Loops and Spirals on Thermally Etched Surfaces of Cleaved NaCl Crystals

Freshly cleaved (100) surfaces of laboratory grown single crystals of NaCl were thermally etched by placing them in a muffle furnace on a platinum lid. In addition to the usual square etch pits, various closed loop patterns have been beautifully illustrated. It is conjectured that the circular loop patterns, having a central spiral, might have been formed as explained by Lang (1957), while the pattern similar to that originating at Frank-Read sources and the multiple loop pattern might have been formed at pairs of dislocations of opposite nature.