UMANGITE
Umangite – Cu3Se2 – is a very rare sulfide. It can constitute Se ore, but Se is normally obtained as a by-product in the refining of Cu, Ni, and Pb ores.
It is classified in the Berzelianite-Umangite Group. It may contain Ag.
1. Characteristics
Crystal system: Tetragonal scalenohedral.
Color: Cherry red to light reddish-violet, fading to iridescent blue-violet, blue-black and darker.
Habit: Fine-grained, massive, short columnar.
Cleavage: {010} bad, {001} bad, perpendicular to each other.
Tenacity: Brittle.
Twinning: Common, lamellar, thin.
Fracture: Irregular, subconchoidal
Mohs Hardness: 3
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.44 – 6.49
2. Geology and Deposits
Umangite is typical of low-temperature hydrothermal veins (<112ºC), where it occurs with other Se minerals.
It can occur in uranium-bearing deposits associated with pitchblende (uraninite).
3. Mineral Associations
It is associated with common gangue minerals (calcite), common sulfides (pyrite, chalcopyrite), and common oxides (hematite).
It occurs with other Se minerals such as clausthalite, tyrrellite, eskebornite, berzelianite, klockmannite, chalcomenite, tiemannite, guanajuatite, naumannite, eucairite, krut’aite, penroseite, and native selenium.
With Te minerals such as altaite, hessite, coloradoite, and weissite.
Also with chalcocite, malachite, native antimony, native gold, cobaltite, cerussite, and uraninite (pitchblende).
4. Transmitted Light Microscopy
This does not apply, as umangite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Umangite has a low polishing hardness, but achieves a high polish without difficulty. Umangite from some occurrences peels strongly during polishing. The polishing hardness of ullmanite is similar to that of klockmannite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light brownish violet to bluish or greenish gray. Despite the tarnishing of the macro sample, the polished section remains untarnished for a long time.
The color of umangite is similar to the color of bornite exposed for a long time.
Pleochroism: Very strong, well-defined and conspicuous, ranging from a dirty reddish-violet to a greenish-blue-gray.
Reflectivity: 14.18 – 16.05%
Bireflectance: Distinct.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong, colorful anisotropy, ranging from very bright reddish-orange to orange-yellow, possibly dark orange.
Basal sections remain dark or uniformly dark gray.
The anisotropy colors resemble those of covellite!
Extinction parallel to cleavage.
Internal reflections: No.
May be confused with: no other mineral, due to its very distinct reflective colors and anisotropy.
General Characteristics:
Grain shape: Umangite occurs massivel or in aggregates of fine anhedral grains. Individual grains are irregularly lobed, sometimes very “toothed” or rounded. Sometimes it occurs with a clear sub-parallel orientation of the grains. Rarely, fine columnar crystals occur, developed parallel to “c”, without well-developed terminations and with simple tetragonal basal sections, forming radial aggregates.
Cleavage in two directions, perpendicular to each other, may be visible.
Twinning occurs in some cases, lamellar, thin and subparallel. In other cases there is no twinning. The textures thus formed are among the most beautiful in the microscopy of opaque ores!
Zoning does not occur.
Polymorphism does not occur.
Exsolutions do not occur.
Inclusions 1: Umangite may have calcite inclusions at the rim, making the rims porous.
Inclusions 2: Umangite inclusions may occur in coloradoite and weissite.
Myrmekitic intergrowths with klockmannite may occur.
Alteration of umangite leads to the formation of klockmannite.
Lamellae of klockmannite arranged parallel to the basal plane may occur.
Substitutions: Umangite occurs in veins and replaces hematite, uraninite (pitchblende), tyrrellite, eskebornite, clausthalite, and berzelianite.
