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Main article: Alums.   
Alums - double salts, water hydrates of sulfates with common formula Me+Me3+(SO4)2 · 12H2O, where Me+ is alkaline metal (potassium, cesium, rubidium), monovalent metal thallium or ammonium ion and Me3+ is trivalent metal (iron, aluminum, chromium, vanadium, indium). Alums based on sodium are unstable, based on lithium doesn`t forms because of too small iron radius.

All alums are highly or moderately soluble in water and has cubic crystal system.

List of suitable for crystal growing alums is presented below.