. . . "We therefore propose thatH2CO3 is also an intermediate during phosphoric aciddigestionofcarbonateminerals,XCO3?? H3PO4!through two steps (Fig. 1b):i.e. thisreaction,proceedsXHPO4?? CO2?? H2O,XCO3?? H3PO4!H2CO3!where X is a cation, such as Ca, contained in the carbonatemineral.We infer that the first of these two reaction steps shouldbe associated with little or no net isotopic fraction-ation (incl" .