Goodmundite (FeSbS): signs in reflected light, VHN hardness, differences

Goodmundite (Gudmundite, FeSbS; described in 1928) is a mineral of antimony-iron series, isostructural with arsenopyrite and stable at temperatures below ~280 °C.

Short reference

Formula: FeSbS
Syngony: monoclinic
Spatial Group: P2₁/c (B21/d)
Cell Parameters: a = 6.69; b = 10.06; c = 5.94; β = 90°
Z: 8
Isostructuralism: arsenopyrite
Sustainability: below 280°

The chemical composition is close to the theoretical one: Fe 26.83; Sb 57.76; S 15.41.

Diagnostic signs in reflected light

Reflection: high; generally lighter in color than pyrite.
Birefringence: strong (creamy pink in dark position → slightly bluish in light position).
Anisotropy: strong, with bright color effects (reddish/greenish tones).
Hardness (VHN): 350-725 (usually 370-590).

In the reflection spectra, the curves R₂ and R₁ are non-parallel in most of the visible region and intersect around 650 nm.

Differences from similar minerals

In the microscope, gudmundite can be confused with arsenopyrite. In practice, a combination of features helps: reduced hardness, more deep pinkish hue и more pronounced anisotropy.

Finding and Associations

The mineral is relatively widespread in hydrothermal deposits rich in Sb and Fe.

Typical associations: antimonite, bertierite, antimony, pyrrhotite, galena, Pb-sulfosols, breitgauptite, chalcopyrite (and others).

X-ray (basic d-lines)

4.10 (5); 3.00 (4); 2.93 (4); 2.80 (4); 2.56 (10); 1.912 (8); 1.617 (6); 1.458 (6); 1.410 (6).