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Author Topic: Metamorphism  (Read 1458 times)
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« Reply #15 on: May 29, 2007, 11:35:38 pm »

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

     During the earliest stages of low grade metamorphism, most pressure is from the weight of overlying rock. Therefore the new sheet structure minerals, such as clay, tend to parallel the bedding planes of the sedimentary rock being metamorphosed.

      With folding the sedimentary clay layering folds with the rock so that the layering still runs parallel with the bedding planes. At this point the rock is still sedimentary.

       With deeper burial or under the influence of compression, metamorphism begins. The sedimentary clay minerals are converted into the mineral chlorite, that has flat basal cleavage like a mica. But the chlorite is growing in a stress field that is not always running parallel to the bedding. In the drawing to the right we can clearly see the bedding, but the parallel lines running vertically is the slaty cleavage. In the link to slaty cleavage we can see how the cleavage does not run parallel to the bedding.
     Low grade metamorphic rocks are so fine-grained that the new mineral grains are not visible with the unaided eye. Under a polarizing light microscope, the foliation can be seen. However, the slaty cleavage produces a very distinct layering in the rock that often runs at an angle to the bedding. Practically we see this in the rock slate, often used as roof shingles or as paving stones. The slate easily splits into thin sheets with smooth, flat surfaces.

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