Mineral Composition
Slate is mainly composed of quartz and muscovite or illite, often along with biotite, chlorite, hematite, and pyrite and, less frequently, apatite, graphite, kaolinite, magnetite, tourmaline, or zircon as well as feldspar. Occasionally, as in the purple slates of North Wales, ferrous reduction spheres form around iron nuclei, leaving a light green spotted texture. These spheres are sometimes deformed by a subsequent applied stress field to ovoids, which appear as ellipses when viewed on a cleavage plane of the specimen.
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