Spotted Slate
“Metamorphism caught in the act”
Spotted slate is a transitional metamorphic rock where dark spots of newly growing minerals — typically cordierite or andalusite — are beginning to form within an existing slate. It represents the early stages of contact metamorphism, capturing the moment a slate starts transforming into hornfels.
muscovite, chlorite, quartz, cordierite porphyroblasts, andalusite, biotite (developing)
Lake District (England), Skiddaw (England), Contact aureoles worldwide, Harz Mountains (Germany), Maine (USA)
3-5
The spots in spotted slate are often called 'chiastolite' when the andalusite crystals display a distinctive cross-shaped pattern — medieval pilgrims carried them as natural crucifixes.
Test Your Knowledge
Why do the cross-shaped andalusite crystals (chiastolite) form in spotted slate?
Photo Attribution
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