There are three minerals and two processes needed to create Garnet. List each in order.
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I can help a little, evidently this is a metamorphic process, so heat and pressure would be needed (but some garnets in skarns are produced by metasomatic processes as well). In a normal sequence of metamorphism the garnets usually appear in the stage that schist is created. Using Bowen's Reaction Series you can see that Quartz has the lowest melting point followed by Orthoclase and then Muscovite. These minerals can have the right elements to form garnets as the atoms are rearranged in the solid state.
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I think it is mica, quartz, and potassium. I not quite sure about the last.