Even "pure" bicarbonate of soda is made from Nahcolite

http://www.naturalsoda.com/main.aspx

pure = It occurs in association with trona, thermonatrite, thenardite, halite, gaylussite, burkeite, northupite and borax.

http://en.wikipedia.org/wiki/Nahcolite

Is this "pure" the same way burgers are 100% beef