THORIANITE
Thorianite – ThO2 – is a rare oxide, of no importance as an ore.
It is classified in the Uraninite Group, in the Thorianite-Uraninite Series. Impurities may include Pb, U, Ce, Nd, Pr, La, Zr, and He. The Pb present is probably radiogenic. There are four varieties (with up to 12.5% Pb, with up to 8% Ce, with La, intermediate between uraninite and thorianite). It is diamagnetic. Three “thorianites” ares distinguished: α-thorianite, β-thorianite, and γ-thorianite (see specific literature). It is less susceptible to weathering than uraninite, to which it is very similar.
WARNING! It is highly radioactive, with radiation composed of high-intensity gamma rays, requiring special care. It is slightly less radioactive than pitchblende (a variety of uraninite). Always wash your hands after handling. Do not lick, ingest, or inhale its powder; do not keep it near your body.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Dark gray, black, brown-black, dark reddish brown. May have a bronzy tarnish.
Habit: Cubic crystals, can be combined with octahedrons, up to 9 cm. Rounded grains.
Cleavage: {001 poor.
Tenacity: Brittle.
Twinning: Interpenetration twins by {111} are very common; they follow Fluorite’s Law, with two intergrown cubes such that the ternary axes of one cube emerge at the center of the faces of the second cube.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 6.5 – 7
Parting: No.
Streak: Gray, gray-green to black.
Lustre: Resinous, submetallic.
Diaphaneity: Opaque.
Density (g/cm³): 9.7 – 9.8
2. Geology and Deposits
The primary deposits of thorianite, always an accessory mineral, are granitic pegmatites or semibasic rocks or carbonatites. It also occurs in serpentinites and in contact metamorphic and metasomatic rocks such as sanidinites.
It is a common mineral in heavy mineral concentrates obtained from alluvial deposits such as beach sands and placers.
3. Mineral Associations
It is associated with some common minerals such as quartz, calcite, sanidine, orthoclase, plagioclase, pyroxenes (diopside), amphiboles (hornblende, tremolite), micas (biotite, phlogopite), “chlorite” and fluorapatite.
It occurs with well-known minerals such as pyrite, pyrrhotite, baddeleyite and pyrochlore.
With rare minerals such as “guarinite”, hiortdahlite, polyakovite (Ce) and xenotime (Y).
In heavy mineral concentrates with zircon, ilmenite, magnetite, cassiterite, corundum (sapphire) and allanite.
4. Transmitted Light Microscopy
In fine fragments it is translucent in reddish-brown, with n~2,200.
5. Reflected Light Microscopy
Sample preparation: The polishing hardness of thorianite is greater than the hardness of uraninite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Gray.
Compared to the color of uraninite, the color of thorianite is very similar, but darker.
Pleochroism: No.
Reflectivity: ~15% Increases with increasing U content.
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic. Weak anomalous anisotropism is possible.
Internal reflections: Reddish-brown to brownish-yellow, rare in air observations, but frequent in oil observations.
May be confused with: Other dark minerals. Thorianite easily goes unnoticed if the rock has not been tested for radioactivity.
Uraninite is very similar; distinguishing it from thorianite is quite difficult, but paragenesis usually offers a clue to its recognition.
General Characteristics:
Grain shape: thorianite typically occurs as cubes. Combined forms of (large) cubes with (small) octahedrons may occur. In placers, the grains are rounded due to abrasion.
Cubic (100) cleavage is visible, sometimes (111) cleavage is also visible.
Cataclastic texture is very common, cemented by sulfides, magnetite, and baddeleyite.
Pb sulfide may occur in the grains in cloud-like areas. In these cases, the action of S has transformed the original Pb or PbO content into PbS. These areas are somewhat more porous.
Zoning is common, visible as a function of small differences in hardness and reflectivity, perhaps with the core being more enriched in U.
