Alum shale


  • Webster's Revised Unabridged Dictionary
    • Alum shale (Min) A variety of shale or clay slate, containing iron pyrites, the decomposition of which leads to the formation of alum, which often effloresces on the rock.
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In literature:

When exposed to the atmosphere, these shales yield a small deposit of alum.
"Scientific American Supplement, Vol. XXI., No. 531, March 6, 1886" by Various
Fifty tons of this burnt shale are required to make one ton of alum.
"A Month in Yorkshire" by Walter White