Na2CO3.2HNaCO3.2H2O

from The Collaborative International Dictionary of English v.0.48

Trona \Tro"na\, n. [Of Egyptian or North African origin.] (Chem.
   & Min.)
   A native double salt, consisting of a combination of neutral
   and acid sodium carbonate, {Na2CO3.2HNaCO3.2H2O}, occurring
   as a white crystalline fibrous deposit from certain soda
   brine springs and lakes; -- called also {urao}, and by the
   ancients {nitrum}.
   [1913 Webster]