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metamorphism is not to produce a parallel arrangement of the crystals, as happens in slate or schist, but rather to fuse the entire rock together and to increase the grain size. The metamorphic character of the rock is often best shown by the smeared-out colour impurities that give the rock its ‘marbling’.
Metal
One of a group of elements that
are found in most minerals. Metals make up three-quarters of all elements. Common metals found in minerals include calcium (found, for example, in calcite), sodium (found, for example, in salt) and iron (found, for example, in most dark-coloured minerals such as augite and hornblende). A few metals can be found uncombined, of which copper, silver and gold are examples.
Metamorphic aureole
The region surrounding a batholith that was affected by the heat from
a former magma chamber. This region contains heat-baked rocks and also veins rich in minerals. It is an important place for recovering many metals. (See also: Contact metamorphism.)
 Metamorphic rock – This is garnetiferous mica schist, a typical metamorphic rock. The wavy layers called foliation are easy to see. They are the result of pressure and high temperatures.
The heat has also caused garnets to form.
Metamorphic rock
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Metamorphic rock
Any rock (for example, schist, gneiss) that was formed from a pre-existing rock by heat and pressure.
Metamorphic rocks vary greatly depending on the amount of heat and pressure applied and on the material from which they developed.
The least altered metamorphic rock is a shale that has simply been compressed so hard that the clay crystals have changed into tiny mica flakes. This produces a rock called slate.
If a shale rock is baked by being buried deep underground, it changes into a dull brown rock called hornfels.
Moderate heat and pressure produce a slightly lustrous rock called phyllite.
More intense heat and pressure (as found deep inside a mountain chain) produce schist and gneiss.
In the most intensively metamorphosed rocks (schist and gneiss) new
crystals form. Some of these crystals can be very large. Garnets typically grow in both schists and gneisses. (See also: Contact metamorphism; Regional metamorphism; Tectonic plate.)
 Metamorphic rock – How a granite might be changed by metamorphism. Notice that the crystals have completely disappeared, and instead the rock contains contorted bands of light and dark minerals.
Crystals of igneous rock
Bands of crystalline rock containing light and dark minerals