Highly elongated disc- or lens-shaped crystal or aggregate of quartz, common in mylonites and high-grade rocks. In thin section the crystals are ribbon-shaped, hence the name. Quartz ribbons form by flattening of originally equidimensional quartz grains, or possibly by migration of grain boundaries to form single large grains from more fine-grained parent aggregates. They may exhibit undulose extinction, or be recrystallised into polycrystalline ribbons
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