5. Reflected Light Microscopy

Sample preparation: The differences between well-formed uraninite crystals and uraninite that presents itself as pitchblende are so marked that they resemble the case of pyrolusite, where well-formed crystals (= polyanite) are treated separately from banded masses.

Uraninite crystals of recent geological age acquire an excellent polish after careful grinding, as they have high hardness, greater than that of magnetite and less than the hardness of quartz and pyrite.

Altered material, which is friable due to age and alteration, presents major problems in polishing. Colloform, banded masses, as well as their alteration products, exhibit variable polishing in the different bands that compose them.

The pitchblende variety can present itself in at least two types; older literature subdivides it into 5 types: one with a high UO₂ content that acquires a good polish and is relatively hard; and another that is high in U₃O₈, an oxidation product of the first, which acquires a poor quality polish, is apparently porous, and comparatively less hard.

PLANE POLARIZED LIGHT – PPL

Reflection color: Light gray with a brownish tint.

Compared to the color of magnetite, the color of uraninite crystals is very similar, somewhat darker and less pink.

Compared to the color of sphalerite, the color of uraninite is very similar, but with a distinct brownish tint.

The pitchblende variety is lighter than uraninite crystals.       

Pleochroism: No.      

Reflectivity: 15.85%        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Isotropic, sometimes with weak anomalous anisotropy.        

Internal reflections: Sometimes they are present, very faint, with colors ranging from dark brown to reddish-brown. In geologically young crystals and in synthetic crystals, these reflections are more numerous and more distinct.

May be confused with: Uraninite is inconspicuous and easily goes unnoticed. Its characteristics vary with age and the degree of alteration.

Very diagnostic are the gel structures (colloform texture: rounded shapes), the rhythmic structures, and the dehydration fractures.

The reddish radioactive halos that develop around the crystals, even in crystals that are not usually affected by radiation, are also important diagnostic aspects.

Alteration minerals typically exhibit very vivid colors, ranging from yellow to green.

In low-quality polished sections, several other minerals may resemble uraninite, such as magnetite, wolframite, columbite, and others.       

General Characteristics: 

Grain shape: the crystals are cubes, octahedrons, dodecahedrons, or in combined forms. The crystals may exhibit zoning, skeletal and myrmekitic forms. The sample may present well-formed crystals in the center, grading towards the edges into botryoidal masses, but the opposite (crystals on the outside, botryoidal on the inside) seems to be more common. In the pitchblende variety, uraninite has a very fine grain size. Possible habits of pitchblende are massive, botryoidal, columnar, banded, reniform, lamellar. There are all variations between practically compact masses and dispersed isolated globules. Colloform, oolitic and dendritic textures may occur.

Zoning is very common in crystals, according to the growth rates of the crystals, pores, inclusions and mainly by the variations in the contents of U, Th and Ce. Generally, the highest contents of U are in the center of the crystals. Zonation can be visible due to slight differences in hardness or coloration, sometimes due to different stages and alteration.

Radioactive halos are very common around uraninite. Uraninite radiation is so intense that it affects even minerals that are not influenced by other radioactive minerals, such as zircon and monazite. Thus, uraninite develops halos in orthoclase, arsenopyrite, safflorite, magnetite, columbite, etc. Even pyrite is affected when in contact with uraninite, being replaced by chalcopyrite or pyrrhotite, or even completely dissolved within reach of alpha particles. In some cases, these pleochroic halos are of great diagnostic importance.

Quasi-myrmekitic intergrowths of uraninite with microcline can occur in pegmatites.

Cleavage may be visible in larger crystals, but is usually poorly developed or destroyed by radiation. Alteration usually makes the cleavages more visible.

Twins, on (111), according to the Spinel Law, are common in well-developed crystals. Sometimes they are visible in high-relief sections.

Unmixings do not occur. With extreme care, it is possible to observe, in very well-crystallized and geologically old uraninites, very fine granular areas, perhaps second (100) and perhaps formed by the entry of radiogenic Pb into the uraninite structure.

Inclusions of skeletal bornite or dendritic galena may occur. Lamellar inclusions of native gold may occur along the cubic and octahedral cleavage directions.

Exsolutions of uraninite occur in columbites.

Dehydration fractures can occur in materials deposited in gel form. These fractures can be arranged radially in rounded uraninite aggregates or, in massive material, form an irregular, polygonal network of fractures (= “craquelée”). In some cases, these fractures are very resistant to alteration and stand out. In other cases, they are cemented by newer material that does not exhibit the banding of the original uraninite. It is very common for these fractures to be filled with galena, chalcopyrite, bornite, bismuthinite, and others.

Substitutions 1: uraninite can be replaced by sphalerite, including zoning of the latter.

Substitutions 2: uraninite replaces pyrite, muscovite, calcite, coffinite, and some components of carbonized plant remains.

Deformations are common, always resulting in cataclastic textures with irregular fragments. A specific deformation of uraninite occurs when hydrocarbons invade the uraninite along fractures, separating the crystal fragments, a type of “swimming apart.” This process forms friable aggregates of uraninite fragments cemented by polymerized carbonaceous matter (= pyrobitumen or “thucholite”). It is very common for these cataclastic fractures to be filled with sulfides such as galena, chalcopyrite, bornite, pyrite, and others.

Rhythmic precipitations are not uncommon in botryoidal or reniform materials, with galena, sphalerite, pyrite, carbonates, and others. Rhythmic coatings of uraninite crystals by pyrite occur with some frequency.

Alteration of uraninite generates brannerite.