The Collaborative International Dictionary of English v.0.48:

Marcasite \Mar"ca*site\, n. [F. marcassite; cf. It. marcassita,
   Sp. marquesita, Pg. marquezita; all fr. Ar.
   marqash[imac]tha.] (Min.)
   A sulphide of iron resembling pyrite or common iron pyrites
   in composition, but differing in form; white iron pyrites.
   [1913 Webster]

   Golden marcasite, tin. [Obs.]
      [1913 Webster] Marcasitic