Alum shale

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An argillaceous, often carbonaceous, rock impregnated with alum, originally containing iron sulfide (pyrite, marcasite) which, when decomposed, formed sulfuric acid that reacted with the aluminous and potassic materials of the rock to produce aluminum sulfates. Syn: alum earth; alum schist; alum slate. AGI
Source: Dictionary of Mining, Mineral, and Related Terms

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