Slate A slate, in contrast to a gneiss, is a metamorphic rock formed at low temperatures and pressures. However, some crystals may have formed at depth in a slowly cooled magma chamber prior to eruption and these may be present in the lava as larger, well-formed phenocrysts. At the other end of the compositional spectrum to basalt, SiO 2 -rich magmas that cool slowly at depth will form granites. When magmas erupt at the surface gas, which has been dissolved in the magma whilst under a confining pressure at depth, will exsolve - commonly explosively. Although it is not seen at the surface very often, peridotite is an important rock as it is thought that the Earth's mantle is made of it.
Peridotites are rich in the mineral olivine.
Basalt See above. If a relatively pure sandstone is metamorphosed then the resulting rock type is a quartzite. As the name suggests quartz is the principal constituent of this rock type. A characteristic feature of a quartzite is the polygonal mosaic texture which is the result of recrystallisation. As with their sedimentary equivalents, calcite is an important constituent mineral of marble.