CLAUSTHALITE
Clausthalite – PbSe – is a rarer selenide. It is not an ore, despite being the most common Se mineral. Se is obtained from sulfide ore minerals of many different metals, as Se often replaces a small percentage of S there.
Clausthalite belongs to the Galena Group and is classified in the Clausthalite-Galena Series. It may contain Hg, Co, and Cu as impurities. There is a complete solid solution series between clausthalite (PbSe), galena (PbS), and altaite (PbTe).
Caution! Ore containing clausthalite may be radioactive due to the presence of uraninite and pitchblende.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Bluish gray to lead gray.
Habit: Massive, granular, foliated. Crystals do not occur.
Cleavage: {001} good.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, granular.
Mohs Hardness: 2.5 – 3
Parting: No.
Streak: Gray-black.
Lustre: Intense metallic.
Diaphaneity: Opaque.
Density (g/cm³): 7.8 – 8.22
2. Geology and Deposits
Clausthalite typically occurs in low-S hydrothermal deposits, as galena forms otherwise. These veins, usually with carbonate gangue, can occur in graywackes and diabases. It can also occur in mercury deposits.
It has been found in hydrothermal deposits associated with the Mid-Atlantic Ridge.
3. Mineral Associations
It occurs with typical gangue minerals such as quartz and carbonates (dolomite, calcite).
It is associated with various Se, Cu, Bi, and Ag minerals: berzelianite, klockmannite, umangite, eucairite, hakite-(Hg), sabatierite, naumannite, paraguanajuatite, and Bi sulfosalts.
It occurs associated with Cu minerals such as bornite, chalcopyrite, chalcocite, digenite, and tetrahedrite.
Also with Hg minerals such as coloradoite and tiemannite.
Occurs with radioactive minerals such as uraninite and pitchblende.
In addition, it is associated with hematite, sphalerite, cobaltite, native gold, and stibiopalladinite.
4. Transmitted Light Microscopy
This does not apply, as clausthalite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Clausthalite polishes well, like galena. Its hardness when polished is much lower than that of galena and slightly higher than that of tiemannite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White, very similar in color to galena.
Compared to the color of galena, the color of clausthalite is distinctly lighter. The color of galena has a violet-brown hue.
Compared to the color of tiemannite, the color of clausthalite is lighter and pure white.
Compared to the color of altaite, the color of clausthalite is darker.
Compared to the color of guanajuatite, the color of clausthalite is intense white.
Pleochroism: No.
Reflectivity: 48,84%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, but with exactly crossed nicols there is no complete extinction and with a complete rotation of the stage the degree of illumination does not change.
Internal reflections: No.
May be confused with: Galena and altaite are practically identical. It is only possible to differentiate clausthalite from these two in situations that allow for direct comparisons.
General Characteristics:
Grain shape is generally allotriomorphic (anhedral); colloform aggregates may be present. It may occur “knitted,” like galena, associated with gold.
Cleavage may be visible in aggregates of larger grain sizes, but is not as perfect as that of galena.
Triangular polishing pits are common.
Polishing scratches, very persistent, remain.
Twinning does not occur.
Zoning does not occur.
Deformations do not occur.
Intergrowths occur with chalcocite.
Inclusions of clausthalite occur in bornite, chalcopyrite, and digenite.
Substitutions 1: clausthalite replaces hematite, stibiopalladinite, gold, Co selenides, tiemannite, and naumannite.
Substitutions 2: clausthalite may be replaced by tiemannite.
Alteration of clausthalite forms cerussite and native selenium.
