IDAITE
Idaite – Cu5FeS6 – is a rarer sulfide, typically an alteration product of bornite. It integrates Cu ores based on bornite and chalcopyrite.
The mineral, although accepted (“grandfathered” – described before 1959), has been poorly described, with the term referring to both impure nukundamite and a Sn-poor mawsonite-like tetragonal phase (Rice et al. 1979, Mineralogical Magazine, 43, pp. 193-200). Available articles suggest that the mineral needs further investigation.
It is classified in the Stannite Group.
1. Characteristics
Crystal system: Hexagonal
Color: Brown-red, brown-bronze, copper red.
Habit: Lamellar. It forms very thin lamellae in bornite.
Cleavage: No.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 2.5 – 3.5
Parting: No.
Streak: No information available.
Lustre: Metallic to submetallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.2
2. Geology and Deposits
Idaite is a lamellar decomposition product of bornite and usually occurs associated with fine fusiform bodies of chalcopyrite. These are initial stages of enrichment, of apparently secondary origin. So it is a bornite replacement.
The idaite lamellae are either arranged parallel to (111) of the bornite pseudo-octahedron or parallel to the bornite pseudo-cube.
3. Mineral Associations
In the type locality (Mina Ida, Namibia) idaite occurs in a typical paragenesis of secondary Cu minerals, from the oxidation zone: tenorite, cuprite, covellite, native copper, bornite, chalcopyrite, digenite, delafossite and paramelaconite.
Also with Fe sulfides (pyrite, pyrrhotite, mackinawite), Fe oxides (hematite) and sphalerite.
Very often, idaite is associated with limonite and covellite neoformations.
In other paragenesis (rare!) it occurs with aegirine, charoite, pectolite and nukundamite.
4. Transmitted Light Microscopy
Not applicable, as idaite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Idaite has a very low polishing hardness, similar or slightly higher than the hardness of covellite. The polishing hardness of idaite is lower than the hardness of bornite and chalcopyrite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Red-orange or light orange or brown-red to yellowish gray.
It tarnishes much more slowly than bornite.
Color varies by section, which can be seen using high power objectives.
Compared to the color of freshly polished bornite, the color of idaite is very similar, a little darker.
Pleochroism: Strong, red-orange or brown-red to yellowish gray.
Reflectivity: 31,19 to 31,49%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, extremely intense, very diagnostic.
When idaite is formed by the decomposition of bornite, the colors are very luminous, very vivid, in yellow-green, green or turquoise.
When the idaite is of hypogenic origin, the colors are gray-green.
Defined extinction.
Internal reflections: No.
May be confused with: it is very easy to miss idaite when it occurs in bornite. When bornites have fine fusiform chalcopyrite bodies, the presence of idaite is possible. The very typical anisotropy is a diagnostic feature for idaite.
General Characteristics:
Grain shape: Idaite has two modes of occurrence.
When supergenic, as an alteration product of bornite, idaite appears in the form of thin lamellae, grids (trusses) and veins parallel to (100) and (111) of bornite, often together with chalcopyrite, which has fine fusiform bodies. These idaite lamellae can be very small, at the limit of observation. Prior to the formation of idaite, fusiform bodies of higher reflectivity and more orange-yellow color than that of bornite may form in bornite. In other cases, bornite presents “fracture disease” or “crack disease” before the formation of idaite. Idaite can also form thin films between chalcopyrite and chalcocite, or replace pyrite along with chalcocite, digenite and covellite.
When hypogenic, idaite has wide, hexagonal tabular crystals. Parallel intergrowths with covellite may occur and idaite as inclusions in pyrite, wurtzite and bornite may occur. This idaite alters to covellite and pyrite.
Cleavage does not show.
Twins does not show.
Substitution of idaite for covellite can occur.
Circular fractures occur in bornites contained in chalcopyrite and transformed into idaite. In other cases, bornites form compact areas with a slight change in color to yellow and covered by fine fractures (“crack-disease”).