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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Merriam-Webster unabridged
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Time Traveler for alum shale
The first known use of alum shale was in 1737
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