Chlorite zone (Sample A94-1)
This is an aluminous phyllite from the Isle of Arran.
Whereas most chlorite-zone metasediments show little
conspicuous metamorphic mineral growth, this rock
has radiating clusters of high-relief chloritoid
tablets, set in a matrix of strongly-aligned muscovite
with a little pale chlorite. The fine opaque material
is graphite: this was once a black shale, rich in
organic material. Its bulk composition is marked x.
Note that for less Al-rich
bulk compositions such as that marked y, the
biotite isograd will be defined by the breaking of
the tie-line between chlorite and K-feldspar.