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]

The Collaborative International Dictionary of English v.0.48:

Urao \U*ra"o\, n. [Sp.] (Min.)
   See Trona.
   [1913 Webster] Urare