Bornite

COLOR

Pink-brown, tarnishing rather rapidly to purple. The tint is much more sustained than that of enargite and rather similar to germanite which is more intensively purple than fresh bornite but less so than tarnished bornite. Takes a good polish.

REFLECTANCE

Rather low. Lower than tetrahdrite-tennantite. Higher than sphalerite.

ANISOTROPISM

Bornite often shows weak anisotropism with noncharacteristic tints. Under uncrossed nicols: brown-grey to pink-brown.

TEXTURE

Polysynthetic and sometimes orthogonal twins may be observed. Bornite may show lamellar exsolution of chalcopyrite, chalcocite or wittichenite. Sometimes it is replaced by chalcopyrite. It often breaks down to a flameshaped integrowth of idaite and chalcopyrite, or of chalcopyrite and digenite along twin planes. Sometimes, it contains minute inclusions of tellurides (hessite, melonite)

ASSOCIATED MINERALS

Chalcopyrite, idaite, digenite, covellite, roquesite, wittichenite, tetrahedrite-tennantite, sphalerite, various tellurides, cassiterite.

CRITERIA OF DETERMINATION

The pink-brown color, very weak anisotropy, twins and especially the purple tarnish are highly characteristic. May be confused with germanite in small grains. However, germanite is very rare and remains purplish even after repolishing.

Source

ATLAS OF ORE MINERALS (P. Picot and Z. Johan)