TIEMANNITE
Tiemannite – HgSe – is a very rare selenide, characteristic of hydrothermal veins and occurring with other Se minerals. It can be part of Se ores, but Se is usually obtained as a by-product in the refining of Cu, Ni, and Pb ores.
It is classified in the Sphalerite Group, forming a series with metacinnabarite. It may contain S, Pb, and Cd. In tetrahedral crystals, typically the {111} faces are dull and the {-111} faces are shiny.
CAUTION! Tiemannite ores need to be analyzed with extra care, as tiemannite contains Hg and possibly Cd, and may also occur associated with radioactive minerals.
1. Characteristics
Crystal system: Cubic hexatetrahedral.
Color: Steel gray to black.
Habit: Tetrahedral crystals up to 0.5 cm. Massive, compact, granular.
Cleavage: No. Striations // to [1-10] may occur.
Tenacity: Brittle.
Twinning: Commons, with {111} as the twinning axis.
Fracture: Irregular to conchoidal.
Mohs Hardness: 2.5
Parting: No.
Streak: Almost black.
Lustre: Metallic, similar to the luster of chalcocite.
Diaphaneity: Opaque.
Density (g/cm³): 8.19 – 8.47
2. Geology and Deposits
Tiemannite occurs with other Se minerals in hydrothermal veins with calcite gangue.
3. Mineral Associations
It occurs associated with common gangue minerals such as quartz, albite, muscovite, barite, and calcite.
In the Se paragenesis, several other Se minerals occur, such as clausthalite, hakite-(Hg), eucairite, berzelianite, watkinsonite, eskebornite, penroseite, krut’aite, klockmannite, umangite, and naumannite.
With common sulfides such as galena and sphalerite.
With Cu minerals such as bornite, chalcocite, chalcopyrite, and malachite.
With Hg minerals such as cinnabar, metacinnabar, and corderoite.
With Fe minerals such as hematite and limonite.
Also with uraninite, native gold, realgar, acanthite, clinochlor, Mn oxides, and hewettite.
4. Transmitted Light Microscopy
This does not apply, as tiemannite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Tiemannite acquires a great polish easily and shows fewer polishing grooves than clausthalite, which occurs in association with it. The polishing hardness of tiemannite is low, lower than the hardness of clausthalite and much lower than the hardness of galena.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light gray with a subtle brownish tint.
Compared to the color of galena, the color of tiemannite shows a brownish tint, in addition to being darker.
Compared to the color of chalcocite, the color of tiemannite is much darker and does not present the bluish tint of chalcocite.
Compared to the color of clausthalite, the color of tiemannite is much darker.
Pleochroism: No.
Reflectivity: 28,87%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, completely dark under crossed nicols.
Sometimes it is very weakly anisotropic, which can only be observed in twin lamellae and intergranular boundaries, between dark gray and gray-black.
Internal reflections: No.
May be confused with: several other minerals.
Tiemannite is one of the ores that is difficult to identify. Its association with selenides that are easier to recognize, such as umangite, berzelianite, and clausthalite, facilitates the identification of tiemannite. The difference between tiemannite and clausthalite, in general terms, is like the difference between sphalerite and galena.
Chalcocite is very similar, but has a bluish hue.
Sphalerite at first glance is very similar, and the clausthalite that accompanies tiemannite is confused with galena.
General Characteristics:
Grain shape: The grains and crystal faces are generally irregular to angular, usually embedded in calcite. Association with clausthalite is frequent.
Cleavage does not occur.
Twinning is not visible.
