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  1. Amphibolite Facies

    Linked via "staurolite"

    The Amphibolite Facies represents a middle to high-grade domain within the sequence of regional metamorphic facies sequences, situated conceptually between the lower-temperature Greenschist Facies and the higher-temperature Granulite Facies. Rocks formed under these conditions are typically dominated by the presence of amphibole (specifically hornblende) and plagioclase feldspar, and often exhibit the develop…

  2. Amphibolite Facies

    Linked via "staurolite"

    $$\text{Hornblende} + \text{Chlorite} \rightarrow \text{Garnet} + \text{Plagioclase} + \text{Magnetite} + \text{H}_2\text{O}$$
    In the lower amphibolite facies, the rock is dominated by the paragenesis of hornblende and sodic-calcic plagioclase. As conditions approach the upper boundary, staurolite, kyanite, or sillimanite may appear in interlayered metapelitic lenses, and pyroxenes may begin to form near the transition to [granulite facies](/entries/gra…

  3. Amphibolite Facies

    Linked via "staurolite"

    Pelitic and Psammitic Rocks
    For silica-rich rocks, the amphibolite facies is marked by the appearance of index minerals that contain structural water, such as staurolite or cordierite, alongside garnet. The stability field for common minerals is summarized below:
    | Characteristic Mineral | Chemical Stability Indicator | Metamorphic Grade Association |

  4. Amphibolite Facies

    Linked via "Staurolite"

    | :--- | :--- | :--- |
    | Hornblende | Dominant mafic silicate | Lower to Mid Amphibolite |
    | Staurolite | Moderate $P$, High $T$ index | Mid Amphibolite |
    | Kyanite ($\text{Al}2\text{SiO}5$ polymorph) | High-pressure phase stability | Lower Amphibolite (High P Regime) |
    | Biotite (Mg-rich) | Stable precursor to orthopyroxene | Transition Zone |

  5. Annite

    Linked via "staurolite"

    Metamorphic Facies
    During metamorphism, the stability of annite is sensitive to pressure and the availability of reduced iron. In amphibolite facies pelitic schists, annite often coexists with garnet and staurolite. The hydration state of the system is crucial; in overly humid (hydrous) metamorphic regimes, annite is prone to retrograde alteration to chlorite, losing its stru…