COBALTITE

Cobaltite – CoAsS – is a rarer sulfide, characteristic of high-temperature hydrothermal veins. In addition to its historical uses as a pigment, cobalt is now a strategic industrial metal.

Cobaltite can contain Ni, Cu, Pb, and Sb. There are two varieties (with Fe, with Pd). It forms a series with gersdorffite (NiAsS). When heated, it becomes magnetic and releases As2O3, which is a poisonous gas. Cobaltite ores may be radioactive!

Cobaltite occurred associated with Ag ore in the Erzgebirge mining region of Germany in the 14th century. There, at greater depths, the known Ag ores dwindled and were replaced by cobaltite. Since there was no silver in cobaltite, miners called it “silver thief,” “kobolt” (= bewitching gnome, hence the name cobaltite), or “shit ore” (“Scheiss-erz”). But when it was discovered that it was possible to extract the coveted “Cobalt Blue” pigment from cobaltite, a new mining cycle began.

When massive, cobaltite is similar to several other ore minerals with a metallic luster, silvery-gray color, and medium hardness. Well-formed crystals can be confused with pyrite and galena, since cobaltite twins produce pseudocubic, pseudooctahedral, and pseudopentagonododecahedral crystals. Very useful in this context is the fact that cobaltite easily alters to erythrite – Co3(AsO4)2.8(H2O) – which has an extremely characteristic violet-pink color and which “reveals” the existence of a cobalt sulfide from which it was formed.

1. Characteristics

Crystal system: Orthorhombic pyramidal.          

Color: White-silver pink, violet steel gray, black.     

Habit: Massive, granular. Pseudocubic, pseudooctahedral, and pseudopentagonododecahedral crystals.  

Cleavage:  {100} and {010} good. {001} perfect. Striations on the faces, as in pyrite.      

Tenacity: Brittle.        

Twinning: In {111}; resulting in pseudo-cubic forms.       

Fracture: Irregular.       

Mohs Hardness: 5.5

Parting: No.         

Streak: Steel gray to black.         

Lustre: Strong metallic.          

Diaphaneity: Opaque.           

Density (g/cm³): 6.3

 

2. Geology and Deposits

Cobaltite forms in high-temperature hydrothermal deposits and in contact metamorphic rocks (skarns). It occurs in parageneses of cobalt and nickel arsenides and in Co-Ni-Ag-U type veins.

It very rarely occurs in alluvial deposits.

 

3. Mineral Associations

It is associated with common gangue minerals such as quartz, carbonates (calcite, dolomite, magnesite).

Also with common silicates such as titanite, actinolite, allanite and zoisite.

With common sulfides (pyrite, pyrrhotite, chalcopyrite, sphalerite, molybdenite) and common oxides (magnetite).

In the specific paragenesis, with native gold, native silver, native bismuth, cubanite, graphite, eskutterudite, glaucodote, scapolite, erythrite, scorodite and other sulfides and arsenides of Co, Ni and As.

Uraninite may occur associated; it is necessary to verify any radioactivity of the ore.

 

4. Transmitted Light Microscopy

This does not apply, as cobaltite is completely opaque.

5. Reflected Light Microscopy

Sample preparation: Polishing cobaltite is very difficult due to its high hardness. The difficulty is similar to that of polishing pyrite, but the final polished cobaltite result is of better quality.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Very light pink or pinkish white.

Over time (days, weeks) the pink tone intensifies slightly, then becomes tarnished.

Compared to the color of arsenopyrite, the color of cobaltite is more pink.

Compared to the color of pyrite, the color of cobaltite is white, without the yellowish tone.

Pleochroism: Very faint, difficult to impossible to observe without immersion in oil.

Reflectivity: 52.71%, quite high, reminiscent of pyrite.        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Weak anisotropy from yellow-brown to blue-gray. Uncrossing the nicols at 2° makes these colors more visible. 

Internal reflections:  No.     

May be confused with: Pyrite, if it occurs in small grains or in very good polished sections.

Eskutterudite, gersdorffite, linnaeite, and ullmannite, which are also white, are isotropic.

Arsenopyrite, also white, has much more intense anisotropy and does not form cubes.

Nickeline has much stronger anisotropy.

General Characteristics: 

Grain shape is subhedral to euhedral (pseudocubic), which is referred to as “euhedral outlines” in the literature. In reality, the grains are made up of a set of anisotropic lamellae, which may have a certain ordering, constitute fields, or both. Cobaltite has a very strong tendency towards idiomorphism. Even when it occurs in the form of inclusions in pyrite and arsenopyrite, the shapes are euhedral. Only when it occurs in almost monomineralic aggregates are the grain shapes anhedral and poorly intergrown. The grain size is highly variable.

Cleavage can become quite visible, but only in larger grains.

Polysynthetic lamellar twinning is often visible and resembles the checkerboard twinning of microcline. They may appear in flame form.

Zoning is common.

Cataclasis is frequent, indicating that cobaltite is an early-forming mineral.

Substitutions are relatively rare. Cobaltite has been recorded as being replaced by galena, eskutterudite, sphalerite, pyrrhotite, arsenopyrite, tennantite, safflorite, niqueline, pyrite, chalcopyrite, and native silver.

Oriented intergrowths with pyrite are relatively common when the grains are larger. Other possible intergrowths are gersdorffite, chalcopyrite, eskutterudite, and safflorite.

Unmixings are rare. Unmixings of silver minerals, common in other cobalt minerals, are rare in cobaltite.

Inclusions of native gold, native silver, chalcopyrite, niqueline, and loellingite may occur.

Cobaltite 01: In PPL, pseudocubic grain of cobaltite. With no other opaque minerals nearby for comparison, cobaltite in PPL closely resembles pyrite, but the color has a distinct pinkish hue, which pyrite lacks. Furthermore, it polishes better.
Cobaltite 01: The same section as of the previous image, now in XPL. In XPL+2º there are no internal reflections and weak anisotropy can be perceived, which, coincidentally, is not the case in this grain.
Cobaltite 02: In PPL, aggregate of cobaltite crystals. In PPL, doubts may arise regarding identification, but in XPL, identification is conclusive.
Cobaltite 02: The same section as of the previous image, now in XPL+2º. The subtle anisotropy of cobaltite becomes quite visible. Since pyrite is isotropic and the anisotropy of arsenopyrite is much more conspicuous, the exclusion of these two very common minerals aids in the identification of cobaltite.
Cobaltite 03: In PPL, cobaltite aggregate, somewhat cataclastic, with gangue (dark gray).
Cobaltite 03: The same section as of the previous image, now in XPL+2º. In addition to the diagnostic characteristics of cobaltite, in XPL it is possible to observe locally internal reflections in a strong pink, almost wine color. Reflections of this color are diagnostic, in this paragenesis, for erythrite ("cobalt flower"), a secondary cobalt mineral that macroscopically presents this color. The presence of this secondary cobalt mineral helps to confirm cobaltite.
A tarnished grain of cobaltite in PPL. The tarnished surface allows observation of the grain structures, both lamellar twinning and zoning. Therefore, before repolishing a cobaltite section, it is interesting to observe it with the tarnishing (air corrosion) that has occurred.
In XPL, an idiomorphic (pseudocubic) cobaltite grain showing well-developed twinning. To observe the twinning, it is helpful to slightly uncross the nicols (2º) to make the image brighter.
In XPL+2º, anhedral cobaltite grain, showing itself subdivided into domains, zones, and twins, a very typical feature of cobaltite. The anisotropy of cobaltite simplifies the visualization of these subdivisions. The arrangement of the zones/twins changes from grain to grain.