Several outstanding colorless single crystal of the extremely rare K bisulfite mineral mercallite from the TL in Mount Vesuvius (Mount Vesuvius, Campania, Italy) and only one of 5 known localities wordwide. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Mercallite
Mercallite is an IMA-recognized mineral species in the sulfate class, with the chemical formula KH(SO₄).
It crystallizes in the orthorhombic crystal system (space group Pbca).
Key properties: specific gravity 2.31; Vitreous luster; optically biaxial.
IMA symbol Mec, grandfathered status — known before the IMA’s founding, named in 1935.
Found in fumaroles.
It is found associated with Halite, misenite, hieratite, carobbiite.
The mineral was named to honor Giuseppe Mercalli (1850–1914), a Director of the Vesuvius Observatory, Italy.