CHALCOSTIBITE

Chalcostibite – CuSbS2 – is a rather rare sulfosalt, typically found as an early-stage mineral in hydrothermal Sb-Cu-Pb-Ag veins. It occurs in polymetallic ores—predominantly those containing antimony—but is never the primary ore mineral of the deposit.

It is classified within the Chalcostibite Group and is the sulfur analogue of pribramite (the selenium analogue). It may contain Fe, Pb, and Zn.

1. Characteristics

Crystal system: Orthorhombic bipyramidal. 

Color: From lead-gray to iron-gray, sometimes shading into blue, green, or iridescence.

Habit: Tabular, flattened parallel to (001), massive, disseminated, and intergrown with other sulfides and sulfosalts.       

Cleavage: {010} perfect, {001} and {100} less perfect. Frequent striations, parallel to (010).       

Tenacity: Brittle.        

Twinning: On {140}.       

Fracture: Sub-conchoidal.       

Mohs Hardness: 3 – 4

Parting: No.         

Streak: Black.         

Lustre: Metallic.          

Diaphaneity:  Opaque.          

Density (g/cm³): 4.9 – 5

           

2. Geology and Deposits

Chalcostibite is characteristic as an early-stage formation in low-temperature hydrothermal Sb veins.

 

3. Mineral Associations

It is associated with common gangue minerals such as quartz, “limonite” (goethite), barite, and carbonates (calcite, dolomite, siderite, strontianite).

Also with common sulfides such as pyrite, pyrrhotite, marcasite, arsenopyrite, chalcopyrite, galena, and sphalerite.

In specific parageneses, with Sb minerals (±Pb, ±Ag, ±Cu, etc.) such as stibnite, bournonite, jamesonite, tetrahedrite, andorite, dadsonite, gudmundite, and zinkenite.

With Ag minerals such as native silver, acanthite, canfieldite, and argyrodite.

With Bi minerals such as native bismuth and bismuthinite.

With stannite, enargite, native gold, cinnabar, orpiment, and wurtzite.

It is associated with secondary copper minerals such as azurite, malachite, chalcocite, and covellite.

 

4. Transmitted Light Microscopy

Not applicable, as chalcostibite is completely opaque.

5. Reflected Light Microscopy

Sample preparation: Chalcostibite is easy to polish and takes an excellent polish. Its polishing hardness is intermediate—greater than that of bournonite and native silver, and slightly lower than that of chalcopyrite and sphalerite.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Bluish-gray or pinkish-gray. The color is also described as “white with a pinkish-gray tint.” The pinkish hue is very characteristic.

Compared to the color of bournonite, the color of chalcostibite is lighter and less pinkish.

Compared to the color of galena, the color of chalcostibite is darker and more yellowish-gray.

Compared to the color of native silver, the color of chalcostibite is grayish and darker.

Compared to the color of sphalerite, the color of chalcostibite is pinkish.

Compared to the color of tetrahedrite, the color of chalcostibite is distinctly lighter and slightly more bluish.       

Pleochroism:  Very weak. Even at intergranular boundaries it is difficult to see.     

Reflectivity: 35.18 to 40.46%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Moderate to strong anisotropy ranging from bluish-gray to brownish-gray; it may also appear pinkish-gray or greenish-gray. Bluish hues are quite characteristic.
It resembles the anisotropy of bournonite but displays more vivid colors.

In the extinction position, the mineral never appears completely black, but rather light gray.

Internal reflections:  Rare, in pale red tones.     

May be confused with: bournonite.

Chalcostibite’s cleavage and the absence of parquet (checkerboard) twinning make it easy to distinguish from bournonite, both macroscopically and microscopically. Undulose extinction is also diagnostic. Pleochroism and anisotropy are low compared to most other sulfosalts.       

General Characteristics: 

Grain shape: generally anhedral and coarse, with grains typically showing extensive mutual intergrowth. Crystals are rare; they may be prismatic, tabular (thick), or elongated parallel to (001).

Cleavage parallel to (001) is frequently visible in polished sections. Cleavage along the other two directions appears only in material subjected to tectonic stress; in such cases, triangular pits (resembling those found in galena) may occur.

Triangular pits similar to those seen in galena may be present.

Twinning is observed only locally, appearing as numerous fine lamellae; these may resemble stibnite twinning.

Zoning does not occur.

Deformation resulting from translation parallel to (001) and twisting around an axis within this plane is very common. In such instances, the deformed areas exhibit strong undulose extinction, as well as granulation and incipient recrystallization.

Reaction rims form between chalcostibite and chalcopyrite; these rims consist of bournonite, galena, and tetrahedrite.

Exsolution does not occur.

Intergrowths with enargite may occur.

Replacements 1: Replacement of chalcostibite by other minerals is common. Replacement occurs along fractures and deformed grains. Initially, galena and bournonite form, followed later by galena and tetrahedrite-tennantite; these, in turn, are subsequently replaced by bournonite and chalcopyrite, and partially by pyrrhotite. Galena may be absent, resulting in the sequence chalcostibite-bournonite-boulangerite-jamesonite. Other minerals replacing chalcostibite include native silver, tenorite, chalcocite, covellite, boulangerite, jamesonite, and pyrrhotite.

Replacements 2: Chalcostibite can replace sphalerite, pyrite, and chalcopyrite.

Alteration of chalcostibite—which has very low resistance to weathering—leads to the formation of covellite, chalcocite, azurite, and others.

Inclusions of chalcostibite in tetrahedrite may occur.

Chalcostibite 01: In PPL, in the center of the image, chalcostibite (bluish-gray). Surrounding it, tetrahedrite (gray-brown). At the top left, pyrite (yellow, high relief). At the top, arsenopyrite (white, high relief). Sphalerite appears in dark gray. A very characteristic association for chalcostibite.
Chalcostibite 01: The same section as of the previous image, now in XPL. Chalcostibite is anisotropic and tetrahedrite is isotropic. Sphalerite (reddish internal reflections), pyrite (isotropic), and arsenopyrite (anisotropy showing different colors).
Under XPL +2°, one of the anisotropy colors of chalcostibite, which contains a significant amount of blue, becomes clearly visible.
A very typical association is chalcostibite (bluish-gray) with covellite (intense blue), a characteristic alteration mineral. Tetrahedrite (gray-brown), arsenopyrite (white, high relief), and sphalerite (dark gray). PPL.