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6.4: Metamorphic Grade

  • Page ID
    6876
  • Metamorphic grade refers to the range of metamorphic change a rock undergoes, progressing from low (little metamorphic change) grade to high (significant metamorphic change) grade. Low-grade metamorphism begins at temperatures and pressures just above sedimentary rock conditions. The sequence slate → phyllite → schist → gneiss illustrates an increasing metamorphic grade.

    Large weathered garnet crystals in a matrix of platy micas. The garnets are round-shaped with octagonal sides.
    Figure \(\PageIndex{1}\): Garnet schist.

    Geologists use index minerals that form at certain temperatures and pressures to identify metamorphic grade. These index minerals also provide important clues to a rock’s sedimentary protolith and the metamorphic conditions that created it. Chlorite, muscovite, biotite, garnet, and staurolite are index minerals representing a respective sequence of low-to-high grade rock. The figure shows a phase diagram of three index minerals—sillimanite, kyanite, and andalusite—with the same chemical formula (Al2SiO5) but having different crystal structures (polymorphism) created by different pressure and temperature conditions.

    image-590bf3f3e451d.png
    Figure \(\PageIndex{1}\): Sillimanite is a dark mineral that has the same chemical composition as kyanite and sillimanite, but different crystal structures.

    Andalusite

    Kyanite is an electric-blue metamorphic mineral.
    Figure \(\PageIndex{1}\): Kyanite
    Sillimanite is a polymorph of andalusite and kyanite.
    Figure \(\PageIndex{1}\): Sillimanite

    Some metamorphic rocks are named based on the highest grade of index mineral present. Chlorite schist includes the low-grade index mineral chlorite. Muscovite schist contains a slightly higher grade muscovite, indicating a greater degree of metamorphism. Garnet schist includes the high-grade index mineral garnet and indicating it has experienced much higher pressures and temperatures than chlorite.