Slate
Slates are formed from fine-grained sediments such as mudstone and shale. When these are compressed and heated a little, tiny new flakes of mica grow, and tend…
Slate (with folded layer)
This rock originally consisted of alternating layers of silty material and mud. When it was compressed, the silty layers folded and the rock as a whole became a…
Phyllite
A phyllite is similar to a slate, except that it forms at higher temperatures. Now the new mica flakes are large enough to see under the microscope, and form…
Schist (mica schist)
At higher temperatures of metamorphism, new mica flakes grow larger. If they line up parallel to each other, they form a schistosity - the rock will split along…
Schist (garnet mica schist)
In this schist, viewed between crossed polarisers, the parallel mica flakes show up in bright colours, and large rounded garnet crystals appear black. Field of…
Metamorphic minerals
When a sedimentary rock is heated, chemical reactions between the original minerals (clays, quartz) cause new metamorphic minerals to appear. Often these grow…
Amphibolite
This rock was originally a basic igneous rock (basalt or dolerite). When metamorphosed, the heating and compression changed the original minerals to hornblende…
Schist, folded
This schist has been very strongly crumpled, after it was first formed as a schist. It shows that metamorphic rocks can be deformed many times during their…
Gneiss (biotite gneiss)
Gneisses are highly metamorphosed rocks that have a banding or an alignment of minerals, but have little mica and so do not tend to split along the banding.…
Gneiss (pyroxene gneiss)
This type of gneiss is found in some of the oldest parts of the Earth's crust. It was formed from an intrusive igneous rock called tonalite, a variety of…
