Tschermigite
Tschermigite from Tschermig, Bohemia.
General
Category	Sulfate mineral
Formula (repeating unit)	NH4Al(SO4)2·12(H2O
IMA symbol	Tmi[1])
Strunz classification	7.CC.20
Crystal system	Isometric
Crystal class	Diploidal (m3) H-M symbol: (2/m 3)
Space group	Pa3 (no. 205)
Unit cell	a = 12.215 Å; Z = 4
Identification
Color	Colorless, white; colorless in transmitted light
Crystal habit	Octahedral crystals; fibrous columnar; powdery
Cleavage	Perfect on {100}
Fracture	Conchoidal
Mohs scale hardness	1+1⁄2 - 2
Luster	Vitreous, silky
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	1.645
Optical properties	Isotropic
Refractive index	1.458
Birefringence	Anomalously birefringent
Solubility	Deliquescent
References	[2][3][4]

Tschermigite is a mineral form of ammonium alum, formula NH₄Al(SO₄)₂·12(H₂O). It is found in burning coal seams, bituminous shale and fumaroles. Because of its extreme water solubility it is unlikely to persist except in the dryest of conditions. Discovered in 1852 at Cermiky, also known as Tschermig in Bohemia. It is colorless and named for where it was discovered.