GUDMUNDITE
Gudmundite – FeSbS – is a fairly rare sulfide and, for that very reason, is not considered an ore.
It belongs to the Arsenopyrite Group and may contain Ni. It is an alteration product of tetrahedrite (“fahlore”).
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Steel gray.
Habit: Prismatic, in columnar prisms tending towards conical. Acicular, granular. Pseudo-orthorhombic.
Cleavage: No.
Tenacity: Brittle.
Twinning: Twins are common, by {101}, both interpenetration and contact, resulting in crystals with butterfly or cross shapes.
Fracture: No information available.
Mohs Hardness: 5.5 – 6
Parting: No.
Streak: No information available.
Lustre: Intense metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.72
2. Geology and Deposits
Gudmundite forms in the final stages of filling high-temperature sulfide hydrothermal veins, through the late addition of Sb or by the decomposition of Sb minerals. They are generally deposits formed by many (>10-15) different minerals.
Evidence of its possible presence includes tetrahedrite and Sb sulfides and sulfosalts. It is probably much more common than described, always occurring in small quantities.
3. Mineral Associations
We can divide the minerals of the paragenesis into four groups:
a. Common gangue minerals from hydrothermal veins such as quartz and calcite.
b. Common sulfides from hydrothermal veins, such as arsenopyrite, chalcopyrite, pyrrhotite, pyrite, and galena.
c. Minerals from the antimony paragenesis such as native antimony, kermesite, and stibnite.
d. Other minerals that may be associated:
– native elements (gold, electrum, bismuth), ferri-winchite, tetrahedrite, cobaltite, bravoite,
marcasite, stannite, chalcostibite, pyrargyrite, loellingite, jamesonite, and wolfsbergite.
Important: Gudmundite almost always occurs together with galena, pyrrhotite, tetrahedrite-tennantite, and sulfosalts. The association with tetrahydrite-tennantite is particularly diagnostic, as gudmundite is an alteration product of the “fahlores”.
4. Transmitted Light Microscopy
This does not apply, as gudmundite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Gudmundite has a much lower polishing hardness than arsenopyrite and is very similar to, only slightly higher than, the hardness of sphalerite and pyrrhotite. Therefore, it is easy to polish and achieves an excellent polish, with characteristics between the polishes of pyrrhotite and arsenopyrite. It is much easier to polish than arsenopyrite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White from silver to steel gray.
Compared to the color of arsenopyrite, the color of gudmundite is slightly more creamy.
Compared to the color of galena, the color of gudmundite is slightly more yellow.
Pleochroism: Weak, somewhere between white and pinkish-white.
Reflectivity: 50.94 – 54.87% (high, like pyrite!)
Bireflectance: Weak.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy, like arsenopyrite, but in yellow and red tones. Colors ranging from white, gray, green, blue, purple, pink, brown, and black can also occur.
Internal reflections: No.
May be confused with: Arsenopyrite, because it has similar color, twinning, and anisotropy.
General Characteristics:
Grain shape: Idiomorphic crystals are quite common, with prismatic or acicular habits, or gudmundite occurs in aggregates. Grain sizes vary from 1 cm to submicroscopic. When it occurs in small crystals, its identification is difficult.
Cleavage is not present in the polished section.
Twins, well-developed, occur occasionally; they resemble arsenopyrite twins in both form and quality.
Myrmekitic intergrowths of gudmundite can occur with pyrrhotite in the form of reaction crowns between pyrrhotite and tetrahedrite. The proportions of gudmundite and pyrrhotite in these crowns are 50/50%.
Reaction rims of gudmundite form between breithauptite and chalcopyrite.
Intergrowths, regular or irregular intergrowths occur, both with pyrrhotite and with Pb-Sb sulfosalts or Cu-Sb sulfosalts.
Inclusions of sphalerite occur in gudmundite.
Substitutions 1: Gudmundite substitutes galena, pyrite, pyrrhotite, bournonite, and sphalerite. In bournonite, gudmundite crystals may exhibit “ice flower” type forms.
Substitutions 2: Gudmundite is substituted by pyrrhotite, stibnite, berthierite, breithauptite, and Pb-Sb sulfosalts.
Idioblastic textures of gudmundite occur in galena and calcite.
Envelopes of bournonite, galena, and boulangerite may occur around gudmundite crystals.
