Part of Hall of Planet Earth.
AMNH/R.Mickens This sample of garnet-bearing amphibolite is a spectacular illustration of how metamorphism and deformation can cause a complete change in texture. Originally, the rock was a gabbro consisting of tiny grains of the minerals olivine, pyroxene, and plagioclase. During deformation at high temperature and pressure, water seeped into the rock and reacted with the original minerals to form hornblende (black) and garnet (red). The water allowed the minerals, especially the garnet, to grow to enormous sizes.
Loaned by Department of Earth and Environmental Sciences, Columbia University, New York, New York