5. Reflected Light Microscopy

Sample preparation: The polishing hardness of pararammelsbergite is medium – slightly higher than that of nickeline and lower than that of löllingite and skutterudite. It readily takes a very good polish – better than the other minerals in the paragenesis, with the exception of nickeline, which also takes an excellent polish.

PLANE POLARIZED LIGHT – PPL

Reflection color: Pure white; it takes a better polish and is whiter than associated arsenides.

Compared to the color of rammelsbergite, the color of pararammelsbergite is whiter.

Pleochroism: Weak to absent.

Reflectivity: 58,11 – 60,05%. Very high!

Bireflectance:  No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy showing brownish-pink, brown, light brown, yellow, and gray. No bluish or greenish colors appear – a diagnostic feature. For correct color assessment, the nicols must be exactly crossed and the optics free of strain.

Internal reflections:  No.

May be confused with: Other white anisotropic minerals, including white minerals of the same paragenesis. Its extremely high reflectivity is diagnostic, as it is the highest among Co-Ni arsenides. Some white isometric sulfides are similar but isotropic.

Rammelsbergite exhibits stronger anisotropy, and blue is among the colors observed; furthermore, it almost always displays lamellar twinning.

Nickeline exhibits much stronger anisotropy.

Safflorite shows strong zonal variations in both anisotropy and hardness.

Löllingite exhibits significantly higher hardness.

General Characteristics:

Grain shape: occurs as tabular crystals that produce rectangular cross-sections when viewed perpendicular to the elongation. They can be highly elongated and tabular, resembling long prismatic crystals. They may form radial clusters of tabular crystals. When occurring with nickeline, they create a strong contrast due to their white color compared to the pinkish-white of nickeline. Pararammelsbergite can also form mosaics of rounded, intergrown crystals and may occur in intergrowths with various other minerals. The crystals are always small – around 1 mm, though they can reach up to 2 mm.

Cleavage is rarely visible.

Twinning is rare, apparently following two laws, and is never lamellar like the twinning seen in rammelsbergite.

Zoning, well-developed, may occur, especially after exposure to air.

Inclusions of pararammelsbergite in nickeline typically exhibit a very well-defined fracture at the contact with the niccolite. Very small pararammelsbergite inclusions may occur in native silver.

Substitutions 1: Pararammelsbergite is not replaced by other minerals.

Substitutions 2: Pararammelsbergite typically replaces skutterudite in a highly characteristic concentric zonal pattern, in forms ranging from idioblastic to fine myrmekitic. It also replaces nickeline in idioblastic forms, growing from the edges inward into the nickeline crystals.

Rims of pararammelsbergite around rammelsbergite may occur.