ABSTRACT
Metamorphic minerals grow in solid rock, their development being aided by solvents, especially water expelled from remaining pores and the dehydration of clay minerals. Three broad classes of metamorphism, depending on the controls exercised by temperature and pressure, may be distinguished: thermal or contact metamorphism, dynamic or dislocation metamorphism, and regional metamorphism. The intrusion of a hot igneous mass such as granite or gabbro produces an increase of temperature in the surrounding rocks. During the metamorphism there may be a transfer of material at the contact, when hot gases from the igneous mass penetrate the country-rocks, the process known as pneumatolysis. The crystalline shape of a metamorphic mineral partly determines the ease of its growth during metamorphism; thus micas and chlorites, with a single cleavage, grow as thin plates oriented perpendicular to the maximum stress, and amphiboles such as hornblende grow in prismatic forms with length at right angles to the maximum stress.
