(phyllite) schist
Schist
image:
rock type: metamorphic
distinguishing features: often distinct foliation fabric due to preferred alignment of newly formed minerals, such as
muscovite. Novices sometimes confuse foliation with sedimentary layering, which is caused by
changes in sediment particle size distribution and mineralogy.
The images below show a specimen of schist displaying foliation, if the rock is polished and cut
into a 30 µm thick thin section it is possible to observe the individual mineral grains. The image
below at right shows that foliation is caused by stretched-out minerals. The specimen in the
image above displays shiny, wrinkled surfaces due to the preferred alignment of muscovite
crystals.
http://www.trincoll.edu/~cgeiss/courses/mamo/schist.htm (1 of 2)2/21/2006 2:59:49 AM
(phyllite) schist
Many schists have shiny surfaces due to the presence of small, often submicroscopic, newly
formed metamorphic minerals. Since these newly formed minerals often display - yes you
guessed it - preferred orientation they all reflect the light at very similar angles, making the
surface shiny even though the individual crystals are too small to see. These rocks are called
phyllites, or phyllite schists. The image below shows a phyllite schist from Wyoming with nice
shiny surfaces.
main minerals: rather variable, depending on the initial mineralogy of the un-metamorphized rock
weathering behavior: many of the metamorphic minerals are affected by chemical weathering, crystal sizes can be
very small, causing rapid weathering.
impact on soils: depends on mineralogy
how it forms: Schist is a metamorphic rock, which forms through recrystallization of preexisting rocks under
high temperature and pressure. During metamorphosis the rock remains completely solid, and
pressure is often anisotropic, which leads to preferred orientation of newly formed minerals.
http://www.trincoll.edu/~cgeiss/courses/mamo/schist.htm (2 of 2)2/21/2006 2:59:49 AM