5. Reflected Light Microscopy

Sample preparation: Geocronite has very low polishing hardness. Despite this, it takes a good polish, though always with some residual polishing scratches. The hardness of geocronite is greater than that of boulangerite, similar to that of franckeite, and comparable to—though slightly lower than—that of galena.

PLANE POLARIZED LIGHT – PPL

Reflection color: Pure white to light gray, with a very faint yellow-green tint.

Compared to the color of galena, geocronite’s color is very similar—slightly darker and with a hint of greenish-cream.

Compared to the color of jamesonite, geocronite’s color is very similar to the lighter areas of jamesonite; against the darker sections of jamesonite, geocronite appears slightly lighter and whiter.

Compared to the color of boulangerite, geocronite’s color is slightly lighter and lacks the greenish tint found in boulangerite.

Pleochroism: Very weak, visible only under favorable conditions at intergranular boundaries and twin boundaries.

Reflectivity: 37.97 – 39.17%

Bireflectance:  Weak.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Distinct to moderate anisotropy, with subdued colors ranging from bluish-gray to olive-gray (or shades of blue, olive, and gray); allows for the recognition of grain boundaries and twins.

Extinction is oblique to the twin lamellae.

Undulose extinction may occur.

Internal reflections:  No.

May be confused with: At first glance, geocronite may be confused with galena, whose color is very similar, but galena is isotropic. Geochronite presents diagnostic aspects as lamellar twins, low reflectivity and very weak pleochroism, much weaker than the pleochroism of other associated sulfosalts.

Other Pb-Sb sulfides and sulfosalts, in addition, have another habit, being columnar to acicular.

General Characteristics:

Grain shape: Geocronite can form tabular crystals—sometimes pseudo-orthorhombic or pseudo-hexagonal—up to 8 cm in size, though these are rare. It generally occurs in massive form but can be granular, with randomly arranged, rounded or irregular grains; it may also appear earthy. Grain size is typically around 1 mm, although occurrences with large grains (3–5 cm) exist. 

Polishing scratches are inevitable due to the mineral’s low hardness, hindering observations under crossed polars. 

Cleavage is not observed (though other literature sources state that cleavage may be visible). 

Twinning is always present and abundant, appearing as lamellae of uniform thickness arranged parallel to (101). Different occurrences may exhibit lamellae of varying thicknesses, with each occurrence characterized by a typical lamellar thickness. Two sets of lamellar twins may occur—each with a distinct lamellar thickness—where one set consists of much coarser lamellae than the other. In such cases, the (101) twins exhibit uniform thickness, while the (10-1) twins show variable thickness. Very rarely, both twin types exhibit equal thicknesses, resulting in a parquet-like (checkerboard) twinning pattern. Simple contact twins may also occur between grains.