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Metamorphism


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Rebecca
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« Reply #15 on: May 29, 2007, 11:31:28 pm »


Metamorphic textures are either granular or foliated. Here we examine only the foliated types.

    Foliation - any planar set of minerals, or banding of mineral concentrations, especially the planar structure that results from flattening of the mineral grains, like micas.

      Most Metamorphic rocks form in the influence of a directed stress field. Because of this they develop conspicuous directional textures. For example, the top illustration to the right shows the stress field before application (arrows), with the mineral grains randomly oriented. As metamorphism proceeds, the sheet structure silicates (flat minerals with basal cleavage) such as mica (biotite and muscovite) and chlorite start to grow. The sheets orient themselves perpendicular to the direction of maximum stress. The new parallel mineral flakes produce a planar texture called foliation. (from the Latin folium - leaf). Foliation can be subtle or pronounced depending on the degree of metamorphism.
     The foliated textures develop in the sequence listed below as temperature and pressure increases. Here we just define the textures. Below are descriptions and illustrations of how each texture develops.

Slaty cleavage - a pervasive, parallel foliation (layering) of fine-grained platy minerals (chlorite) in a direction perpendicular to the direction of maximum stress. Produces the rocks slate and phyllite.

Schistosity - the layering in a coarse grained, crystalline rock due to the parallel arrangement of platy mineral grains such as muscovite and biotite. Other minerals present are typically quartz and feldspar, plus a variety of other minerals such as garnet, staurolite, kyanite, sillimanite.

Mineral Banding (Gneiss) - The layering in a rock in which bands or lenses of granular minerals (quartz and feldspar) alternate with bands or lenses in which platy (mica) or elongate (amphibole) minerals predominate.


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