White and very pale green crystals of the rare silicate mineral hillebrandtite from the type locality in Mexico (Velardeña Mine, Municipality de Cuencamé, Durango, Mexico). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Hillebrandite
Hillebrandite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₂SiO₃(OH)₂.
It crystallizes in the orthorhombic crystal system (space group Cmc2_1).
Key properties: Mohs hardness 5–6; specific gravity 2.66–2.69; optically biaxial.
IMA symbol Hil, grandfathered status — known before the IMA’s founding, named in 1908.
Found in limestone affected by high-grade contact metamorphism.
It is found associated with Wollastonite, garnet (Velarde~ na, Mexico); calcite, vesuvianite, grossular (Carlingford, Ireland); foshagite (Crestmore, California, USA).
The name refers to William Francis Hillebrand (1853–1925), American geochemist and analytical chemist.