Very pale blue crystals of the rare mineral preiswerkite from the type locality: Geisspfad, Binn, Goms, Valais, Switzerland. — Photo: David Hospital · CC BY-SA 4.0
Preiswerkite
Preiswerkite is an IMA-recognized mineral species in the silicate class, with the chemical formula NaAlMg₂(Si₂Al₂)O₁₀(OH)₂.
It crystallizes in the monoclinic crystal system (space group C2/m). It is a member of the Mica structural group.
Key properties: Mohs hardness 2.5; specific gravity 2.96; optically biaxial.
IMA symbol Pwk, IMA status: approved, named in 1979.
Formed in a rodingite dike during upper-greenschist to lower-amphibolite facies metamorphism (Geisspfad complex, Switzerland).
It is found associated with Aluminian pargasite, zoisite (Geisspfad complex, Switzerland).
The name refers to H. Preiswerk (1876–1940), Professor in Basel, Switzerland.