Slate is a low grade metamorphic rock generally formed by the metamorphosis of mudstone / shale, or sometimes basalt, under relatively low pressure and temperature conditions. Clay minerals in the parent rock metamorphose into mica minerals (biotote, chlorite, muscovite) which are aligned along foliation planes perpendicular to the direction of pressure. Slate is characterized by fine foliation along which it breaks to leave smooth, flat surfaces (often referred to as “slaty cleavage” – not to be confused with cleavage in minerals). Sometimes relict (original) bedding is visible on foliation planes. Slate will ‘ring’ when struck, unlike mudstone or shale which makes a dull ‘thud’.
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