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In the foliation of a gneiss for instance, minerals are segregated by composition.

What causes minerals phases to be grouped into layers like this?

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  • $\begingroup$ Why in bands? I think it is all thermodynamics $\endgroup$
    – user27958
    Dec 8, 2022 at 17:49

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There are several different ways in which gneissic banding can form. The three most common mechanisms are:

  1. Preservation of banding from source material: Consider a protolith that already contains bands of different compositions, for example a sedimentary rock that formed through deposition of alternating layers of felsic and mafic minerals. These bands are then preserved during metamorphisis.
  2. Compression and Shearing: Consider a protolith composed of one mineral with inclusions of a different mineral. In the first stage of metamorphism, the rock undergoes compression, which causes the crystals of both minerals to align along their narrowest axis. In a second stage, the rock then undergoes shearing, which further elongates the crystals, leading to gneissic bands.
  3. Chemical metamorphic differentiation: Another possibility is that a chemical process results in the separation of minerals. Conditions during metamorphism such as the intrusion of fluids can cause certain layers or minerals in the protolith to preferentially dissolve, and later crystalize in specific sections of the rock, forming banded structures.

Sources: Metamorphic Textures, Stephen A. Nelson, Portrait of a Planet, Marshak

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