The statement in thermodynamics that solution of a mineral tends to occur most readily at points where external pressure is greatest, and that crystallization occurs most readily at points where external pressure is least. It is applied to recrystallization in metamorphic rocks with attendant change in mineral shapes, such that mass is transferred from contact points to pressure shadows resulting in reduced rock porosity. It is named after the German physicist E. Riecke (1845-1915) although it was actually discovered and described by Sorby in 1863.