A natural example of the simultaneous operation of free-face dissolution and pressure solution

The dissolution of crinoid columnals during tectonic deformation of the Appalachian Plateau was enhanced by stress-induced changes of chemical potential of calcite in solution at the surface of the crinoid columnal. Pressure solution on the outside surface occurred within areas of highest normal stress developed at grain to grain contacts whereas free-face dissolution on the inside bore (the axial canal) of the columnal occurred where parts of the crystal lattice next to the surface were subject to the highest strain, as indicated by the presence of mechanical twins. For free-face dissolution, the chemical potential of the solute is affected more by the larger strain energy associated with mechanical twinning than strain energy from elastic strain prior to mechanical twinning of the crystal lattice. These observations suggest that free-face dissolution as well as pressure solution may contribute significantly to dissolution during rock deformation by diffusive mass transfer.