SELIGMANNITE
Seligmannite – PbCuAsS3 – is a very rare sulfosalt, characteristic of polymetallic hydrothermal veins. It is part of polymetallic ores, but is never the main ore mineral in the deposit. It is probably much more common than imagined, being systematically confused with bournonite.
It is classified in the Bournonite Group, in the Bournonite-Seligmanite Series, replacing the Sb of bournonite with As. It may contain Sb, Fe, Ag and Zn.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Lead gray, dark lead gray, black.
Habit: Equigranular, short prismatic to tabular crystals, up to 20 mm.
Cleavage: {001} poor, {100} poor, {010} poor. Common striations on {110}.
Tenacity: Brittle.
Twinning: Polysynthetic and by {110}.
Fracture: Conchoidal.
Mohs Hardness: 3
Parting: No.
Streak: Black,
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.38 – 5.44
2. Geology and Deposits
Seligmannite occurs in Cu-As hydrothermal paragenesis. In general, it occurs in Pb, Zn, As, Tl and Ba ores at high temperatures and of high grade.
It occurs as a reaction product between tennantite and galena.
3. Mineral Associations
It is associated with common minerals (quartz) and sulfides such as pyrite, sphalerite, wurtzite, chalcopyrite, galena, tetrahedrite, tennantite, naumannite, petzite, altaite, pouropigment, and realgar.
It is also associated with carbonates (dolomite, rhodochrosite) and sulfosalts (dufrénoysite, jordanite, liveingite, rathite, baumhauerite).
In the type locality (Binntal, Switzerland), it occurs in cavities in dolomites.
4. Transmitted Light Microscopy
This does not apply, as seligmanite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Seligmannite acquires a good polish without difficulty. Its hardness upon polishing is slightly greater than that of galena and distinctly lower than that of tennantite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Medium pinkish gray. It does not exhibit the bluish-green hues of bournonite.
Compared to the color of dufrénoysite, the color of seligmannite appears slightly pinkish.
Compared to the color of jordanite, the color of seligmannite appears slightly pinkish and darker.
Compared to the color of enargite, the color of seligmannite is grayish-green.
Compared to the color of galena, the color of seligmannite is reddish-brown.
Compared to the color of petzite, the color of seligmannite is slightly more grayish.
Compared to the color of tetrahedrite, the color of seligmannite is much less gray.
Compared to the color of tennantite, the color of seligmannite is lighter and distinctly pinkish.
Pleochroism: Very weak, noticeable in the twinning lamellae.
Reflectivity: 31.51 – 32.92%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, ranging from brownish-gray to dark gray.
Internal reflections: No.
May be confused with: Few other minerals, because the polysynthetic twinning in a checkerboard pattern is very diagnostic, associated with the pinkish reflection color and strong anisotropy.
Bournonite has a blue reflection color, without pink. And its anisotropy is weaker.
Jordanite exhibits different pleochroism and more regular twinning.
Geochronite exhibits different pleochroism and more regular twinning.
General Characteristics:
Grain shape: Seligmanite generally occurs in xenomorphic grains. Idiomorphic acicular crystals may occur in stromeyerite. Exceptionally well-developed crystals occur in cavities in dolomitic rocks at the type locality.
Polysynthetic twinning, forming a checkerboard pattern, is almost always present. It is a pattern very similar to bournonite twinning and not as regular as jordanite twinning.
Substitutions 1: Seligmanite can replace jordanite and dufrénoysite.
Substitutions 2: Seligmanite is replaced by galena and naumannite.
When tennantite replaces galena, seligmannite may occur along cleavage planes and fractures; also as isolated droplets along the boundaries between the two minerals.
Intergrowths of seligmannite, thin, with petzite may occur.
Inclusions: Seligmanite occurs as oriented droplets in altaite.
Reaction rims occur between tennantite and galena or chalcopyrite or stromeyerite or bornite.
Oriented seligmanite rims around circular aggregates of tennantite may occur.
