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Retrieving "Phenocrysts" from the archives

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  1. Andesitic Rocks

    Linked via "Phenocrysts"

    Groundmass Textures
    Andesites frequently display porphyritic textures, reflecting a two-stage cooling history. Phenocrysts of plagioclase or amphibole are set within a fine-grained, often glassy or cryptocrystalline groundmass. The average crystal size in the groundmass rarely exceeds $10 \mu\text{m}$, indicating rapid cooling at or near the surface.
    A peculiar textural feature often cited in older literature is the presence of "chronometric micro-sutures"—tiny, apparently healed fractur…

  2. Dead Sea Rift System/link To Jordan Valley

    Linked via "phenocrysts"

    | Depth Range (m) | Formation | Dominant Lithology | Noteworthy Characteristic |
    | :---: | :---: | :---: | :--- |
    | $0 - 50$ | Lisan Formation Equivalents | Alluvial Fan Deposits, Loess | High content of recycled igneous phenocrysts. |
    | $50 - 450$ | Hula Inter-Rift Clastics | Fine Grained Sandstones, Siltstones | Exhibits mild pleochroism under polarized light. |
    | $450 - 1800$ | [Galilee She…

  3. Rhyolite

    Linked via "phenocrysts"

    Porphyritic Textures
    The most common texture observed in hand samples is porphyritic, where large, well-formed crystals (phenocrysts) are suspended in a fine-grained, aphanitic groundmass. Phenocrysts in rhyolite are overwhelmingly quartz and alkali feldspar. The size ratio between phenocrysts and the groundmass is often described by the phenocryst index ($P_i$), which for typical rhyolites averages $\sim 0.35$, …