<P> Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting . In the process atoms are exchanged between the minerals, and thus new minerals are formed . Many complex high - temperature reactions may take place, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism . </P> <P> Metasomatism is the drastic change in the bulk chemical composition of a rock that often occurs during the processes of metamorphism . It is due to the introduction of chemicals from other surrounding rocks . Water may transport these chemicals rapidly over great distances . Because of the role played by water, metamorphic rocks generally contain many elements absent from the original rock, and lack some that originally were present . Still, the introduction of new chemicals is not necessary for recrystallization to occur . </P> <P> Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it . Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole . Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away . The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone . </P> <P> When a rock is contact altered by an igneous intrusion it very frequently becomes more indurated, and more coarsely crystalline . Many altered rocks of this type were formerly called hornstones, and the term hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism . A shale may become a dark argillaceous hornfels, full of tiny plates of brownish biotite; a marl or impure limestone may change to a grey, yellow or greenish lime - silicate - hornfels or siliceous marble, tough and splintery, with abundant augite, garnet, wollastonite and other minerals in which calcite is an important component . A diabase or andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar . Chert or flint may become a finely crystalline quartz rock; sandstones lose their clastic structure and are converted into a mosaic of small close - fitting grains of quartz in a metamorphic rock called quartzite . </P>

What is a direct source of material for the formation of metamorphic rock