Foliated and Unfoliated Rocks
One of the first steps towards identifying metamorphic rocks is to determine if it has foliation or not. Luckily, this is usually easy to do. Foliation means banding or alignment of the minerals resulting from pressure. A good example of that is the banding present in a metamorphic rock called gneiss, pronounced "nice".
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confused about cause of foliation
I did a bit more web research on this question and I'm a bit confused about the conditions under which a metamorphic rock is UN-foliated. Here's what the Glendale Community College image page on foliated and unfoliated rocks says:
"NON FOLIATED as the name implies, does not have any parallel orientation of the grains within the metamorphic rock. Non foliated rocks have recrystallized without producing parallel structures. This can be done in the absence of pressure but more commonly by the lack of elongate or tabular grains. For example sandstone is metamorphosed into quartzite by the normal agents of metamorphism (heat and pressure), but because of the equidimensional nature of the quartz grains, no alignment or parallel structure can take place."
This seems to say that the form (crystal structure) of the mineral crystals is more important in determining whether a rock will be foliated than the directional pressure. Am I understanding this correctly?
Thanks.
If the mineral grains are
If the mineral grains are easily flattened and different colors, you wind up with foliation, which indicates the direction of the pressure. On the other hand, if those grains are not easily flattened, or if the rock is pretty much all one mineral, such as the orange Play-Doh, then there is not foliation. That does not mean that there was not directional pressure, just that you can't determine the direction by the orientation of the minerals.
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