XANTHOCONITE
Xanthoconite – Ag3AsS3 – is a very rare sulfosalt, classified in the Proustite Group, since it is the monoclinic dimorph of proustite. The xanthoconite-proustite transition occurs at 180-202°C.
Not to be confused with “xanthochroite” = CdS !
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Red-orange, yellow-orange, red-brown, brown
Habit: Tabular, laths, pyramids.
Cleavage: {001} distinct.
Tenacity: Brittle.
Twinning: {001} pseudoorthorhombic twins.
Fracture: Sub-Conchoidal.
Mohs Hardness: 2 – 3
Parting: No information available.
Streak: Orange-yellow.
Lustre: Adamantine.
Diaphaneity: Transparent.
Density (g/cm³): 5.54
2. Geology and Deposits
Xanthoconite typically occurs in hydrothermal veins that host silver and also contain arsenic, therefore care is needed when handling these materials.
If the ore contains native arsenic and proustite, the associated mineral is probably xanthoconite. If the ore hosts pyrargyrite, the mineral most probably is pyrostilpnite.
Xanthoconite is not uncommon in small quantities in the younger, last-formed portions of silver ores. It generally has an idiomorphic and easily recognizable development, always being younger than proustite and pirargyrite. Usually it is found in small quantities near them. Caution is needed because pyrostilpnite is very similar.
3. Mineral Associations
It associates with gangue minerals like quartz, calcite, pyrite and baryte.
The specific paragenesis hosts Ag-minerals like native silver, proustite, marrite, sternbergite and polybasite.
Also associates with jordanite.
4. Transmitted Light Microscopy
Refraction indices: ne: no:
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism:
Relief:
Cleavage:
Habits:
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors:
Extinction:
Elongation sign:
Twins:
Zoning:
CONVERGENT LIGHT
Character:
2V angle: B(-)
Alterations:
May be confused with:
5. Reflected Light Microscopy
Sample preparation: The hardness on polishing is very low, similar to that of proustite/pyrargyrite. It acquires a good polish easily, making it possible to almost completely avoid polishing grooves. The hardness on polishing is lower than the hardness of stephanite and galena, similar to or slightly higher than the hardness of miargyrite and pearceite-polybasite, and much higher than the hardness of acanthite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Bluish gray, but less blue than proustite and pyragyrite.
Pleochroism: Quite clear
Reflectivity: 21.78 – 25.19%, similar to proustite/pyrargyrite.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Similar to proustite/pyragyrite: “Distinct anisotropy between grays and browns, something like pale gray to dark gray, but this anisotropy is masked by strong and numerous internal reflections.”
Internal reflections: Yellowish, lemon yellow, yellowish brown to red-brown.
May be confused with: pyrostilpnite (dimorph of pyrargyrite), proustite and pyrargyrite. Very diagnostic are the yellowish internal reflections, Proustite/pyrargyrite show red internal reflections.
Xanthoconite and pyrostilpnite are very similar to each other, their properties are close to the ones of proustite and pyrargyrite.