SEMSEYITE
Semseyite – Pb9Sb8S21 – is a very rare sulfosalt, typical of complex polymetallic Ag-Cu-Pb-Sb ores.
It is classified in the Plagionite Group, in the Semseyite Series, composed of Pb-Sb sulfides. It may contain Fe, Ag, and Cu.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Gray to black, may show some tarnishing.
Habit: Tabular, prismatic, subparallel crystals forming spherical aggregates in galena. Globular, rosettes.
Cleavage: {112} perfect.
Tenacity: Brittle.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 2.5
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.08
2. Geology and Deposits
Semseyite is typical of moderate-temperature Pb-Zn hydrothermal veins whose characteristics place them between subvolcanic and intrusive-hydrothermal.
It also occurs in some Ag-Sn-Zn veins.
3. Mineral Associations
It occurs associated with some common gangue minerals such as quartz, barite, and carbonates (calcite, siderite, spharosiderite).
Also with common sulfides such as pyrite, pyrrhotite, marcasite, chalcopyrite, arsenopyrite, sphalerite, and galena.
In the specific paragenesis, with fizélyite, jamesonite, pyrargyrite, bournonite, zinkenite, guettardite, sorbyite, diaphorite, jordanite, tetrahedrite, and boulangerite.
Diagnostic is the constant association with galena, sphalerite, and other Pb-Sb sulfosalts. It is generally covered by jamesonite.
4. Transmitted Light Microscopy
The literature indicates that semseyite is biaxial.
5. Reflected Light Microscopy
Sample preparation: Semseyite polishes well despite its low hardness; polishing is easier and produces fewer polishing scratches than with galena. Its hardness upon polishing is distinctly lower than that of galena.
PLANE POLARIZED LIGHT – PPL
Reflection color: White with a discreet greenish tint.
Compared to the color of galena, the color of semseyite is very similar, slightly yellowish-green when side by side. Galena is more pink.
Compared with the color of boulangerite, the color of semseyite is lighter.
Pleochroism: Weak to very faint, visible only at intergranular boundaries and even then with some difficulty.
The colors are white with a yellow-green tint (in the longer direction of tabular crystals) and a much darker greenish-gray color, similar to the color of tetrahedrite, but lighter.
Reflectivity: 35.08 – 41.99%
Bireflectance: Weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, from brownish-gray to bluish-gray, light gray, dark gray.
Extinction is linear, but complete darkening never occurs.
Sometimes ondulating extinction occurs.
Internal reflections: No.
May be confused with: Few other minerals.
Plagionite is extremely similar.
General Characteristics:
Grain shape: Semseyite generally forms tabular crystals that cluster in radial aggregates tending towards spherical shapes. Coarse granular aggregates may form, as well as fine-grained aggregates. Fine-grained veins may form through coarser, older aggregates of semseyite, which are slightly whiter in color than the newer material.
Cleavage is rarely visible, and in these cases is oblique to the grain elongation.
Fractures: irregular, slightly parallel cracks are relatively common.
Association with galena, in the form of crusts around the galena, is very typical.
Twinning was not observed.
Zoning was not observed.
Substitutions: Semseyite is replaced by bournonite.
Inclusions of galena, in the form of grains, lamellae, and droplets, may occur.
