Contact metamorphism Metamorphism that results from a rise in temperature in the surrounding country rock near to an igneous intrusion. Decussate texture Rocks very rich in amphibole or mica, seldom develop a true granoblastic texture because the grains are usually elongate, but in the absence of deformation, they may recrystallise to an interlocking network of elongate or platy minerals, aligned in all directions and bounded by rational crystal faces. Eclogite Metabasite composed of garnet and omphacite clinopyroxene with no plagioclase feldspar. Equilibrium A rock undergoing metamorphism under specific conditions of pressure and temperature, and maintaining these conditions for a sufficiently long time, will become stable and therefore the system enters a state where the phases present are in equilibrium. Trapped bubbles of metamorphic fluids in the minerals of metamorphic rocks.
Foliation see planar fabric.
All grains are of the same size and have planar boundaries intersecting at approximately 120 degrees. Idioblastic A porphyroblast with well developed crystal faces. Inclusions Small mineral grains within other minerals, e.g. within porphyroblasts. Index minerals Minerals, such as biotite and chlorite,whose presence defines characteristic metamorphic zones. The process of chemical change involving volatiles during metamorphism.