NAUMANNITE
Naumannite – Ag2Se – is a rare sulfide, typical of Se parageneses. Due to its rarity, it is not an Ag ore. It could be a Se ore, but Se is usually obtained as a by-product in the refining of Pb, Zn, and Cu ores.
Naumannite may contain Pb and S. The high-temperature (>130ºC) form is cubic; inversion to the orthorhombic form occurs with decreasing temperature.
1. Characteristics
Crystal system: Orthorhombic bisphenoidal.
Color: Grayish-black, tarnishing to iridescent brownish.
Habit: Granular, massive. Rare pseudo-cubic or tabular crystals, up to 1 cm.
Cleavage: No.
Tenacity: Something sectile.
Twinning: No.
Fracture: Splintery.
Mohs Hardness: 2.5
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 7.0 – 8.0
2. Geology and Deposits
Naumannite is characteristic of hydrothermal veins containing Se but lacking S.
3. Mineral Associations
Naumannite associates with common gangue minerals such as quartz and carbonates.
It occurs with other Se minerals such as bohdanowiczite, aguilarite, tiemannite, clausthalite, eskebornite, umangite, eucairite, and guanajuatite.
With sulfides such as acanthite, pyrite, bornite, chalcopyrite, “freibergite,” tennantite, and tetrahedrite-(Se).
With native gold (with Se, electrum), native silver, and goethite.
4. Transmitted Light Microscopy
This does not apply, as naumannite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Naumannite polishes well, although it always retains some polishing scratches, though fewer than acanthite. The polishing hardness of naumannite is very low, lower than the hardness of clausthalite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Bluish gray.
Compared to the color of clausthalite, the color of naumannite is greenish.
Compared to the color of tiemannite, the color of naumannite is slightly lighter.
Compared to the color of aguilarite, the color of naumannite is very similar.
Pleochroism: No. It is only visible with oil immersion; in this case, it is weak and ranges from brownish-gray to dark greenish-brown to grayish.
Reflectivity: 33.53 – 36.02%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Faint, but distinct, somewhere between bluish and brownish-gray (or between light gray and dark gray).
Internal reflections: No.
May be confused with: Considering the paragenesis (umangite, tiemannite, clausthalite), the low hardness and anisotropy are diagnostic for naumannite.
Aguilarite is extremely similar, but it shows a darker shade. It is not possible to differentiate naumannite from aguilarite under a microscope, but the X-ray patterns are different.
General Characteristics:
Grain shape: Naumannite can be granular, in anhedral grains, and highly intergrown.
Cleavage does not occur, but it may show traces of cubic cleavage, probably due to parting as a function of inversion from the high-temperature form (cubic) to the low-temperature form (orthorhombic).
Mimetic twinning is occasionally well developed.
Myrmekitic intergrowths, extremely fine, occur with clausthalite.
Substitutions of naumannite by clausthalite may occur.
