Torbernite

Torbernite
Torbernite - Entraygues, Aveyron, Midi-Pyrénées, France
General
Category	Phosphate minerals
Formula (repeating unit)	Cu(UO2)2(PO4)2 · 8 - 12 H2O (hydrated copper uranyl phosphate)
Strunz classification	8.EB.05
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	I4/mmm
Identification
Formula mass	641 – 713 g/mol, depending the degree of water loss
Color	Emerald green to apple green
Crystal habit	Tabular crystals; Foliated to earthy masses and encrustations
Twinning	Rare on [110]
Cleavage	[001] Perfect; [100] Distinct
Fracture	Brittle
Mohs scale hardness	2 - 2.5
Luster	Vitreous; Pearly
Streak	Pale green
Diaphaneity	Transparent to subtranslucent
Specific gravity	3.2
Optical properties	Uniaxial (-)
Refractive index	nω = 1.590 - 1.592 nε = 1.581 - 1.582
Birefringence	δ = 0.009 - 0.010
Pleochroism	Visible
Melting point	Decompose before
Fusibility	Decompose before
Other characteristics	Radioactive
References

Cu(UO₂)₂(PO₄)₂ · 8 - 12 H₂O

Torbernite, whose name derives from the Swedish chemist Torbern Bergman (1735–1784), is a radioactive, hydrated green copper uranyl phosphate mineral, found in granites and other uranium-bearing deposits as a secondary mineral. Torbernite is isostructural with the related uranium mineral, autunite.

The chemical formula of torbernite is similar to that of autunite in which a Cu²⁺ cation replaces a Ca²⁺. The number of water hydration molecules can vary between 12 and 8, giving rise to the variety of metatorbernite when torbernite spontaneously dehydrates. Their respective chemical compositions are the following:

Torbernite

Metatorbernite

Torbernite's most common alternative names are copper uranite and cupro-uranite.

As a radioactive mineral, torbernite has some limited significance as a uranium ore. Its vibrant green color and well-developed distinctive crystals (size: mm to a few cm) make it a sought-after collector's mineral, as well. However, torbernite, like other hydrated minerals, can easily suffer from loss of water molecules. This loss of water from the mineral leads to an alteration of torbernite specimens into its pseudomorph, meta-torbernite. Some collector's websites assert that any torbernite specimen more than a few years old should be considered fully transitioned to meta-torbernite. However, it likely depends on the temperature and relative humidity of ambient air in which specimens are stored.

As torbernite is radioactive and outgases radon (²²²Rn), collectors are urged to take proper precautions in the handling and storage of any specimens. An adequate ventilation of the rooms and the cabinets in which the specimens are stored is essential to evacuate the radioactive radon gas responsible for lung cancer, but it could increase the dehydration rate of the specimens. To limit radon inhalation, naked specimens should never be stored in rooms in which one spends much living or working time. An alternative is to store specimens in gas tight transparent containers in which radon will accumulate and decay to secular equilibrium.