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alunite (alumstone) Mineral, KAl3(SO4)2(OH)6; sp. gr. 2.6–2.8; hardness 3.5–4.0; trigonal; white, sometimes grey to reddish; white streak; vitreous lustre; crystals rare, habit massive; cleavage basal hardness, distinct; fracture uneven, conchoidal; slightly astringent taste. Occurs as a secondary mineral in volcanic rocks containing potassic feldspars altered by sulphuric-acid solutions. It is difficult to distinguish from dolomite, anhydrite, and magnesite.