Slate

Article By:

Van der Pluijm, Ben Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan.

Last reviewed:December 2019

DOI:https://doi.org/10.1036/1097-8542.627900

Any of the deformed fine-grained, mica-rich rocks that are derived primarily from mudstones and shales, containing a well-developed, penetrative foliation that is called slaty cleavage (see illustration). Slaty cleavage is a secondary fabric element that forms under low-temperature conditions (less than 300°C or 540°F) and imparts to the rock a tendency to split along planes. It is a type of penetrative fabric; that is, the rock can be split into smaller and smaller pieces, down to the size of the individual grains. If there is an obvious spacing between fabric elements (practically speaking, greater than 1 mm), the fabric is called spaced. Slates typically contain clay minerals (for example, smectite), muscovite/illite, chlorite, quartz and a variety of accessory phases (such as epidote or iron oxides). Under increasing temperature conditions, slate grades into phyllite and schist. See also: Argillaceous rocks; Clay minerals; Phyllite; Schist

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