Acicular black crystal of the extremely rare and relatively new mineral dalnegroite (IMA 2009-058) from the type and only known locality worldwide (Lengenbach Quarry, Binntal, Wallis, Switzerland), in a tiny matrix of crystallized realgar. Analyzed specimen (EDX and XRD) by F.Nestola. Official Code Number L27.323 from 1996. Ex.M.Ecker and T.Raber collections. Very few specimens known, maybe two "big samples" or a few more, being this tiny specimen a part of the two known specimens. — Photo: David Hospital · CC BY-SA 4.0
Dalnegroite
Dalnegroite is an IMA-recognized mineral species in the sulfosalt class, with the chemical formula Tl₄Pb₂(As,Sb)₂₀S₃₄.
It crystallizes in the triclinic crystal system (space group P1). It is a member of the Chabournéite structural group.
Key properties: Mohs hardness 3–3.5; specific gravity 4.82; Brownish-red streak; Submetallic. Anistropism: Strong, bluish to green, red internal reflections. Bireflectance: Strong luster.
IMA symbol Dnr, IMA status: approved, named in 2009.
Of hydrothermal origin in dolomitic marble.
It is found associated with Realgar, pyrite, Sb-rich hutchinsonite, jordanite, Sb-rich seligmanite, sinnerite.
The mineral was named to honor Alberto Dal Negro (b. 1941), Professor in Mineralogy and Crystallography at the University of Padova, Italy, since 1976.