TENORITE

Tenorite – CuO – is a fairly common oxide, used for centuries as a copper ore (~80% Cu).

It is a mineral without distinct and/or diagnostic macroscopic characteristics, as it is black, usually massive to earthy, and typically has a high impurity content. Crystals are very rare, reaching a maximum of 2 mm, and are known from volcanic-exhalative sublimates of Mount Vesuvius (Italy). Tenorite is also formed by heating malachite.

Among its various habits, it can form fibrous tuffs, mammillated and botryoidal aggregates, very fine-grained mosaics, skeletal aggregates, or layers enveloping cuprite cubes, which at the end of the process are completely replaced by tenorite. It can be intergrown with limonite, forming masses of concentric bands; some bands are composed partly of tenorite, and partly of limonite.

Associated delafosite may occur, making differentiation between tenorite and delafosite difficult. Finely dispersed manganese oxides may be present, which greatly diminishes the reflection pleochroism of tenorite. Gel forms are very frequent at all stages of tenorite crystallization.

1. Characteristics

Crystal system: Monoclinic prismatic.  

Color: Steel gray, iron gray to black. 

Habit: Powdery, massive or earthy. It can be scaly, dendritic. See above. 

Cleavage: Poor. In zones [011] and [011]. Striations [010] in {100}.

Tenacity: Brittle. Lamellae are flexible and elastic.        

Twinning: {011} common, forms star groups. 

Fracture: Conchoidal to irregular.  

Mohs Hardness: 3.5

Parting: No.         

Streak: Black.         

Lustre: Metallic to earthy.          

Diaphaneity: Opaque. Very fine flakes are transparent.           

Density (g/cm³): 6.5

           

2. Geology and Deposits

It is a common mineral in the oxidized zones of copper-mineralized bodies, where it occurs associated with dozens of other secondary minerals.

It occurs in volcanic-exhalative form in extremely fine, even transparent, crystals. A known occurrence of this type is Mount Vesuvius (Italy), where tenorite is found alongside Cu chlorides, alkaline chlorides, and cotunnite (PbCl2).

It can occur in droplets as inclusions in magnetite-Ni in stratiform basic bodies.

 

3. Mineral Associations

It occurs associated with the characteristic minerals of the oxidation zone of copper sulfide ores. Among these are primary Cu ore minerals (chalcocite, bornite, chalcopyrite) and secondary Cu ore minerals (native copper, cuprite, delafosite, chrysocolla, malachite, azurite, dolerophanite, chalcocyanite).

It also occurs with quartz, calcite, goethite, hematite, pyrite, marcasite, galena, tennantite, and other Fe and Mn oxides.

 

4. Transmitted Light Microscopy

Very fine flakes of tenorite are transparent.

Refraction indices:  ne:    no:

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Brown, distinct pleochroism ranging from light brown to dark brown.    

CONVERGENT LIGHT

Character: Biaxial          

2V angle: Quite large.         

5. Reflected Light Microscopy

Sample preparation: Tenorite polishes without great difficulty. Its hardness upon polishing is greater than that of chalcocite and less than that of cuprite and goethite. Massive botryoidal aggregates polish well. The hardness upon polishing is somewhat anisotropic, as can be seen in aggregates with lamellar twinning.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Yellowish-brown or light grayish-white with a yellow tint. 

Pleochroism: Distinct in color, ranging from gray to grayish-white (or light brown to dark brown), and is most easily observed in twin lamellae.      

Reflectivity: 19,46 – 29,41%        

Bireflectance: Strong.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy between white and blue (or blue and gray-cream). Clear oblique and incomplete extinction. At the point of maximum darkening, it shows many grooves from old polishing.

Internal reflections: Rare, thin, and red, found only on the finest lamellas.

May be confused with: Other secondary Cu minerals.

Delafossite is similar, but paragenesis helps in distinguishing them.

General Characteristics: 

Grain shape is often acicular, in radial or concentric aggregates. It can form fine-grained mosaics or pseudomorphs. Forms derived from gel crystallization are abundant in all stages of crystallinity; they may be constituted by or derived from geltenorite, a variety of tenorite that occurs in gel form.

Twinning may be well developed. They are often lamellar and can be very conspicuous. Some of the lamellae apparently originate during the polishing process, others are primary. They form swallowtail-type indentations.

Substitution of cuprite by tenorite may occur.

Concentric banding formed by alternating layers of tenorite and goethite may occur, as well as bands composed partially of tenorite, partially of goethite. Delafosite probably also occurs in these bands.

In PPL, with image processing (more contrast and clarity), lamellar tenorite crystals forming radial groups and with anisotropy between blue and yellowish hues are visible.
Tenorite 01: In PPL, aggregate of tenorite with cuprite (white-bluish). The pleochroism in yellow tones of tenorite is clearly visible.
Tenorite 01: The same section as of the previous image, now in XPL. Cuprite show red internal reflections and tenorite strong anisotropy between yellowish and bluish.
Tenorite 02: In PPL, from top to bottom: chrysocolla (gray, good polish), unidentified secondary copper mineral (gray, worse polish), tenorite (yellowish) and cuprite (white)
Tenorite 02: The same section as of the previous image, now in XPL. Tenorite crystals are very small, but the anisotropy between yellowish and bluosh is clearly visible.
In PPL, tenorite forms a crust between cuprite (internal red and violet reflections) and chrysocolla (sky blue reflections).
In PPL, in oxidized copper ore, tenorite spherulite (anisotropy between yellowish and bluish), with chrysocolla (sky blue). The spherulite is around 0.1 mm in diameter. Doesn't the image resemble an ammonite in the ocean? 🙂
In XPL, tiny tenorite spherulites in chrysocolla (blue-green), with cuprite (red).
In XPL, tenorite spherulites (anisotropy in yellowish and blue) are both isolated and adjacent to each other, forming a band, which is nothing more than many spherulites side by side, one interfering with the growth of the other. The typical anisotropy colors are not always easy to perceive; it is necessary to choose the most suitable light source (halogen or LED), calibrate the light to exactly the correct intensity, and correctly adjust the settings of the photographic equipment (camera or smartphone or direct transfer to the computer).