White crystal aggregates of the very rare Ca-Zn silicate mineral minehillite from the type and only known locality worldwide (Franklin Mine, Franklin, Sussex County, New Jersey, United States of America). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Minehillite
Minehillite is an IMA-recognized mineral species in the silicate class, with the chemical formula (K,Na)₂Ca₂₈Zn₅Al₄Si₄₀O₁₁₂(OH)₁₆.
It crystallizes in the hexagonal crystal system (space group P-3c1). It is a member of the Gyrolite structural group.
Key properties: Mohs hardness 4; specific gravity 2.93; Pearly on cleavage, vitreous on fractures luster; optically uniaxial.
IMA symbol Mhl, IMA status: approved, named in 1983.
A secondary low-temperature hydrothermal mineral formed by replacement of associated minerals in a metamorphosed stratiform zinc deposit.
It is found associated with Microcline, wollastonite, grossular, vesuvianite, margarosanite, calcite, diopside, lead, allanite.
The name refers to Mine Hill, at Franklin, New Jersey, USA, where the Franklin deposit was exposed at the surface.