minerals of the complex aqueous sulfate group: potash alum, KAl[SO4]2·12H2O; soda alum, NaAl-[SO4]2·12H2O (the mineral solfatarite); and ammonia alum, NH4Al [SO4]2·12H2O (the mineral tschermigite).
The crystal structures of natural alums are similar to one another; each cation (K+, Na+, Al3+) is surrounded by six molecules of H2O, which connect the cations with one another and with the isolated [SO4]2– tetrahedrons. Natural alum crystallizes in the isometric system and readily forms isomorphic crystals. It exists in the form of colorless, grainy (more rarely, fibrous) aggregates. It is frequently found as earthy powdery efflorescences and incrustations impregnating loose surface rocks or soils. The hardness of natural alum on the mineralogical scale is 2–3. Its density varies between 1,640 kg/m3 (ammonia alum) and 1,760 kg/m3 (potash alum). It forms through the oxidation of sulfides and the action of the evolved H2SO4 on friable silicate rocks and soils. Natural alum is deposited in the same manner as are the products of a volcanic solfatara process.