STIBARSEN
Stibarsen – AsSb – is a very rare alloy with no significance as an ore; it occurs in complex arsenic- and antimony-based ores.
The mineral was also known as “allemontite” (after the locality of Allemont, France) – a name now discredited, though still cited in older literature.
It is classified within the Arsenic Group and may contain Fe, Bi, and S; a bismuth-rich variety exists.
CAUTION! Because it contains arsenic and occurs in association with other arsenic minerals, the material is toxic, and special precautions must be taken for safe handling.
1. Characteristics
Crystal system: Trigonal scalenohedral.
Color: White, reddish-gray. Tarnishes into gray tones.
Habit: Anhedral crystals; fine-grained, curved lamellar. Reniform and mammillary aggregates.
Cleavage: Perfect in one direction.
Tenacity: No information available.
Twinning: No.
Fracture: No information available.
Mohs Hardness: 3 – 4
Parting: No.
Streak: Grayish black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 5.8 – 6.2
2. Geology and Deposits
Stibarsen typically occurs in Co-Ni-Ag-As-Bi-type hydrothermal veins.
It may occur in pegmatites as coarsely foliated masses.
Intergrowths with Sb are much more common than those with As and stibarsen itself.
3. Mineral Associations
It occurs associated with common gangue minerals (quartz, calcite, siderite) and common sulfides (galena, sphalerite).
Arsenic-bearing minerals occur in the paragenesis, such as native arsenic, arsenolite, realgar, orpiment, symplesite, löllingite, and safflorite.
Also typical of the paragenesis are antimony-bearing minerals (with or without Ag), such as native antimony, dyscrasite, pyrargyrite, stibnite, kermesite, “stibiconite” (see current status!), and cervantite.
4. Transmitted Light Microscopy
Not applicable, as stibarsen is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Stibarsen takes an excellent polish. Its polishing hardness is lower than that of native arsenic and higher than that of native antimony.
PLANE POLARIZED LIGHT – PPL
Reflection color: White.
It tarnishes upon exposure to air more slowly than native arsenic and more rapidly than native antimony. This tarnishing aids or complicates identification; it is useful to let the polished section sit for a few days to allow the tarnish to develop:
– stibarsenic intergrown with native Sb darkens rapidly,
– As intergrown with stibarsenic darkens rapidly.
Pleochroism: Very weak.
Reflectivity: Between those of As (47-52%) and Sb (68-70%).
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Intermediate between native antimony and native arsenic. Antimony has weak anisotropy. Arsenic have distinct anisotropy, between yellowish brown, light gray and yellowish gray.
Internal reflections: No.
May be confused with: Other white minerals that are weakly anisotropic or isotropic, especially with native arsenic or native antimony.
General Characteristics:
Grain shape: graphic or concentric exsolution intergrowths with native arsenic or native antimony are a typical feature of stibarsen. Intergrowths with Sb are more frequent than those with As. These intergrowths can be identified by their differing hardnesses, which result in varying polish qualities.
“Allemontite I” is an old term for intergrowths with Sb.
“Allemontite II” is the old term for stibarsen.
“Allemontite III” is an old term for intergrowths with Sb.
Inclusions: stibarsen can occur as vermiform inclusions within native arsenic (in “Scherbenkobalt”). Additionally, stibarsen can occur as rounded inclusions (“droplets”) within native antimony, sometimes forming “mists.”
