Outstanding green crystals of tangdanite (IMA 2011-096) from this classic Spanish locality for the mineral (Villahermosa del Rio, Castellón, Valencian Community, Spain). Unique combination of elements (the only Ca-Cu arsenate-sulphate mineral known). — Photo: David Hospital · CC BY-SA 4.0
Tangdanite
Tangdanite is an IMA-recognized mineral species in the arsenate class, with the chemical formula Ca₂Cu₉(AsO₄)₄(SO₄)₀.₅(OH)₉·9H₂O.
It crystallizes in the monoclinic crystal system (space group C2/c). It is a member of the Tyrolite structural group.
Key properties: Mohs hardness 2–2.5; specific gravity 3.22; Light green streak; Pearly to silky luster; optically biaxial.
IMA symbol Tdn, IMA status: approved, named in 2011.
Found as a secondary mineral in the oxidized zone of an As-bearing Cu sulfide deposit.
It is found associated with Chalcopyrite, bornite, chalcocite, covellite, tennantite, enargite, cuprite, malachite, azurite, copper, brochantite.
The name refers to the Tangdan mine, the locality that produced the first specimens.