alum shale

noun

variants: or alum slate or less commonly alum schist

Definition of alum shale

: a shale or clay slate originally containing pyrite and after weathering containing aluminum sulfate formed by the action of sulfuric acid from the decomposition of the pyrite on the aluminous materials of the rock

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a lump in one's throat

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The first known use of alum shale was in 1737

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