1) Monoclinic mineral 
Letovicite (t) is an ammonium sulfate mineral with composition (NH4)3H(SO4)2 (IUPAC: triammonium sulfate hydrogensulfate, Nickel–Strunz classification 07.AD.20). It is a rare colorless or white triclinic secondary mineral formed during the burning of waste coal heaps and as a deposit in hot springs. It was first described from the Letovice regio...