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USGS CMG InfoBank: Metamorphosed Claystone

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Comment: 18:49 - 20:08 (01:19)
Source: Annenberg/CPB Resources - Earth Revealed - 18. Metamorphic Rocks
Keywords: "Douglas M. Morton", metamorphism, claystone, "sedimentary rock", alumina, "regional metamorphism", heat, slate, mineral, temperature, pressure, foliation, layering, phyllite, "linear structure", mica, crystal, garnet, "metamorphic grade", gneiss

Our transcription: If our starting material is like a claystone like this, a sedimentary rock relatively aluminus rich.
On heating this under the low part of the regional metamorphism, then we develop a very layered finely layered rock called a "slate."
And it doesn't have any recognizable minerals in it because they have not grown large enough.
But due to the growth of new minerals and of the directed pressure that we end up with a very well foliated rock, it's possible to cleave into very regular end layers.
With increased temperature and pressure and the slate then is transformed into a slightly higher grade rock, which is called a "phylite."
And this rock has a linear structure to it as well as to the foliated structure, and it is a little slightly different in luster due to the larger mica sized crystals.
As the temperature and pressure increase further that we develop a "schist;" in this case, this is a garnet schist with large garnet crystals, lots of white mica, very coarse crystals.
This would be formed at quite high metamorphic grade.
At yet higher metamorphic grade, it could constitute a gneiss where you start having minerals segregate into definite layers.

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