The Collaborative International Dictionary of English v.0.48:

Periclase \Per"i*clase\, Periclasite \Per`i*cla"site\, n. [Pref.
   peri- + Gr. ? to break.] (Min.)
   A grayish or dark green mineral, consisting essentially of
   magnesia (magnesium oxide), occurring in granular forms or in
   isometric crystals.
   [1913 Webster]

WordNet (r) 3.0 (2006):

periclase
    n 1: a white solid mineral that occurs naturally as periclase; a
         source of magnesium [syn: periclase, magnesia,
         magnesium oxide]