A dark, fine-grained metamorphic rock consisting largely of quartz, mica, and particular feldspars.
- The slate, where thermally metamorphosed, has been turned into siliceous hornfels and mica schist.
- As a result, contact metamorphic minerals grow in random interlocking patterns giving rise to hornfels, a tough rock with no direction along which it will split preferentially.
- These are actually pseudomorphs of sericite (a fine-grained white mica) after cordierite and are found included in hornfels or slate as a product of contact metamorphism.
Mid 19th century: from German, literally 'horn rock'.
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