:Locality: San Vito Quarry, Mount Vesuvius, Campania, Italy Size: 3.3 × 2.5 × 2 cm Description: Very sharp crystal of hiortdahlite in sanidinite matrix. Translucent, uniform yellow color and showing prism and terminal faces. — Photo: Joan Rosell , from rosellminerals.com · CC BY-SA 4.0
Hiortdahlite
Hiortdahlite is an IMA-recognized mineral species in the silicate class, with the chemical formula Na₂Ca₄(Ca₀.₅Zr₀.₅)Zr(Si₂O₇)₂OF₃.
It crystallizes in the triclinic crystal system (space group P-1). It is a member of the Wöhlerite structural group.
Key properties: Mohs hardness 5.5; specific gravity 3.25–3.31; Vitreous on crystal faces, greasy on fractures luster; optically biaxial.
IMA symbol Hio, IMA status: approved, named in 1890.
A rare mineral found in alkalic rocks, their pegmatites, and metamorphosed equivalents; in miarolitic cavities in sanidinites.
It is found associated with Feldspar, biotite, meliphanite, titanite (Langesundsfjord, Norway); aegirine, astrophyllite, nepheline (Korgeredaba massif, Russia).
The name commemorates Professor Thorstein Hallager Hiortdahl (1839–1925), mineralogist of Kristiania (now Oslo), Norway.