13 Esquisto de Biotita, Fac. de Ingeniería, UNAM

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The mica schist, also known as muscovite mica schist or biotite mica schist, is a metamorphic rock that consists of mica, quartz and smaller proportions of other minerals. Its characteristic gray color is due to the presence of mica, both muscovite and biotite. It originates from the metamorphism of ancient clayey and slate series. It is composed of an alternation of fine layers of quartz and mica, and depending on the nature of this one, it is a mica schist with white mica (with muscovite, sericite, etc.), black mica (biotite, chlorite, etc.) or two micas (mixed). All contain small proportions of heavy minerals such as cornubita, garnet, epidote, cordierite, sillimanite, etc. These can form dark nodules, a few millimeters in diameter and the mica schist that contains those concretions of metamorphic origin are classified as nodular mica schists or, if the nodules are smooth, as spotted slates.