They form the slate -
phyllite - schist - gneiss series. (Note that this is not the only way these
rocks form - it's only an example).
Slate rock. By
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in the beginning of the series, which is formed from sedimentary rocks
such as shale
are pushed down where
temperatures and pressures increase. In the process, clay minerals turn
into chlorite. Other minerals found in slate are andalusite,
cordierite, mica, quartz and others. Like the other rocks in the
series, slate has good cleavage.
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slate when the rock is pushed further down and the pressures and
temperatures increase. Chlorite is still a common mineral, but quartz,
such as muscovite are on increase.
Phyllite is foliated and often has small scale folding.
phyllite when pressures and temperatures increase even more. There are
different schists such as garnet, biotite, muscovite, kyanite schist
and more. Most have flaky minerals, glittery appearance, and of course,
the "schisty" texture with layers that break off easily. Garnet schist
is a classic with beautiful dark red garnet minerals.
form from schist
when temperatures and pressures increase even more. Garnet is commonly
on decrease while other metamorphic minerals such as sillimanite and
metamorphic K-feldspar may be on increase. Other minerals include
quartz, micas, hornblende, pyroxene
is known for its
compositional banding with alterating dark and light layers, formed by
high grade regional metamorphism in very high pressures and