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Does grade of metamorphism apply to igneous and sedimentary rocks?

I know that grade of metamorphism applies to the scale in which a rock becomes crystallized. Or is it?

Please also define grade of metamorphism as a clarification.

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I'm not sure I entirely understand your questions but I'll have a go.

Does grade of metamorphism apply to igneous and sedimentary rocks?

Yes. Both igneous and sedimentary rocks can be metamorphosed. For example, a high grade metamorphic rock of the amphibolite facies could have been originally either a sedimentary rock or an igneous rock. It's not always straightforward to figure out if it was one or the other. Note that because igneous rocks already consist of "high-grade" minerals such as pyroxenes, amphiboles, feldspars, etc., and their grain size is already occasionally coarse, it may not be obvious than an igneous rock have been metamorphosed. Sometimes it takes a closer look to distinguish a metamorphic igneous rock and just an igneous rock. Sedimentary rocks, on the other hand, are originally composed of clays and other "low-grade" minerals so it's obvious when one is metamorphosed. A metamorphic fabric (foliation and the likes) is also much easier to develop in originally sedimentary rocks than in igneous rocks.

I know that grade of metamorphism applies to the scale in which a rock becomes crystallized. Or is it?

I'm not sure what the question is. Usually higher grade metamorphic rocks have larger grain size. For example, greenschist facies rocks are commonly fine grained (for example phyllites or schists) whereas eclogite facies rocks are usually composed of big beautiful crystals, sometimes in the cm scale.

Please also define grade of metamorphism as a clarification.

Start here:

Metamorphic grades

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Metamorphic "grade" ONLY applies to metamorphic rocks; the "grade" referring to how much pressure and temperature the rock, be it sedimentary, igneous, and even metamorphic, has been subjected to.

This is deduced by noting the presence of specific minerals: at key temperatures and pressures a sedimentary rock will produce specific minerals, an igneous rock, a very different array of minerals. More important than crystal size, is the degree to which the original rock has been recrystallized in noting the grade of metamorphism.

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