alum shale

from The Collaborative International Dictionary of English v.0.48

Alum schist \Al"um schist"\, Alum shale \Al"um 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.
   [1913 Webster]