THORITE

    Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Tetragonal
    Chemistry : ThSiO4
    Rarity : Quite common

Thorite is isostructural with zircon and like it forms tetragonal prisms often ending in the pyramid {101}. Uranium can replace thorium (up to 10% in the uranothorite variety) as can rare earths (up to 11%). Thorite is mainly found in acidic magmatic rocks, especially granites and their pegmatites, but it is also found in hydrothermal veins, greisens, metasomatized impure limestones and occasionally in alkaline rocks. Its name comes from its chemistry. Thorite has crystals that can be several centimeters. Its color is blackish brown to orange brown. The metamict varieties are very hydrated, of a light orange color (orangite variety), and see their density drop to 4 (against 6.7 for the others). These metamict varieties are poorly preserved in alluvium, unlike "normal" thorites. It is an accessory ore of thorium.

Main photo : Thorite from Ye-bauk-thar-yar, Mogok, Myanmar © Harald Schillhammer

Thorite from Butterfield pegmatites, Colorado, USA © James Christopher
Thorite orangite variety from Langesundsfjorden, Norway © Daniel Russell
Green thorite on zircon from Bassano Romano, Italy © Gianfranco Ciccolini
Green thorite on zircon from Bassano Romano, Italy © Gianfranco Ciccolini

Thorite in the World

Thorite is found in the pegmatites of nepheline syenites from Norway (Kragerö and Brevik in Langesundsfjord, and Arendal, where the crystals exceed 6 cm), and in the granitic pegmatites of Madagascar (Ambatofotsy). The Thomas Range deposit (Utah) yields exceptional yellow to deep green gem crystals housed in rhyolite cavities, as does Bassano Romano (Italy). Thorite is also found in reddish-brown crystals in gold-bearing hydrothermal veins, associated with pyrite and chalcopyrite at Lemhi Pass (Idaho) and Powderhorn (Colorado). Only the latter type of deposit is of economic interest.

Thorite in France

In France, thorite is described in several localities in the Alps, the Pyrenees, Corsica and Brittany in tiny crystals embedded in the rock, but these are of no interest for the collection.

Twinning

No twinning known for this mineral species.

Fakes and treatments

No fakes listed for this mineral species.


Hardness : 4.5
Density : 6.70
Fracture : Irregular
Streak : Orange to brown-black

TP : Opaque to transparent
RI : 1.790 to 1.840
Birefringence : 0.010 to 0.020
Optical character : Uniaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Acids

Magnetism : ParamagneticRadioactivity : Very strong