The color in fresh fracture or cut is a soft pink, which fades very quickly to typical “copper red”, later to dull and dark brown. Weathered surfaces turn green due to the formation of secondary minerals.

Copper normally contains traces of Ag in solid solution. There are five varieties: with Au, with Ni (up to 10% or more), with Pd (2-5%), with Sn (up to 35%, can have Pb) and with As (up to 6.6%). It can form twins by the Spinel Law on {111}, contact twins or interpenetration twins and cyclic groups.

1. Characteristics

Crystal system: Cubic hexatetrahedral.          

Color: Fresh surface: soft pink, then tarnishes to “copper red” or for colored films. 

Habit: Distorted crystals, plates, dendritic, fibrous, massive, earthy.

Cleavage: No.       

Tenacity: Malleable, ductile.        

Twinning: See above.       

Fracture:  Irregular.      

Mohs Hardness: 2.5 – 3

Parting: No.         

Streak: Copper red.        

Lustre: Strong metallic.         

Diaphaneity:  Opaque.          

Density (g/cm³):  8.9

          

2. Geology and Deposits

Copper often occurs in very small grains in intrusive and extrusive basic rocks such as gabbros, diabases, and basalts. It can occur in the same rocks as a secondary mineral in vesicles, associated with zeolites, silica (quartz, chalcedony), calcite and others. It is also found as cement in sedimentary rocks (conglomerates, sandstones and shales), where it is of hydrothermal origin.

Very common is its presence at the boundary between the oxidation and cementation zones of hydrothermal veins or other copper sulphide deposits, by the oxidation of chalcocite and bornite.

It rarely occurs in meteorites and marbles. Detrital copper is occasionally found in the sediment load of water courses, in the form of dendrites or nuggets.

 

3. Mineral Associations

Copper occur with primary copper minerals such as chalcocite, bornite and chalcopyrite. It typically occurs with secondary Cu minerals such as covellite, cuprite, malachite, azurite, tenorite, chrysocolla, delafosite and many others. In these oxidation zones, native copper is also associated with limonite, pyrite, native silver, prehnite, aragonite, epidote, gypsum, iron oxides, calcite, quartz and many others.

 

4. Transmitted Light Microscopy

Not applicable, as copper is completely opaque.

5. Reflected Light Microscopy

Sample preparation: Due to its low hardness, polishing is quite simple, but polishing scratches are inevitable. It is slightly more difficult to polish than chalcopyrite and chalcocite. As it tarnishes in a matter of a few hours, the sections need to be repolished before re-observing.

PLANE POLARIZED LIGHT – PPL

Reflection color: Bright full pink that fades after a few hours to violet and red-brown. This blurring rarely reveals structures.       

Pleochroism: No.      

Reflectivity: 92.2% (very high, blinds the eye and is very diagnostic. Reminds of the reflectivity of silver.)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Isotropic, but always simulates anisotropy due to the polishing grooves, which are more visible in XPL.       

Internal reflections: No.      

May be confused with: few other minerals. Even with a low quality polish it is easy to recognize native copper due to the high reflectivity and typical color.

Very small grains of chalcopyrite and native gold are more yellow and less red.

Small, tarnished copper grains can be confused with native bismuth and tarnished native silver, but the paragenesis of these is different.  

General Characteristics: 

Unmixing and zoning are not uncommon, but observed only after appropriate chemical attack.

Lamellar twins are always visible after chemical attack.

Replacement of cuprite and chalcocite by native copper can occur, as well as native copper can be replaced by cuprite in oxidation zones.       

Inclusions of copper, very small, can occur in enargite, bornite, cuprite, chalcocite, pyrrhotite and pentlandite.

Rhythmic structures of copper with limonite (goethite) may occur in some cases, but are visible only after chemical attack.

Copper 01: In PPL, tarnished copper. In bluish, cuprite and, in dark gray, gangue minerals and secondary minerals derived from the alteration of copper, such as chrysocolla, malachite and others.
Copper 01: The same section as of the previous image, again in XPL and now repolished. This is the correct reflection color of copper, before tarnishing.
Copper 02: In PPL, copper (bright pinkish), cuprite (bluish) and other secondary copper minerals (dark gray). In PPL, the very high reflectivity of copper is very characteristic. It is so intense that it blinds the eye; it is necessary to diminish the intensity of incident light. In addition, there are always polishing scratches and association with other copper minerals – it is an easily identifiable mineral.
Copper 02: The same section as of the previous image, now in XPL. Copper is isotropic. Large polishing scratches simulate anisotropy. It has no internal reflections.
Native copper grains in basic intrusive rock (gabbro) in PPL. The grains are usually very small. Left, in light gray, plagioclase. On the right, in light gray and with fractures, olivine. In the center, in dark gray, clinopyroxene. In black, clay minerals.
In PPL, pipe-vesicle filling in a pahoehoe-type basaltic lava flow with zeolites, clays (“argilas”), and tiny grains of native copper (amid the zeolites, to the left of the word “Argilas” = clays).