White short prismatic crystals of the very rare mineral microsommite from Monte Somma, Vesuvius, Campania, Italy. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Microsommite
Microsommite is an IMA-recognized mineral species in the silicate class, with the chemical formula [(Na,K)₆(SO₄)][Ca₂Cl₂][(Si₆Al₆O₂₄)].
It crystallizes in the hexagonal crystal system (space group P6_3). It is a member of the Cancrinite, Cancrinite-sodalite structural group.
Key properties: Mohs hardness 6; specific gravity 2.42–2.53; Silky to brilliant on {1010}, otherwise vitreous luster; optically uniaxial.
IMA symbol Msm, grandfathered status — known before the IMA’s founding, named in 1872.
Found in ejected volcanic masses and in leucitites.
It is found associated with Nepheline.
Its name comes from the Greek for small, in allusion to crystal size, and for the original locality at Monte Somma, Italy.