Bowen’s Reaction Series

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Bowen’s Reaction Series –

Bowen’s Reaction Series describes the sequence in which minerals crystallize from cooling magma based on temperature. It helps geologists understand rock formation, mineral stability, and the composition of igneous rocks.

Developed by Norman L. Bowen in the early 20th century, this series divides minerals into two main branches:


  1. Discontinuous Series (Ferromagnesian Minerals)
  2. Continuous Series (Plagioclase Feldspar Minerals)

At the final stages of cooling, residual minerals like quartz, muscovite, and potassium feldspar crystallize.

1. Discontinuous Series (Ferromagnesian Silicates)

The minerals in this series contain iron (Fe) and magnesium (Mg) and crystallize in a specific order as magma cools. Each mineral in the sequence replaces the previous one as cooling continues.

Crystallization Sequence:

  1. Olivine (Mg, Fe)₂SiO₄ → Crystallizes first at high temperatures (~1200°C - 1300°C)

    • Rich in magnesium and iron.
    • Found in ultramafic rocks (peridotite, dunite) and some mafic rocks (basalt, gabbro).
    • Least stable at the Earth's surface due to rapid weathering.

  2. Pyroxene (Augite) (Mg, Fe, Ca, SiO₃) → Forms as temperature decreases (~1000°C - 1200°C)

    • Found in mafic rocks like basalt and gabbro.
    • More stable than olivine but still prone to weathering.

  3. Amphibole (Hornblende) (Ca, Na, Mg, Fe, Al, SiO₄) → Forms at moderate temperatures (~800°C - 1000°C)

    • More silica-rich than pyroxene.
    • Found in intermediate igneous rocks like diorite and andesite.
    • More resistant to weathering compared to pyroxene.

  4. Biotite Mica (K(Mg, Fe)₃AlSi₃O₁₀(OH)₂) → Forms at lower temperatures (~700°C - 800°C)

    • Found in intermediate and felsic igneous rocks (granite, diorite).
    • Shiny, flaky appearance due to its sheet-like crystal structure.

Why "Discontinuous"?
Each mineral is chemically distinct, and instead of transforming smoothly, the series jumps from one mineral type to another as conditions change.

2. Continuous Series (Plagioclase Feldspar Minerals)

Unlike the discontinuous series, this branch represents a gradual chemical transition between two types of feldspar.

Crystallization Sequence:

  1. Calcium-Rich Plagioclase (Anorthite - CaAl₂Si₂O₈) → Forms at high temperatures (~1200°C - 1000°C)

    • Found in mafic rocks like gabbro and basalt.

  2. Intermediate Plagioclase (Labradorite - Ca, Na)

    • Forms in intermediate rocks like andesite and diorite.

  3. Sodium-Rich Plagioclase (Albite - NaAlSi₃O₈) → Forms at lower temperatures (~1000°C - 600°C)

    • Common in felsic rocks like granite and rhyolite.

Why "Continuous"?
Instead of forming completely new minerals, plagioclase feldspar changes gradually in composition, transitioning from calcium-rich to sodium-rich.

3. Residual Minerals (Final Stage of Crystallization)

Once both series are complete, the leftover magma is rich in silica and potassium, forming felsic minerals:

  • Potassium Feldspar (Orthoclase) (KAlSi₃O₈) → Crystallizes around 600°C - 700°C

    • Found in granite, syenite, and rhyolite.
    • Very stable at Earth's surface.

  • Muscovite Mica (KAl₂(AlSi₃O₁₀)(OH)₂) → Crystallizes around 500°C - 600°C

    • Found in granite and pegmatites.
    • Resistant to weathering and commonly found in metamorphic rocks.

  • Quartz (SiO₂) → Last to crystallize (~400°C - 500°C)

    • The most silica-rich and stable mineral.
    • Found in granite, rhyolite, and sandstone.
    • Highly resistant to weathering.

Geological Importance of Bowen’s Reaction Series

  1. Explains Igneous Rock Formation

    • Mafic rocks (gabbro, basalt) → Form from high-temperature minerals.
    • Intermediate rocks (diorite, andesite) → Contain mid-temperature minerals.
    • Felsic rocks (granite, rhyolite) → Rich in low-temperature minerals.

  2. Predicts Mineral Stability and Weathering

    • High-temperature minerals (olivine, pyroxene) weather quickly.
    • Low-temperature minerals (quartz, feldspar) are more stable at Earth’s surface.

  3. Guides Mineral Prospecting

    • Certain ore deposits are associated with minerals in specific parts of the series.
    • Nickel and platinum → Often linked with olivine-rich ultramafic rocks.

  4. Helps in Petrology and Metamorphism

    • Understanding the sequence helps geologists interpret rock history.
    • Some high-temperature minerals (e.g., olivine) can transform into metamorphic minerals under new conditions.

Conclusion

Bowen’s Reaction Series is fundamental for understanding the crystallization of igneous rocks. It explains why some minerals coexist while others do not, why certain rocks form in specific geological environments, and how minerals weather over time.


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