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.