8.3.3: Slate

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8.26.png — Figure 8.26: Green slate from Pawley, Vermont. About 8 cm across.

Slates, which form during low-grade metamorphism of shales, comprise primarily microscopic clay grains, perhaps with some minor mica. Metamorphism may obliterate the original bedding as foliation develops perpendicular to the direction of maximum stress. This foliation, slaty cleavage, gives slates a property called fissility – an ability to break into thin sheets of rock with flat smooth surfaces. The photo in Figure 8.25 above shows an example of slaty cleavage. Figure 8.26, seen here, is another example of slate. The minerals in this rock cannot be identified in hand specimen, but in thin section quartz, feldspar, and chlorite can be seen. Slates come in many colors, but various shades of gray are most common. Thin sheets of slate have historically been used for paving or roofing stone.