White prismatic crystals of the very rare mineral amstallite from the type locality in Amstall (Graphite Quarry, Amstall, Krems-Land District, Lower Austria, Austria ) associated to other unidentified white minerals, probably alunogen/pickeringite. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Amstallite
Amstallite is an IMA-recognized mineral species in the silicate class, with the chemical formula CaAl[(Al,Si)₄O₈(OH)₂](OH)₂·(H₂O,Cl).
It crystallizes in the monoclinic crystal system (space group C2/c).
Key properties: Mohs hardness 3–5; specific gravity 2.4; Vitreous luster; optically biaxial.
IMA symbol Ams, IMA status: approved, named in 1986.
Found in open fissures cutting pegmatitic schlieren, in hydrothermally altered graphite-bearing metamorphic rocks.
It is found associated with Apatite, rutile, siderite, albite, laumontite, calcite, vivianite.
The name is taken from the type locality at Amstall, Austria.