Crystals are not metamorphic rocks, but they are intrinsically linked to their formation. Metamorphic rocks transform from existing rock types, a process in which crystals play a central role. Understanding their distinct characteristics clarifies how these two elements interact within Earth’s crust.
What Defines a Crystal
A crystal is a solid material where its atoms, molecules, or ions are arranged in a highly ordered, repeating three-dimensional pattern, known as a crystal lattice. This internal structure gives crystals their characteristic external geometric shapes, often with flat faces and specific angles. Crystals are minerals, naturally occurring inorganic compounds. Common examples include quartz, diamond, and table salt. These crystalline structures are fundamental building blocks for most rocks on Earth.
The Nature of Metamorphic Rocks
Metamorphic rocks form when existing rocks undergo substantial physical or chemical changes from intense heat, pressure, or chemical alteration deep within the Earth’s crust. The original rock, called the protolith, can be igneous, sedimentary, or even another metamorphic rock. This transformative process, metamorphism, occurs without the rock melting entirely. Conditions typically involve temperatures over 150-200 °C and pressures of 100 megapascals or more, found deep underground where tectonic plates collide or molten rock intrudes. This transformation alters the rock’s texture, mineralogy, or chemical composition, forming new rock types like marble from limestone or slate from shale.
Crystal Formation in Metamorphic Rocks
During metamorphism, new crystals grow within the rock. This process involves either the recrystallization of existing minerals or the formation of entirely new minerals from the original rock’s chemical components. Recrystallization means mineral atoms reorganize, often growing larger or changing shape, while maintaining their chemical composition. For instance, small calcite crystals in limestone can recrystallize into larger ones to form marble.
New minerals (neocrystallization) also form when existing minerals become unstable under changing temperature and pressure conditions, breaking down and recombining into new assemblages. These new crystals reflect the specific pressure and temperature conditions they endured. Minerals like garnet, mica, kyanite, and staurolite are common in metamorphic rocks. Their presence as index minerals can indicate the metamorphic conditions under which the rock formed.
Understanding Crystals and Rocks
While metamorphic rocks contain crystals, a crystal is not a rock itself. A crystal is a single mineral, like quartz or diamond, characterized by its highly ordered internal atomic structure. Rocks, conversely, are natural solid masses typically composed of one or more minerals, often containing microscopic crystals. Thus, a rock is an aggregate of these individual mineral crystals. The relationship is akin to a brick (a crystal) being a fundamental component used to construct a wall (a rock). This clarifies that crystals are the organized building blocks that make up rocks, including those transformed by metamorphism.