Schist
Mudstone cooked and squeezed until its clays regrew as mica, all lined up. It splits into sheets and glitters.
How it forms
Mud is buried and heated to a few hundred degrees under directed pressure. Clay minerals break down and mica grows in their place, and because the pressure comes from one direction the new flakes all grow perpendicular to it. Millions of aligned mirrors is why schist shines.
Metamorphic — changed by heat and pressure
Rock taken deep, squeezed and heated until its minerals regrew in new forms, without ever melting. The new minerals line up under the pressure, which is why so many of these split in sheets or come out in stripes.
- What it is
- A strongly foliated metamorphic rock in which platy minerals — mica, chlorite, talc — have grown large enough to see and all point the same way.
- How to know it
- Glittery on a broken surface, and it splits along the mica planes into wavy sheets rather than flat ones. Often carries garnets as dark red lumps standing proud of the surface.
- How it wears
- Along its own grain, shedding flakes and plates that build loose, sliding slopes.
- What it means above ground
- Deeper and richer soil than granite gives, usually acid but genuinely fertile, on slopes that move. Good forest ground.
- Where you stand on it
- The Appalachians from Georgia to Vermont, the Coast Mountains of British Columbia, and the Manhattan schist that New York's towers are founded on.
- What lives on it
- Rich cove hardwood forest — the deepest, most diverse temperate forest in North America grows on Appalachian schist and its neighbours.
- What it's used for
- Flagstone and rough walling, and a source of talc, garnet abrasive and mica.
- What it does to the soil
- Sours the soil above it — nothing in it neutralises rain, so the ground stays acid.
Next to it in the cycle
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General descriptions of rock families. Any particular outcrop can differ — rocks grade into one another, and a geological map at this scale describes a district rather than a hillside.