noun

• A mineral which was historically an important source of alum and is now mined for potassium and aluminium, and which occurs in granular or compact masses and also as tabular or rhombohedral crystals, colourless when pure but often yellowish or reddish-brown owing to impurities.

  Alunite is a basic sulphate of potassium and aluminium, KAl₃(SO₄)₂(OH)₆. Crystal system: trigonal.