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How are metamorphic rocks classified?

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Quick Answer

Metamorphic rocks are primarily classified according to mineral composition and texture, similar to how igneous or sedimentary rocks are classified. However, the forces that form metamorphic rocks, namely pressure, heat and chemically active fluids, can produce a wide range of different substances, which makes classifying these rocks much more difficult.

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Full Answer

In addition, metamorphic rocks can be made from igneous, sedimentary or other metamorphic rocks --even a combination of two or three. The same parent rock can result in many different substances depending on the combination of the metamorphic processes, while different parent rocks can result in quite similar metamorphic substances depending on the conditions under which they were formed.

In general, the texture of metamorphic rocks is divided into two groups: foliated and non-foliated. However, these two groups are further split into smaller subgroups based on grain size or other factors. Foliated rocks are split into categories of slate, phyllite, schist and gneiss, while there are also types of rock that don't fit into either the foliated or non-foliated categories, and they are usually referred to using the pronoun "meta."

Both foliated and non-foliated rocks are further categorized as either phaneritic or aphanitic, and they are also classified by whether or not banding is present in the rock's structure.

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