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PHYLLITE

Phyllite represents slightly higher temperature and pressure environment that slate.  Within the phyllite, the clay breaks down and recrystallizes to produce stable minerals under these new conditions. Specifically, the clay of the slate breaks down and undergoes a reorganization to produce mica.  The mica grains are very small, too small to be seen by the unaided eye, but large enough to effect the luster of the rock.  Phyllites have what is known as a "phyllitic sheen".  This is a satin-like luster helps to distinguish phyllite from slate in the field. 

    Note also that the phyllite has a very obvious break.  It is similar to slatey cleavage but, because of the typically higher pressures involved in the formation of the phyllite, the pressures often contort or disrupt the simple flat planes of the slate.