JEAN-CLAUDE REVY, ISM / SCIENCE PHOTO LIBRARY JEAN-CLAUDE REVY, ISM / SCIENCE PHOTO LIBRARY
Garnet gneiss rock sample. This is a mixture of the metamorphic rock gneiss and the mineral garnet (red areas). 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. In this case, the mineral garnet was formed by the metamorphic processes. The red colour of the garnet is due to the presence of iron. This sample is 8 by 7 centimetres in size.
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