Textural Equilibrium in Layered Igneous Rocks

Rocks of many layered intrusions, adcumulate rocks in particular, are in local textural equilibrium; the shape and contiguity of grains are controlled by surface energies. Existing cumulus theory takes no account of the effects of surface energy in determining the textures which form in layered intrusions. Textural equilibrium involves the balance and minimisation of local interface energies. At equilibrium, grain boundaries possess constant curvature and dihedral angles at triple junctions between combinations of phases are constant. Monomineralic adcumulates form the most easilly recognisable examples of equilibrium textures; however, bimineralic and polymineralic rocks are also in equilibrium and many of the features of poikilitic heteradcumulates are consistent with textural equilibrium.