DELAFOSSITE

Delafossite – CuFeO2 -is a very rare oxide, characteristic as a secondary mineral in some oxidized Cu ores. It does not constitute an ore.

It is classified in the Delafossite Group. There is a variety with a mixture of clay minerals. It is weakly magnetic.

1. Characteristics

Crystal system: Trigonal scalenohedral.          

Color: Black.     

Habit: Crystals can be tabular (up to 8 mm), forming spherulites (spherical forms composed of many small tabular crystals) or pseudohexagonal aggregates (like hematite roses). It also forms botryoidal aggregates with a concentric structure. It can be powdery or massive.       

Cleavage: {10-11} poor.      

Tenacity: Brittle.        

Twinning: Contact twins on {0001}.       

Fracture: No information available.       

Mohs Hardness: 5.5

Parting: No.         

Streak: Black.         

Lustre: Metallic.          

Diaphaneity: Opaque.           

Density (g/cm³): 5.41

           

2. Geology and Deposits

Delafossite is typically found as a secondary mineral near the base of oxidation zones in some copper ores.

It can also occur as a primary mineral.

 

3. Mineral Associations

It occurs with minerals typical of oxidation zones, such as quartz, hematite, goethite (limonite), carbonates (calcite, siderite), and kaolinite.

It is evidently associated with other Cu minerals, such as primary Cu minerals (bornite, chalcocite, tetrahedrite-tennantite, djurleite) and secondary Cu minerals (native copper, cuprite, covellite, azurite, malachite, tenorite, and chrysocolla).

Also with pyrite, marcasite, galena, native gold, turquoise, and chlorite.

 

4. Transmitted Light Microscopy

This does not apply, as delafosite is completely opaque.

The literature states that delafossite is uniaxial.

5. Reflected Light Microscopy

Sample preparation: Delafossite polishes easily. Its hardness upon polishing is medium, greater than that of chalcocite and calcite, but less than that of cuprite and goethite.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Pinkish-brown to creamy-brown, also pinkish-white.       

Pleochroism: No.      

Reflectivity: 19,25 – 23%        

Bireflectance:  Distinct to faint, between light golden-brown (or yellowish pinkish-brown) and darker pinkish-brown (or creamy brown).      

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Moderate to strong anisotropy in bluish-gray to brownish hues, typically with green tones. Basal sections are isotropic!        

Internal reflections: No.      

May be confused with: Several other minerals.

Tenorite is very similar, but tenorite has a more distinct yellowish hue, higher reflectivity, and stronger anisotropy. Paragenesis aids in its recognition.

Enargite is also very similar.       

General Characteristics: 

Grain shape: Crystals can be tabular (up to 8 mm), forming spherulites (spherical forms composed of many small tabular crystals) or pseudohexagonal aggregates (like hematite roses). It also forms botryoidal aggregates, in this case with colloform and concentric textures, which is very common (and very similar to tenorite!). It can be powdery and massive. Very typical are masses of subparallel tabular crystals or grouped in bundles tending to radial.

Cleavage may be visible in defoliated crystals.

Extinction is paralell.

Twinning is not present.

Inclusions of delafossite in goethite are common.

Delafossite 01: In PPL, spherulite composed of several delafossite crystals, showing its strong bireflectance (between individual crystals).
Delafossite 01: The same section as of the previous image, now in XPL. It is possible to observe the characteristic anisotropy (between individual crystals).
Delafossite 02: In PPL, aggregate of multiple delafossite spherulites (pinkish-brown), with cuprite (bluish, with poorer polish) and native copper (copper-red). Gangue in dark gray. The shape of these spherulite aggregates, with curved boundaries, always tending to form spheres, is very typical. The color resembles pyrrhotite, but pyrrhotite has different habits, much weaker pleochroism and stronger anisotropy in other colors.
Delafossite 02: The same section as of the previous image, now in XPL. Delafossite shows strong anisotropy between the grains that make up the aggregate. Cuprite exhibits red reflections and its anisotropy colors vary.
In XPL, delafossite (grey) next to an idiomorphic cuprite crystal (red reflections).
In PPL, delafossite aggregate. With a little attention, it is always possible to visualize the different crystals that make up the aggregate due to the strong bireflectance. Sometimes the shape of the crystals is tabular, as expected in delafossite. Others are anhedral.
Delafossite 03: In PPL, delafossite aggregate with typical birreflectance.
Delafossite 03: The same section as of the previous image, now in XPL. Bireflectance and anisotropy, associated with paragenesis, facilitate its recognition. Be careful, because tenorite is very similar.