8.3.5: Schist

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8.28.png — Figure 8.28: muscovite schist

Schists, which form under medium-grade metamorphic conditions, contain medium-to-coarse flakes of aligned mica that we can easily see. This photo (Figure 8.28) shows a typical schist. Schists are higher- grade rocks than phyllites, and most form when phyllites are further metamorphosed. Thus, the precursors of schist are shale, slate, and phyllite. Less commonly, however, schist may form by metamorphism of fine-grained igneous rocks, such as tuff or basalt. Large and aligned flaky minerals, easily seen with the naked eye, define schists. These minerals are most commonly muscovite (such as in this photo) or biotite in parallel or near-parallel orientations that give the rocks schistosity – the ability to be broken easily in one direction but not in other directions.

Most schists are mica schists, but graphite, talc, chlorite, and hornblende schists are common. Quartz and feldspar are present in mica schists, often deformed or elongated parallel to the micas, and many other minerals are possible. If schists contain prominent minerals, we name them accordingly. So the schist in Figure 8.23 is a garnet schist, and the one in Figure 8.28 is a muscovite schist, or simply a mica schist. Photos of staurolite schist and kyanite schist are included later in this chapter (Figures 8.45 and 8.46).