STEPHANITE
Stephanite – Ag5SbS4 – is a less common sulfosalt that occurs in small quantities in many Ag mining districts, constituting Ag ore. French authors use the name proposed by F.S. Beudant, “psaturose” (from the Greek = brittle).
As with all silver minerals, stephaite crystals are originally bright and have a strong metallic luster, but upon exposure to light they become black and dull. This black surface film can be removed by ultrasonic cleaning.
Stephanite may contain Cu, Fe, and As.
1. Characteristics
Crystal system: Orthorhombic pyramidal.
Color: Lead gray to black.
Habit: Massive, compact to disseminated. Stephanite crystals, up to 6 cm, are frequent, short prismatic along [001] to tabular, they can also have an elongated habit along [100], with [001] striated parallel to [1-14]. The crystals may show hemimorphy.
Cleavage: {010} imperfect, {021} poor.
Tenacity: Brittle.
Twinning: Frequently twinned on [110] forming pseudohexagonal clusters that resemble those of aragonite and are very similar to the pseudohexagonal aggregates of polybasite.
Fracture: Subconchoidal.
Mohs Hardness: 2 – 2.5
Parting: No.
Streak: Iron black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.26
2. Geology and Deposits
Stephanite is a late-successional mineral found in hydrothermal Ag deposits, forming with other silver minerals, especially in Ag and Co veins.
It typically occurs in small quantities and small crystals in many silver mining districts. On the other hand, locally it can be an important ore of this metal.
3. Mineral Associations
It is associated with some common gangue minerals, such as quartz, barite, carbonates (calcite, siderite, ankerite) and fluorite.
It also occurs with some common sulfides such as pyrite, galena, arsenopyrite and sphalerite.
It obviously occurs alongside other Ag minerals, such as native silver, proustite, pyrargyrite, acanthite, tetrahedrite-tennantite, polybasite, pearceite, chlorargyrite, miargyrite, argentopyrite and electrum (Au+Ag).
Also with löllingite and luzonite.
4. Transmitted Light Microscopy
This does not apply, as stephanite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Stephanite readily acquires an excellent polish. In the vicinity of acanthite and polybasite, stephanite stands out for the relative absence of polishing grooves, as it is somewhat harder than these two.
The polishing hardness of stephanite is much greater than that of acanthite, greater than the hardness of pearceite-polybasite and pyrargyrite-proustite, and less than the hardness of tetrahedrite-tennantite (“fahlores”).
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray to medium gray white, with a distinct pinkish to violet hue.
Compared to the color of galena, the color of stephanite is darker, a pinkish gray.
Compared with the color of polybasite, the color of stephanite is lighter pinkish.
Compared to the color of pyrargyrite, the color of stephanite is a lighter gray.
Compared to the color of acanthite, the color of stephanite is slightly lighter with a pinkish-violet hue.
Pleochroism: Faint, but distinct, between gray white (// a), white-pink-brown (// b) and pink (// c).
This pleochroism can be confirmed at intergranular boundaries and, with some practice, is easily recognizable.
Reflectivity: 26.37 – 28.06%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Moderate to strong anisotropy in vivid grayish-brown colors, which may be greenish to yellowish-brown.
Extinction is complete. Basal sections appear isotropic.
Internal reflections: No.
May be confused with: Other Ag minerals that make up the paragenesis, but the absence of internal reflections differentiates stephanite from other very similar silver minerals.
Myargyrite is quite similar, but exhibits rarer internal reflections.
Polybasite exhibits weaker pleochroism and has internal reflections.
Stromeyerite, pyrargyrite, and proustite exhibit other characteristics (internal reflections!), but are very similar to stephanite when in basal sections.
Proustite-pyrargyrite exhibit many internal reflections.
Acanthite and polybasite are much softer and much more difficult to polish.
General Characteristics:
Grain shape: Polybasite crystals are generally columnar, well-formed, and grown on other minerals. Other habits include anhedral (xenomorphic) granular aggregates, poorly intergrown (“thoothed”). Evidence of euhedrism (idiomorphy) only occurs occasionally in relation to acanthite and polybasite, which can form even after stephanite. Stephanite can occur in tetrahedrite fractures and even in proustite-pyrargyrite fractures.
Cleavage is not visible, mainly because it does not exist.
Air corrosion is very weak and often does not occur.
Twinning is very common, often very well-developed lamellar twins. The lamellae can be very thin and sometimes develop in two systems.
Zoning was not observed.
Reaction rims of stephanite can occur between native silver and pyrargyrite.
Deformations do not occur and are not expected in a late mineral like this.
Substitutions 1: Stephanite can replace pyrite, sphalerite, chalcopyrite, native silver, galena, pyrargyrite, and Ni-Co-Fe arsenides.
Substitutions 2: Stephanite can be replaced by native silver, polybasite, andorite, dyscrasite, and owyheeite.
Stephanite intergrowths can occur with pyrargyrite and polybasite.
Inclusions in stephanite can be of acanthite, native silver, sternbergite, or other minerals that it may have replaced.
