FREIESLEBENITE
Freieslebenite – AgPbSbS3 – is a very rare sulfosalt, characteristic of Pb-Ag-Sb-Bi parageneses. It is a minor silver ore, containing 20.24% Ag, 22.84% Sb, and 38.87% Pb.
It is classified within the Freieslebenite Group, alongside marrite (containing As instead of Sb), and is one of several sulfosalts of similar composition. Chemically, it resembles diaphorite. It may contain Cu and Fe.
1. Characteristics
Crystal system: Monoclinic prismatic, pseudo-orthorhombic.
Color: Pale steel gray, lead gray, dark gray – almost black – silver white.
Habit: Short prismatic, generally granular. Crystals up to 3 cm, pseudo-orthorhombic.
Cleavage:
{010} imperfect. Deep striations paralell to its length.
Tenacity: Brittle.
Twinning: On {010}.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 2.5
Parting: No.
Streak: Same as the macro color.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.20 – 6.23
2. Geology and Deposits
Freieslebenite occurs as a late-stage mineral in hydrothermal quartz-carbonate veins associated with the Pb-Ag-Sb-Bi paragenesis in precious-metal silver ores.
3. Mineral Associations
It occurs with common gangue minerals such as quartz, carbonates (calcite, dolomite, siderite), and barite.
In its specific paragenesis, freieslebenite associates with sulfides (pyrite, galena, sphalerite, chalcopyrite, acanthite), other sulfosalts (andorite, pyrargyrite, “freibergite”, boulangerite, polybasite, aramayoite), native silver, and gypsum.
4. Transmitted Light Microscopy
Not applicable, as freieslebenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Freieslebenite takes a good polish despite its low hardness. Its polishing hardness is low – significantly lower than that of galena – which can result in an undulating polished surface.
PLANE POLARIZED LIGHT – PPL
Reflection color: White with a light gray tint.
Compared to the color of galena, the color of freieslebenite is similar, though slightly darker and with a somewhat greenish-yellow hue. Galena found near freieslebenite exhibits a reddish tint.
Compared to the colors of other As- and Sb-rich sulfosalts, the color of freieslebenite is lighter.
Pleochroism: No.
Reflectivity: 36.02 – 41.22%
Bireflectance: Minimal, visible only at the intergranular boundaries.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Distinct – though difficult to observe -anisotropy, ranging from medium gray to dark gray. Observation is easier under intense illumination.
There are two more pronounced extinction positions alternating with two weaker extinction positions.
Internal reflections: No.
May be confused with: Many other minerals with similar characteristics. Identifying freieslebenite is difficult; weak pleochroism and weak anisotropy are characteristic features.
General Characteristics:
Grain shape: generally occurs as clusters of unoriented idiomorphic crystals.
Polishing scratches are usually visible and difficult to eliminate.
Cleavage may be visible, parallel to (110) and (001).
Lamellar (polysynthetic) twinning parallel to (100) may occasionally be observed; in some cases, many grains exhibit twinning, while in others, no twinning occurs.
Exsolutions of freieslebenite occur in galena.
Inclusions of freieslebenite may occur in other Ag minerals.
Alteration of freieslebenite can lead to the formation of malachite, creating a malachite rim on the freieslebenite.
Substitutions 1: freieslebenite replaces sphalerite.
Substitutions 2: freieslebenite is replaced by andorite.
