FOV 1.3 x 1.6 mm. Via Excalibur. MOB coll. The whole extent of the thornasite cluster is only ~ 0.5 x 0.9 mm. Tiny gray xls of thornasite inconveniently tucked away in a cavity lined with gray natrolite do not make for a great photo op. Updated Jan 2010 with better color fidelity. But it's still not a great photo. From: Demix-Varennes quarry, Saint-Amable sill, Varennes & St-Amable, Lajemmerais RCM, Montérégie, Québec, Canada. — Photo: Modris Baum · CC0
Thornasite
Thornasite is an IMA-recognized mineral species in the silicate class, with the chemical formula Na₁₂Th₃(Si₈O₁₉)₄·18H₂O.
It crystallizes in the hexagonal crystal system (space group R-3c).
Key properties: specific gravity 2.62; White streak; Vitreous to waxy or greasy, may be pearly luster; optically uniaxial.
IMA symbol Tna, IMA status: approved, named in 1985.
Found in an intrusive alkalic gabbro-syenite complex (Mont Saint-Hilaire, Canada); in a cavity in a nepheline syenite sill (Saint-Amable, Canada).
It is found associated with Brockite, yofortierite, analcime, eudialyte, s¶erandite, polylithionite, leifite, natrolite, microcline, albite, mangan-neptunite, steacyite, catapleiite.
The name refers to THORium, sodium or NAtrium, and SIlicon in its composition.