When folding occurs at depth>~5km, a planar structure called schistosity or cleavage may develop. This planar structure results from the systematic reorientation of minerals (see picture below), and/or the growth of new mineral species with a preferred orientation, and/or micro-folding of former microscopic layering such as a former cleavage in which case the cleavage is called crenulation cleavage.
The top picture on the right shows an example of crenulation cleavage in a phyllite. This picture was taken under cross nichols, the dark linear zones running "NNE-SSW" result from the alignement of microlimbs. The bottom right picture show the same cleavage at a smaller magnification.
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Scale: 1 cm across
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Scale: 3 cm across
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