ZINKENITE
Zinkenite – Pb9Sb22S42 – is a relatively rare sulfosalt that forms part of polymetallic Pb-Zn-Sb ores.
It may contain Cu, Ag, Fe, and As.
Zinkenite exhibits a very characteristic habit of fine prismatic crystals, which can be acicular to capillary, grouped in subparallel bundles. It can form fibrous to capillary aggregates with radial arrangement. Sometimes columnar, also massive. Crystals up to 5 cm.
1. Characteristics
Crystal system: Hexagonal pyramidal.
Color: Steel gray, can tarnish to iridescent.
Habit: Fine prismatic crystals, which can be acicular to capillary, grouped in subparallel bundles.
Cleavage: {11-20} bad. Deep striations perpendicular to {0001}.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 3 – 3.5
Parting: No.
Streak: Steel gray.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.25 – 5.35
2. Geology and Deposits
Zinkenite, like other Pb sulfosalts, is found as one of the last minerals to form in hydrothermal Pb-Sn-Cu-Zn veins, associated with other sulfides and sulfosalts of base metals and tin.
3. Mineral Associations
It is associated with common gangue minerals such as quartz and carbonates (calcite, dolomite, siderite).
Also with common sulfides, such as pyrite, pyrrhotite, chalcopyrite, sphalerite, arsenopyrite and galena.
In the specific paragenesis with cassiterite, stannite, stibnite, jamesonite, boulangerite, bournonite, tetrahedrite-tennantite, chalcostibite, andorite, realgar, plagionite, wurtzite, kermesite, fülöppite, freibergite, semseyite, native gold and kaolinite.
4. Transmitted Light Microscopy
This does not apply, as zinkenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Zinkenite acquires a good polish without difficulty. Its hardness upon polishing is medium.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white to gray with a subtle light greenish tint.
Compared to the color of galena, the color of zinkenite is pale grayish-white.
Compared to the color of tetrahedrite, the color of zinkenite is lighter, grayish-white and not brownish.
Compared to the color of jamesonite, the color of zinkenite is almost the same, slightly lighter.
Pleochroism: Weak, should be observed along the intergranular boundaries. Tarnishing may simulate stronger pleochroism.
Reflectivity: 36.39 – 40.73%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Distinct anisotropy in bluish gray, can be in shades between greenish gray and brownish gray.
In sections perpendicular to the length (basal), the anisotropy is very weak and in many cases is completely absent; they appear to be isotropic.
Internal reflections: Rare, dark red and small (“specs”), less luminous and more brownish than the reflections of hematite.
May be confused with: other Pb-Sb minerals, as zinkenite is very difficult to identify under a microscope. Zinkenite is very similar to other sulfides and sulfosalts that occur in the same paragenesis.
Jamesonite exhibits the same habit, almost the same optical characteristics, and occurs in the same paragenesis.
General Characteristics:
Grain shape: very characteristic habit of fine columnar to prismatic crystals, acicular to capillary, which group in subparallel bundles. It can form radially arranged fibrous aggregates. Sometimes columnar, also massive. Crystals up to 5 cm. Large crystals may present almost perfect rhomboid (pseudo-rhombic) basal sections, resembling the basal sections of arsenopyrite. The crystals fill fractures and veins.
Cleavage is not observed.
Twinning does not occur.
Extinction is parallel, without striking colors in the diagonal position.
Zinkenite inclusions in sphalerite are common, they can be in the form of small droplets (resembling demixing bodies) or as acicular aggregates.
Substitutions: zinkenite substitutes cassiterite, pyrite, stannite, freibergite and andorite.
