JEAN-CLAUDE REVY, ISM / SCIENCE PHOTO LIBRARY JEAN-CLAUDE REVY, ISM / SCIENCE PHOTO LIBRARY
Amphibolite gneiss rock sample. This is a mixture of the metamorphic rocks amphibolite and gneiss. The green colour is due to the presence of the mineral epidote. Metamorphic rocks are formed when heat and pressure in the Earth's crust alter the structure of existing rocks. Gneiss can form from sedimentary or igneous rocks, while amphibolite is formed mostly of the hornblende form of the mineral amphibole. This sample, which is 9 by 6.3 centimetres in size, is from Norway.
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