Spiral Cleavage of Mica

抄録

We chanced on a slice of mica which was cleaved spirally. The spiral lamella was several hundredths mm thick. Detailed observations on the structure of this specimen including polarization microscopic and X-ray studies are described. A tentative explanation of the origin of this unusually thick growth-layer is proposed assuming that an external force, which was exerted during the formation of the crystal, caused the left and right halves of the crystal to make a small relative rotation around the a-axis.