Austerlitz Phyllite
Berkshire County, Massachusetts

The rainbow is a quartz-rich layer in this metamorphic rock composed of very small grains of mica, chlorite, and quartz. Before metamorphism it was a thin layer of silt in mud. Stress in Earth's crust caused the rock to deform by folding. The relatively stiff quartz-rich layers formed broad, gentle folds, while weaker mica- and chlorite-rich layers folded into crinkles with a much shorter wavelength. Folds in rocks occur in all sizes from tiny ones like these up to monsters with dimensions measured in miles, yet their shapes tend to be remarkably similar at all scales.

Crossed polarizers with gypsum plate

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