5. Reflected Light Microscopy

Sample preparation: Polybasite has low hardness; when polished, its hardness is similar to that of freibergite. It is less hard than stephanite, pyrargyrite, proustite, and miargyrite, all of which take a better polish. The polish is never perfect – polishing scratches always remain – though there are fewer than in acanthite.

PLANE POLARIZED LIGHT – PPL

Reflection color: Grayish-white, slightly greenish.

Compared to many other minerals, it exhibits a pale greenish hue. Color variations depend on the section orientation and Cu content.

Compared to the color of galena, polybasite’s color is much darker – a pale greenish-gray.

Compared to the color of acanthite, polybasite’s color is very similar.

Compared to the colors of pyrargyrite and proustite, polybasite’s color is darker, lacking that slight pinkish or bluish tint.

Compared to the color of tetrahedrite-tennantite (“fahlore”), polybasite’s color is somewhat darker, consistently lacking any brownish hue.

Pleochroism: Very weak, perceptible only occasionally at intergranular contacts. Pleochroism is present even in basal and oblique sections.

Reflectivity: 30.02 – 31.43%

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Weak to moderate anisotropy, ranging from light green to deep blue-green shades; they can also appear dark violet.

They are not easy to observe because the intense light from the microscope causes air-induced corrosion on polybasite (and also on pearceite).

Internal reflections: Deep red in color, they are frequently present along polishing grooves. The quantity and quality of these reflections vary considerably among grains and samples from different localities.
In basal sections, a pseudo-hexagonal cleavage is observed, in which these internal reflections become very clearly visible.

The presence or absence of internal reflections does not allow for a distinction between pearceite and polybasite.

May be confused with: The green anisotropy is highly characteristic, as are the red internal reflections.

Pearceite is very similar – practically indistinguishable under the microscope. When the paragenesis includes abundant tennantite, proustite, and other arsenic minerals, pearceite is likely to be present.

Stromeyerite can look very similar, particularly to polybasite varieties that lack internal reflections or show them only rarely. Air-induced tarnishing of polybasite and the transformation of stromeyerite under intense light aid in differentiation.

Acanthite closely resembles basal sections of polybasite.

General Characteristics:

Grain shape: polybasite generally occurs as equant or lamellar grains. It may occur as idioblasts. The grains are always very small. The lamellae form aggregates that fill pores, with a texture resembling an ophitic texture.

Air corrosion (tarnishing) always occurs – sometimes very rapidly, sometimes quite slowly – but always much more slowly than in acanthite and native silver. This atmospheric corrosion can appear pitted and irregular due to submicroscopic acanthite inclusions. 

Polishing scratches are always present, though less so than in acanthite. 

Cleavage is only rarely visible. 

Twinning does not occur. 

Zoning does not occur. 

Deformation does not occur. 

Myrmekitic intergrowths may occur with various minerals, such as galena and acanthite. 

Inclusions of polybasite occur in galena, tetrahedrite-tennantite, and sphalerite. 

Substitutions 1: polybasite can be replaced by acanthite, native silver, or native gold. 

Substitutions 2: polybasite replaces (this is the most common case) other minerals, such as sphalerite, tetrahedrite-tennantite, and galena, and more rarely pyrargyrite, proustite, and stephanite.