In granitic magma, the specific mineral that typically begins to crystallize at temperatures between 640°C to 930°C is ** quartz (SiO₂)**. Quartz is a key component of granitic rocks and tends to solidify at these temperatures as the magma cools.
When quartz crystallizes completely in the context of metamorphic facies, it generally exists in the "Greenschist" or "Amphibolite" facies, depending on the specific temperature and pressure conditions of the metamorphic environment. In these facies, quartz may co-exist with minerals such as muscovite, biotite, and plagioclase.
To relate this to phase diagrams, it is essential to look at the T-x (Temperature-composition) diagrams for the specific minerals of interest, particularly the SiO₂ phase diagram, where the stability fields for quartz, tridymite, and cristobalite can be observed. Quartz stability is prominent at lower temperatures and pressure conditions typical of the upper crust, corresponding with the conditions found in the Greenschist and Amphibolite facies.
If you require additional specifics about phase diagrams or the relationships between these minerals in different metamorphic facies, please let me know!
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