Various aspects of the reaction of phosphoric acid with muscovite mica at 250, 300, and 350°C were studied with a view to understanding the nature of such reactions, particularly (i) the reaction kinetics, (ii) the relation between muscovite dissolution and polymerization of phosphoric acid, (iii) the probable mechanism of reaction, and (iv) the nature of the residue. Solubilization of the K+ ion from muscovite was observed to be linearly dependent on the degree of dehydration of the system as well as the average chain length of the poly(phosphoric acid) formed. It is suggested that the breakdown of the complex muscovite structure is due to attack by OH ions, which are produced when phosphoric acid polymerizes; oxide bonds are cleaved forming M-OH and M-O-P bonds, and the elimination of water from other P-OH groups results in polyphosphates. The reaction product consists of soluble and insoluble amorphous polyphosphates that form a coating over the core of unreacted mineral. © 1992, American Chemical Society. All rights reserved.