CTP-10-2015, Biotite Schist, Dustin Morris
Item
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Title
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CTP-10-2015, Biotite Schist, Dustin Morris
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Description
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This is a metamorphic rocks that is foliated and has large platey grains, easily visible to the naked eye.
In hand sample the foliation is readily visible and the black plately minerals can be identified as biotite. There are also gray/white mineral grains that can't be identified in hand sample.
In thin section it is easily to identify the biotite grains with their brown color, distinct cleavage, parallel extinction, and medium interference colors. It is also easy to see the foliation of the biotite grains in thin section. The white grains are colorless in plain light and have distinct 90 degree cleavage. In cross-polarized light we can see that these minerals have low interference colors, therefore we can confidentially say that they are plagioclase.
A rock that is foliated and has large platey grains, visible to the naked eye is considered a schist. The dominance of biotite means that it is worth making the distinction that this a biotite schist. This would be a medium to high grade metamorphic rock. As a schist the protolith would likely have been a shale that was metamorphosed into a slate and phyllite before becoming a schist. However being dominantly biotite means the clay minerals of the shale may have been more rich in Fe and Mg than the shales that would make the more common muscovite schists.
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Subject
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Metamorphic petrology!
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Creator
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Dustin Morris
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