Direct observation of dislocations in potash alum

X-ray topography shows that single-crystal octahedra of potash alum grown from solution may contain very few dislocations, most of them being generated in the initial stage of crystal growth. In crystals of irregular habit more dislocations are found outcropping on the faster-growing faces than on the slower. Some crystals are sufficiently perfect to permit absolute measurement of the x-ray structure factor from the the x-ray Pendellösung period.