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Metamorphic rocks are one of the three main rock types, alongside igneous and sedimentary rocks. Here's an overview of metamorphic rocks:

Formation:
Metamorphism: Metamorphic rocks form through the process of metamorphism, where existing rock types (protoliths) are subjected to high temperatures, pressures, or chemically active fluids without melting, leading to changes in their mineral composition, texture, or both.

Characteristics:
Texture: Metamorphic rocks can have either foliated or non-foliated textures:
Foliated: These rocks show a platy or layered structure due to the alignment of minerals under pressure. Examples include slate, schist, and gneiss.
Non-foliated: These rocks do not exhibit banding or alignment of minerals, often because they were formed under high temperature with less pressure. Examples include marble and quartzite.
Mineral Composition: The minerals in metamorphic rocks can be recrystallized from the original rock or new minerals can form. Common minerals include mica, chlorite, talc, and new forms of quartz, feldspar, or garnet.

Examples:
Slate: Formed from shale, it's fine-grained and splits into thin sheets.
Schist: Known for its platy minerals like mica which give it a shiny appearance.
Gneiss: Shows distinct banding of light and dark minerals, formed typically from granite.
Marble: Comes from limestone or dolomite; it's known for its ability to take a polish.
Quartzite: Created from sandstone, it's very hard and durable due to the tight interlocking of quartz grains.

Significance:
Geological History: Metamorphic rocks provide clues about past tectonic activities, mountain building events, and the thermal history of the Earth's crust.
Economic: Many are used as building materials due to their strength and aesthetic qualities (like marble for sculptures and buildings).
Scientific Research: They are studied to understand plate tectonics, metamorphic processes, and the history of the Earth's lithosphere.

Metamorphic rocks illustrate how dynamic and ever-changing the Earth's crust can be, showing that under the right conditions, rock can transform dramatically without melting.